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PHOTOGRAPHY 
 
 BY 
 
 LIEUT. HENRY A. REED, U. S. ARMY, 
 
 *> 
 
 Assistant Professor of Drawing, U. S. Military Academy, West Point, N. Y. 
 
 NEW YORK: 
 
 JOHN WILEY & SONS, 
 
 15 ASTOR PLACE. 
 1888. 
 
COPYRIGHT, 1888, 
 BY JOHN WILEY & SONS. 
 
 DRUMMOND & NET, 
 
 Electrotypers, PRESS OF 
 
 1 to 7 Hague Street, 
 
 New York. FERRIS BROTHERS. 
 
PREFACE. 
 
 IT would seem from the scarcity of published information on the subject of this 
 treatise, that its practical value has not been justly appreciated. In foreign countries, 
 and particularly in France, photography has been for a considerable period applied to 
 surveying ; and the results show a saving of time and labor that cannot be attained by 
 any other known means ; and, as in ordinary surveying, an accuracy that is directly propor- 
 tioned to the quality of the instruments employed and to the care exercised in the field- 
 work. In the United States, photographs are sometimes used as addenda to an important 
 map, because they are recognized as conveying information to be otherwise gained only 
 by actual inspection of the tract or subject represented ; and when it is considered that in 
 themselves they present all the data necessary for the construction of a map, thus rendering 
 field-work other than that required for their production unnecessary, then certainly this 
 method needs but to be better known to be appreciated. 
 
 In his own practice its value is so strongly impressed, that the author feels it a 
 duty to try and present the subject in a plain and concise manner, trusting that others 
 may so improve and enlarge upon it, that this method of surveying will not fail to become 
 more generally understood and practised. 
 
 Of the numerous foreign authors whose recent works have been referred to for in- 
 formation may be mentioned Laussedat, Girard, Gossin, Hannot, Bornecque, Javary, Tis- 
 sandier, Dufaux, Pizzighelli, Keucker and Colson. Acknowledgment is also due for 
 information obtained from the periodicals La Revue cTArtillene, La Nature* and the 
 Bulletin de la Socie'te' Fran^aise de Photographic. 
 
 WEST POINT, N. Y., January 23, 1888. 
 
CONTENTS. 
 
 SECTION I. 
 INTRODUCTORY. HISTORICAL SKETCH AND GENERAL PRINCIPLES, ..... i 
 
 SECTION II. 
 METHOD BY PLANE PERSPECTIVES. INSTRUMENTS AND MATERIALS, FIELD-WORK AND PLOTTING, 9 
 
 SECTION III. 
 METHOD BY CYLINDRIC PERSPECTIVES. INSTRUMENTS, FIELD-WORK AND PLOTTING. . . 36 
 
 SECTION IV. 
 METHOD BY RADIAL PERSPECTIVES. INSTRUMENTS, FIELD-WORK AND PLOTTING, . . 42 
 
 SECTION V. 
 THE CAMERA WITHOUT A LENS, . ........ 50 
 
 SECTION VI. 
 TELESCOPIC AND BALLOON PHOTOGRAPHY, . ....... 52 
 
 SECTION VII. 
 VARIOUS APPLICATIONS OF PHOTOGRAPHIC SURVEYING, AND SOME OF ITS ADVANTAGES, . 62 
 
 INDEX, . ............67 
 
 PLATES. 
 I. PHOTOGRAPHIC MAP, . . . . . . . . . End of Vol. 
 
.Cd 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 SECTION I. 
 
 INTRODUCTORY. -HISTORICAL SKETCH AND GENERAL PRINCIPLES. 
 
 HISTORICAL SKETCH. 
 
 I. PERSPECTIVE drawings were used as data for the construction of plans long before 
 the practical application of photography. Near the beginning of the present century, 
 Beautemps-Beaupr called attention to their use in topographical surveying. The instruc- 
 tions prepared by him for the officers of the frigate Bonite, preceding a voyage around the 
 earth, recommended them for the survey of the surroundings of places visited, and of the 
 coasts which were either passed by or could not be reached. His method was simply to 
 make drawings or careful sketches of the subject from any two stations, of which the 
 distance apart was determined ; then, by measuring with a sextant or other means, at 
 each station, the angle included by a visual ray to any point and the right line joining 
 the stations, data were afforded for orienting the sketches on the plot ; and a simple 
 geometrical construction, the reverse problem of perspective drawing, then sufficed to 
 locate the details in plan. The accuracy of the result evidently depended upon skill in 
 sketching. Colonel Leblanc subsequently endeavored to apply this method to military 
 topography ; but found, even under the most favorable circumstances, that the results 
 were only approximately exact ; and, having but few advocates, it was no longer em- 
 ployed. The result of these attempts induced Colonel Laussedat, professor of geodesy 
 at the polytechnic school, to try ana find some means whereby ordinary skill in draw- 
 aing would suffice for the production of accurate maps; and, in 1854, his efforts ended 
 successfully in his well-known adaptation to the purpose of Wollaston's camera lucida ; 
 and later in the telemetrograph, a combination of the camera lucida and telescope. 
 
 In 1839, when Arago presented Daguerre's famous discovery to the French Chamber 
 of Deputies, he said, " That the subjection of photographic images in their formation to 
 geometrical rules will enable us with very few data to determine from these images the 
 exact dimensions of the highest objects, of the most inaccessible structures. . . . We 
 could give some ideas we have entertained of rapid means of investigation that the 
 topographer will be enabled to borrow from photography. 
 
 In 1847, Martens, a French engraver, constructed a panoramic apparatus consisting 
 of a camera with a cylindrical daguerreotype plate vertical. y disposed ; the plate remained 
 stationary, while the part containing the objective revolved ; the light acted upon the plate 
 through a small vertical slit in a diaphragm pkued opposite the objective. Sharply- 
 
2 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 defined images were thus produced; but the preparation of the .curved plate, and sensitiz- 
 ing it by the collodion process, were attended with too much difficulty to make the 
 apparatus useful. 
 
 Martens' instrument was modified by Garella, L. Schuller, and also by Brandon, 
 so that the image was received upon a flat plate. The camera, by means of clock-work, 
 was made to revolve about a vertical axis through the optic centre of the objective, 
 carrying with it, and in rear of the objective, a screen which contained a vertical slit ; a 
 narrow sector of light was thus admitted ; at the same time the plate was made to move and 
 occupy positions such as to be continuously tangent to the cylinder described by the 
 base of the sector. Vertical lines were faithfully represented, but horizontals, not con- 
 tained in the horizon of the instrument, were represented by curves. 
 
 Similar modifications were made by J. Martens and by Koch; and Silvy substituted 
 for the glass and metal plates a sensitized paper which was made to roll from one cylin- 
 der to another. 
 
 In 1856, M. Chevallier made his first researches in this direction, and succeeded in 
 forming upon a single plate, horizontally disposed, an entire tour of the horizon. As 
 the camera he employed belongs to a class having certain special merits, a detailed 
 description of it in its present improved condition is given in the text, par. 46. Mangin's 
 camera also belongs to this class, and its description follows that of the former instru- 
 ment. 
 
 In 1858, M. Porro, a maker of instruments of precision, proposed an ingenious 
 apparatus intended especially for topographical purposes ; but the transformation of the 
 images necessary to the construction of the plan was so tedious and complicated, that it was 
 never brought into use. 
 
 Other modifications or improvements in panoramic cameras were made in 1857 by 
 Ross and Brooman, in 1861 by Sutton, in 1864 by Johnson, about 1865 by Busch, and 
 quite recently by Liesegang of Dusseldorf ; these instruments receiving various names, 
 such as the " pantoscopic camera," " metoscopic camera," and the " camera of rotation." 
 
 About 1858, M. Carette invented a camera giving views about two inches square, and 
 insured the verticality of his plates by means of a circular level. While he made the 
 exposures, an assistant measured with a pocket sextant the angles necessary to fix the 
 photographic stations. The negatives were used directly to obtain the coordinates of 
 different points of the perspective. This camera was employed simply as an accessory 
 to the ordinary topographical instruments to fix some of the points upon the sketch. 
 
 In 1864, Colonel Laussedat, now Director of the National Conservatory of Arts 
 and Trades in Paris, published in the Memorial de VOfficier du Gtnie a very thorough 
 description of photographic surveying, in fact so exhaustive that at the present date no 
 treatise upon this subject can be complete without making use of the results of his 
 researches, and no practice perfect without applying the principles that he established. 
 He may be called the inventor of practical photographic surveying. Two of his most 
 extensive undertakings were a partial survey of Paris in 1861, and of Grenoble and its 
 suburbs in 1864, the latter in charge of Captain Javary. In each case the topographical 
 maps were characterized by great accuracy ; in the former, agreeing precisely with the 
 regular survey made by the engineer in charge of roads and bridges, and in the latter the 
 
HISTORICAL SKETCH AND GENERAL PRINCIPLES. 3 
 
 differences of level as compared with the regular survey nowhere exceeding 19 inches, 
 which for ordinary scales is of course within the limit of permissible error. 
 
 During this period, and while Col. Laussedat's method was practised in France, 
 experiments were made in Germany by Meydenbauer, the counsellor in charge of public 
 buildings, with a view to applying photography to land and architectural surveying. 
 Apparently unaware of the researches of Beautemps-Beaupre and Laussedat, he first 
 resorted to photography for the construction of plans and elevations of certain structures, 
 difficult of access ; and like that of Beautemps-Beaupre, his method consisted in determin- 
 ing the orthographic projection from the photograph by reversing the direct perspective 
 problem. In land-surveying, his method was based upon the same principles as Lausse- 
 dat's ; but on account of the very narrow field of view which, in the objectives then 
 employed, corresponded to a true image, he would soon have abandoned his very costly 
 experiments had it not been for the optician Busch of Rathenow, who at this juncture 
 invented the pantoscope. This instrument, presumably similar to the modification of 
 Martens' heretofore described, gave a true image for a field of view of 105 ; the only 
 fault consisting in an unequal brilliancy of the centre and edges, which however could be 
 avoided by securing a favorable illumination of the subject. (Steinheil's apparatus, now in 
 use, is exempt from this fault.) Meydenbauer's work now began to attract much atten- 
 tion, and " photogrammetry," as the method with both the pantoscope and the or- 
 dinary camera was termed, was more extensively applied. One of his trial surveys was 
 that of Fribourg and its suburbs, covering an area about half a mile square, which was 
 plotted to a scale of T oVu> w ' tn IO f eet contours, by himself and a draughtsman who 
 had not taken part in the field-work. The field-work required 2 days ; there were 6 sta- 
 tions and 21 plates; the office-work was performed in 3 weeks: and the resulting map 
 was accurate in every respect. Elevations of buildings were also obtained with like 
 success. 
 
 During later years, several different instruments for the purpose have been invented, 
 of which the most important appear to be Duboscq's Polyconograph, Bertsch's Auto- 
 matic Camera, and Dubroni's Apparatus. In the first mentioned, the inner surface of 
 the shield of the plate-holder was subdivided by raised strips into fifteen equal squares, 
 and the strips being of such thickness as to touch the sensitive plate, the latter was 
 practically subdivided in a like manner. A metal part of the camera containing the 
 lens could be fastened at pleasure opposite any one of these squares, and an exposure 
 made by means of a corresponding opening in the shield. The focussing was effected 
 with the aid of a ground glass placed in the prolongation of the sensitive plate, the lens 
 part being attached to it for this purpose. This instrument was of very small dimensions, 
 the tripods when folded serving as a cane. 
 
 The term " automatic" was applied to Bertsch's camera because it was focussed per- 
 manently for distant views. The camera consisted of a four-inch metal cube, a lens of 
 four-inch focal distance, and the sensitive plates were two and one half inches square. 
 The proper direction for any view was given by means of an alidade with attached level, 
 having at one extremity an open rectangle which, with the other extremity of the alidade 
 as the -point of observation, exactly limited the field. The negatives produced were so 
 sharply defined as to admit of a useful enlargement of from 100 to 500 times their super- 
 
4 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 ficial area. A field laboratory, contained in a box fifteen inches square, accompanied 
 the camera. 
 
 The main features of Dubroni's Apparatus were its easy manipulation, and the use 
 of the camera itself for developing the plates. The camera was a box with yellow glass 
 sides, enclosed in another box of wood ; openings were provided as usual for the lens and 
 for introducing the plate. The latter having been collodionized and introduced, the box 
 was inverted, lens upward, and the silver solution was poured in through a tube. The 
 plate being thus sensitized, the instrument was then pointed and the exposure made ; 
 the box was again inverted and the plate developed by introducing the required solution ; 
 the fixing was delayed till the day's survey was finished. Since the focal distance was 
 adjusted by the instrument-maker, and rules for the manipulation accompanied each 
 instrument, the work could be performed by those not at all skilled in photography. 
 
 Doubtless other important inventions of this nature exist ; but those mentioned give 
 an idea of the progress made in the past, and, together with the instruments described in 
 the text, will serve to give a fair knowledge of the development of this very important 
 aid to surveying. 
 
 GENERAL PRINCIPLES. 
 
 2. Range and Object of the Metliod. Although the range of Photographic Surveying 
 extends to geodetic measurements, it is limited in the present treatise to ordinary plane 
 and topographical work. 
 
 As in surveying with compass or transit, this method has for its object the measurement 
 of angles and distances necessary to the location of points, for a graphical representation of 
 the important features of a limited portion of the earth's surface. Like an ordinary topo- 
 graphical map this representation is a horizontal projection with added references for heights ; 
 but beyond this, the means employed furnishes, without additional labor, a complete view 
 of the tract surveyed, with its details as they would be naturally presented to the eye, which 
 
 D 
 '-o 
 
 FIG. i. FIG. 2. 
 
 addition evidently increases the value of the result. Its accuracy, as will hereafter be shown, 
 may be said to be sufficient for the purposes above stated. 
 
 3. Location of Points. Two ways of locating points are used. 
 
 I. By the intersection of two or more right lines through given points ; thus, C 
 (Fig. i) is fixed in position if from two given points, A and -B, the directions Aa and 
 Bb are known; and if from a third point, D, Dd is also given, and Dd intersects C, a 
 
((UNI Vi 
 
 HISTORICAL SKETCH AND GENERAL PRINCIPLES. 5 
 
 check upon the work is afforded. It is evident that curved lines serve the same purpose 
 if their positions are known. 
 
 C may also be fixed if, as shown in Fig. 2, the distance A' B' and the angles C'A'B' 
 and C'B'A' are given ; and, as before, a third point D', being fixed, and the angle C'D'B' 
 given, a check is also afforded. 
 
 II. By an Azimuth and a Distance. Thus, in Fig. 3, the point E, of the right line 
 EF, given; G is fixed, if the azimuth z and the distance EG are known. A check is 
 had either by another azimuth and distance, or by the application of (I), using other 
 known points and lines. 
 
 The foregoing constructions apply to plane-photographic surveying. To locate a 
 point completely requires that not only shall its position in plan, or its projection upon a 
 horizontal plane, be determined ; but also that its elevation or depression with reference 
 to this plane shall be known. Thus, in Fig. 4, let K be the required point ; its projection 
 k in plan may be found by (I) or (II), e.g., as indicated, by the azimuth z and distance 
 Hk\ but its complete determination requires the elevation or distance //above this plane. 
 This is ordinarily found in photographic surveying, by measuring the angle of elevation 
 
 A* 
 
 / 
 
 k 
 7 
 
 /H 
 
 FIG. 3. FIG. 4. 
 
 @ by means hereinafter explained ; then h = Hk tang. @. The position of a point 
 below the horizontal plane is similarly determined by measuring the angle of depression. 
 
 4. Relation of Photographs to Perspective Drawings: Definitions of Terms. The ready 
 application of photography to surveying requires that the photographs taken in the 
 field shall be accurate representations of the features included in the field of view ; 
 in other words, that they shall be true perspectives. The definitions of certain terms 
 used in perspective drawing may here be found appropriate. In Fig. 5> the picture V 
 represents the distant topographical features as shown, and O the eye of the observer. 
 V is called the plane of the picture, and, for convenience in the application of geometric 
 rules, is taken in a vertical position; O is the point of sight; P, the point in which a 
 visual ray perpendicular to V pierces it, is called the principal point ; the horizontal line 
 HH' through P is the horizon of the picture, or simply the horizon, it is evidently the 
 intersection with V of the observer's horizon ; and the points in which visual rays, or 
 right lines from distant points to O, pierce V, are the perspectives of these points; thus 
 a, b, . . . . are the perspectives of A, B, . . . . These elements will be frequently desig- 
 nated hereafter by their distinguishing letters as above given. 
 
 5. To Determine the Direction of a Point from a Perspective Representation. If the 
 different points of a landscape and the visual rays intersecting them be vertically pro- 
 
PHOTOGRAPHY APPLIED TO SURVEYING 
 
 O 
 
 FIG. 5. 
 
 jected into the plane of the horizon, the angles included by the projections of the 
 
 visual rays are the true horizontal angles; the same that would be measured with a 
 
 theodolite or any other horizontal angle-meas- 
 urer: Thus, in Fig. 5, OP and HH', since 
 they are in this plane, are their own projec- 
 tions ; OA and OB are horizontally projected 
 in OA' and OB' . . . .; and a' Ob' is evidently 
 the horizontal angle included by OA' and OB'\ 
 a'Oc' the angle included by OA' and OC .... 
 Now let V, Fig. 6, represent a perspective, as 
 in Fig. 5 ; Since the positions of HH' and P, 
 and the distance OP, are always known ; if V 
 be laid upon a flat surface, PO can be set off 
 perpendicularly to HH', the different points a, 
 b, . . . . may be projected vertically into HH', 
 see a', b' , . . . . in the figure, and the hori- 
 zontal angle between any two points will then 
 be included by the right lines joining their 
 projections with O. Thus a Ob' is the true 
 horizontal angle of the points A and B, a 
 and b, in the figure. Hence the direction from 
 
 O of the projection of any point on the horizontal plane may be determined from the 
 
 perspective. 
 
 FIG. 6. 
 
HISTORICAL SKETCH AND GENERAL PRINCIPLES. 7 
 
 These angles may be measured with a protractor ; or, since Pa' , Pb' .... are the 
 tangents, to the radius OP, of the angles which they respectively subtend, any angle may be 
 determined by first measuring its tangent with a scale of equal parts; then the distance 
 thus obtained, divided by OP, is the natural tangent of the angle, and the angle is 
 
 p a ' _j_ Pb' 
 
 given in a table of natural tangents: Thus, tang, a' Ob' -^ . . .; b'Oc' is ob- 
 tained by determining POc' and POb' separately, then b'Oc' POc' POV . 
 
 The vertical angles are similarly determined ; e.g., to find the angle of elevation of 
 A (a in the figure) ; set off a' a" perpendicular to Oa! and equal to a' a, then measure 
 
 a"Od with a protractor ; or, since a! a" is the tan. a" Oa' to the radius Oa' , measure 
 Oa' with a scale of equal parts, then tan. a" Oa' -^ -. 
 
 Therefore, since its azimuth and vertical angle (see also Fig. 4) are given, the 
 true direction of any point with reference to O may be determined from a perspective 
 representation. 
 
 6. To Determine the Position of a Point from a Perspective Representation. The other 
 
 FIG. 7. 
 
 element required to fix a point completely is its distance from O, or, which is equivalent, 
 its direction from some other point of which the position with reference to O is known. 
 The latter method is generally used in photographic surveying; in other words, in this 
 process, points are fixed by intersections from known points. 
 
 This method requires that the points of observation O and O', Fig. 7, of two views, 
 V and V 1 , shall be fixed, and the directions of OP and O' P known : if the positions 
 of the horizons HH' are then given, it is only necessary to draw right lines from O 
 and O' through the projections of any point on these horizons and produce them to 
 intersection : Thus, to fix A in plan; produce Oa' till it intersects O'a', and similarly 
 for any other point represented on both views. The view from a third known point, 
 
8 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 embracing points on the other two views, would serve to check the accuracy of the plot. 
 
 To avoid repetition, the orientation of the views, or the determination of the 
 required directions of OP and the consequent positions of HH' for the different views, 
 is deferred to Plotting, par. 24 et seq. 
 
 The remaining element to determine is the reference, or the distance of any point 
 above or below the horizontal or datum plane. Referring to Fig. 8 ; the distances Oa! 
 
 and a' a being measured on the view, and OA' measured on the plot; then Oa' : OA' 
 \\a'a : A' A, whence A' A -~, ; denoting A A by H, and OA' by D, and since -~ , 
 
 is the tang. @ (see figure) to the radius Oa', there results the general expression for the 
 
 reference of a point 
 
 H=D tang. @; 
 
 the -f- and signs referring respectively to distances above or below the datum plane. 
 
 Since the position of a point may thus be found, any point may be completely 
 determined from a perspective representation. 
 
 The next step is to describe the instruments and means employed in the Field-work. 
 
SECTION II. 
 
 o 
 
 METHOD BY PLANE PERSPECTIVES.-INSTRUMENTS AND MATERIALS, FIELD-WORK 
 
 AND PLOTTING. 
 
 INSTRUMENTS AND MATERIALS. 
 
 THE instruments and material required to obtain a photographic perspective are the 
 camera with its objective, a compass, level, certain other appliances which will be de- 
 scribed in their appropriate places, and the sensitive plates. 
 
 7. Camera for Plane Perspectives. Fig. 9 represents the kind of camera used for this 
 purpose; it is the kind generally used for photographic work. Its principal parts are the 
 back B, front F, bellows b, 
 objective O, objective-slide 
 D, ground-glass G, rails R, 
 and tripod T. H is the 
 plate-holder, s its shield, 
 and / the needle-points, 
 of which the use is here- 
 inafter described. Any care- 
 fully-constructed camera of 
 this type can be used with 
 good results. The special 
 requirements are that the 
 different parts shall be so 
 fitted to each other as to 
 admit of rigid and uniform 
 adjustment. The tripod 
 legs should be quite stiff, 
 and when of the ordinary 
 telescopic pattern, as in the 
 figure, the levelling of the 
 instrument is very quickly 
 effected ; the binding screw 
 which secures the camera 
 to the tripod-head should 
 be large and strong. In 
 focussing with the special 
 pattern shown in the figure, 
 the back is fixed to the FlG 
 
 rails, while the front is made 
 
 to traverse by means of the milled head shown at the base of the front; but the 
 ordinary pattern, in which the back is made to traverse, evidently serves the same purpose. 
 
io PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 When adjusted for taking a view for surveying purposes, the optic axis of the 
 objective should be horizontal, and the film surface of the sensitive plate the plane of 
 the picture (par. 4) perpendicular to it. This condition is generally assured by levelling 
 the camera when the ground-glass and objective-slide are parallel ; and it also requires that 
 the plate-holder should so fit the back, that the sensitive plate when inserted shall 
 exactly replace the ground-glass. In a well-constructed camera these conditions either 
 exist permanently, or are capable of adjustment as will be hereinafter explained. The 
 form shown in Fig. 9 is called a "double swing-back," because the back can be turned a 
 few degrees about either a horizontal or a vertical axis, and the plate may thus be placed 
 and clamped in positions oblique to the optic axis ; this appliance is very convenient 
 for ordinary photographic work, but unnecessary for the present purpose ; in fact, with a 
 swing-back, an additional adjustment is required to make the ground-glass and objective- 
 slide parallel ; a plain, rectangular, bellows camera, firmly constructed, is the simplest and 
 best for surveying purposes. (See also close of pars. 20 and 24.) 
 
 8. The Objective. An objective is required that will produce upon a plane surface a 
 
 true perspective of a landscape, or at least one sufficiently exact 
 for the present purpose. There are doubtless other suitable forms, 
 but those known to the author are Dallmeyer's Rapid Rectilinear, 
 his Wide-angle Rectilinear, Steinheil's Aplanatic, and the Hermagis 
 90 Angle objectives ; they are respectively English, German, and 
 French patterns adapted to surveying purposes; and the accuracy 
 of representation resulting from their use is directly proportioned 
 
 to the care exercised in the field- and office-work. Each consists 
 FIG. io. 
 
 of a double combination of lenses ; each combination being made 
 
 by joining a concavo-convex lens, , Fig. TO, of which the concave face has the greater 
 curvature, and a meniscus, &, Fig. io, of which the convex face has the greater 
 curvature. In the Dallmeyer combinations, the former is of flint, and the latter of crown- 
 glass ; while in the Steinheil they are of flint-glass, but of different density. 
 
 Fig. ii shows the manner in which the lenses are arranged in the objective-tube, 
 thus forming a symmetrical double combination. By this construction and the use of 
 proper diaphragms, or "stops," one of which, s, is shown inserted, these objectives are 
 corrected for spherical and chromatic aberration and are free from distortion; therefore 
 by accurately focussing, the images formed are true and distinct throughout. 
 
 Fig. 12 represents the Rapid Rectilinear 10X12, the greater number indicating in 
 inches a side of the largest square plate which may be effectively covered by the image: 
 the dimensions 10X12 are the trade size of the plate. The lens-tube T is attached 
 by means of a screw-thread to the flange F, which is fastened by screws to the objec- 
 tive-slide; the cover or cap fitting the extremity E of the tube, and used in making 
 ordinary exposures, is usually of leather lined with velvet; it should fit closely, and at 
 the same time permit of removal without jarring the camera, a pneumatic shutter, ad- 
 justable for either time or instantaneous exposures, is much to be preferred. 
 
 9. Compass and Level. A convenient form of an attached compass is that in which 
 the degree graduations are numbered from o to 360, from right to left, or in the 
 inverse order of the numbers on a watch-dial, and the o-:8o line is in the vertical 
 
JO 
 V 
 
 METHOD BY PLANE PERSPECTIVES. 
 
 plane of the optic axis of the objective, or in a plane parallel to it, the o being to the 
 front; for in this case, when the camera is pointed toward the N.E. quadrant, the indi- 
 cations will be in the natural order from o -to 90, and so on throughout a revolution, 
 thus requiring no computation, and that the numbers only of the graduation be recorded. 
 
 FIG. ii. 
 
 FIG. 12. 
 
 FIG. 13. 
 
 In the absence of an attached compass, a hand-compass will serve the purpose, and for 
 convenience while reading may be rested upon the top or on the rails of the camera. 
 
 The level may be fixed to the camera by the instrument- 
 maker or detached, and in either case the circular form, Fig. 
 13, is very convenient. If attached, it is desirable to have two 
 of this form, one on the back and the other on the front of 
 the camera: one is shown in position at /, Fig. 9. 
 
 10. Sensitive Plates and C hanging-boxes. During most of 
 
 the period considered in par. I, rapid dry plates were unknown, consequently the work 
 was obstructed by cumbersome apparatus and the delay incident to long exposures and 
 field development. At the present time, not only have dry plates been brought to a 
 state of great perfection ; but, where very light outfits and great rapidity of manipu- 
 lation (as instanced in the roller plate-holder) are elements of prime importance, paper 
 has replaced glass. It is well to state, however, in this connection, that for exact 
 work in surveying, glass negatives have not yet been equalled; they retain their shape, 
 and the images are thus undistorted by chemical manipulation ; (see also par. 32.) 
 
 Any of the standard dry plates are suitable; but with reference to detailed repre- 
 sentation, a 5X8 (inches) is the smallest that can be used to advantage. They are taken 
 into the field in their original packing-boxes, the plate-holders, of which six double 
 ones are a convenient number, having first been filled in the dark-room. A negative- 
 box for exposed plates is necessary; also a "field-tent," or a thick orange or ruby- 
 colored cloth, underneath which the plates may be transferred as needed from the 
 plate-holders to the negative-box, and from the packages to the plate-holders. A chang- 
 ing-box is also used for this purpose; the plates sliding directly from it to the holder, 
 and the converse, making the tent or cloth unnecessary. 
 
12 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 A recent invention, due to Capt. de Torres of the Spanish army, consists of a 
 wooden box of sufficient dimensions to contain 24 plates, which are held parallel and 
 at a small distance apart by lateral grooves. One of its sides, double the length of 
 the box, slides in longitudinal grooves and contains a central vertical slit, large enough 
 to permit a plate to pass freely through it; and an indexed scale and button on one of 
 the edges serve to place and fasten the slit opposite any plate. A corresponding slit 
 is in one extremity of the plate-holder, and each slit is furnished with a stiff cloth 
 screen which opens and closes automatically. To transfer a plate: the end .of the plate 
 holder is placed against the side of the box, so that the slits are opposite each other; 
 the box is tilted, and the plate slides into the holder, where it rests upon four small 
 triangular pieces of steel placed midway of the sides of the latter, and which also serve the 
 same purpose as vhe needle-points (par. 7): the plate is held firmly in place by a clamp- 
 screw of which the milled head projects at the back of the holder. As an additional 
 safeguard against the admission of light during the transfer, the end of the holder is 
 made to fit grooves on either side of the slit in the box. A plate after exposure is 
 transferred back in a similar manner. 
 
 Another and very simple device consists in making the camera-back of metal and 
 of sufficient size to contain, say, a dozen plates, each plate contained in the thin flexible 
 holder which is in common use. The plates in place, a leaf-spring attached to the 
 rear inner face of the box or back, and which bends downward, forces the plates to 
 the front. The cover of the box consists, in addition to the lid, which is closed for 
 transportation, of a rubber cloth, large enough to admit of the shield being withdrawn 
 within it from any holder, and also to permit a holder, after exposure of its contained 
 plate, to be transferred from the front to the rear of the series. The last plate-holder 
 of the series is readily distinguished by a notch cut in its shield. 
 
 11. Tests and Measurements Required. It has been observed that for good work the 
 different parts of a camera should be susceptible of rigid and uniform adjustment; 
 appliances are also needed in the field operation of levelling, to determine the horizon 
 of a view; certain preliminary tests and measurements are therefore necessary, and these 
 taken in their appropriate order are : 
 
 I. The Test for Register. 
 II. The Measurement of OP, or the Focal Distance. 
 
 III. The Determination of HH' for any View. 
 
 IV. The Measurement of the Field of View. 
 To these is added a Test for Distortion. 
 
 Since the accuracy attained in field-work and plotting will depend almost entirely 
 upon the result of these operations, hardly too much care can be exercised in their per- 
 formance. 
 
 12. Test for Register. When the plate-holder is inserted, the camera should "regis- 
 ter;" that is to say, the film surface of the sensitive plate should exactly replace the 
 rough surface of the ground-glass. To test for this: focus an object, a few yards distant, 
 accurately on the latter, using a large stop; then substitute for the sensitive plate a 
 plate of ground-glass, or of other glass covered with a thin translucent film, the film to 
 the front; withdraw the shield and observe if the image is as distinct as before; if not, 
 
METHOD BY PLANE PERSPECTIVES. 13 
 
 note and mark on the rails the distance the back has to be moved to the front or rear, 
 to make it so, and adjust accordingly in making exposures. As in cases of other faults 
 of construction, when this test is made at 'first and the defect discovered, the instru- 
 ment-maker should remedy it. 
 
 13. Measurement of t/te Focal Distance, or OP. For general work in photography, 
 the focal distance of an objective, the distance of the focus from the optic centre, is vari- 
 able, depending upon the distance of the subject from the optic centre; but, in survey- 
 ing, since the nearest points of the features to be represented are usually so far distant 
 that the rays of an incident pencil from any point may be regarded as parallel, the 
 focal distance for parallel rays is required. This for a combination of lenses is called 
 the equivalent focal distance, but for the sake of brevity the term " equivalent " is herein- 
 after omitted. 
 
 All objectives of the same name and size are supposed to have the same focal 
 distance, which is usually stated by the maker; but slight variations in their manu- 
 facture give different values for it, and it is therefore best to measure it for each one. 
 
 For a simple convex lens, double or piano convex, the position of the optic centre 
 
 S 1 
 
 s 
 
 FIG. 14. 
 
 is easily observed, and it is only necessary to measure, with a scale of equal parts, the 
 perpendicular distance from it to the rough surface of the ground-glass; but for a double 
 combination lens this operation is evidently impracticable and any of the following 
 methods may be employed: 
 
 I. Set up and level the camera; place a staff, S, Fig. 14 (a horizontal projection), 
 vertical and about 75 yards from G, the rough surface of the ground-glass; and another 
 staff, S', also vertical about 10 yards from S, and so that 55' shall be perpendicular to 
 PS. Having drawn a vertical line through the middle point of G, focus 5 upon it; 
 measure PS, the horizontal distance between G and 5, 55' and ss' : then from the 
 similar triangles thus formed, Os or OP : ss' :: OS : 55'; OP : ss' :: OP : ss' ; whence 
 2OP : 2ss' :: OS+OP : SS' + ss'; by substitution, OP : ss' :: PS : SS' + ss' ; therefore 
 
 OP = 
 
 PS X ss' 
 
 SS'-\-ss' 
 
 75 X 36 X 2 
 
 e.g.: P5=75 yards, SS' 10 yards, and ss' = 2 inches; then OP= "T~ ! 4-9 2 
 
 
 inches, very nearly, near enough for practical purposes, ss' can be measured either by 
 exposing a sensitive plate and applying a scale of equal parts to it after development; 
 or on the rear surface of the ground-glass, by carefully marking off on the edge of a 
 
I4 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 strip of white paper, the distance ss', and applying the scale ; in any case, ss' and SS' 
 should be measured in the same direction. 
 
 II. Having set up and levelled the camera, focus two points, as 5 and S', Fig. 14, 
 75 or 100 yards distant, and so disposed that their images on the ground-glass will be 
 separated horizontally by a distance of about one third of its breadth. Make one image, 
 as s, coincide with the middle vertical of G; and measure the distance ss'. Remove the 
 camera; set up a transit or other angle-measurer at O, and measure the angle SOS' = 
 sOs', which denote by a\ then 
 
 OP = - = ss' cot. a. 
 tang, a 
 
 III. It is a well-known principle of optics that when the sizes of image and object 
 are equal, the distance between them is four times the equivalent focal distance of 
 the lens. Therefore, to measure OP: construct upon a sheet of white paper two right 
 lines at right angles to each other; from the point of intersection as a centre, and with a 
 radius of about one third of the shorter side of the ground-glass, describe the circumference 
 of a circle, and fasten the sheet to a vertical wall or board ; set up and level the camera 
 in such a position that. the image of the centre of the circle shall be in the optic axis, 
 the middle point of the ground glass when the objective-slide is in its normal position 
 (see par. 14); and focus so that the construction on the paper, and its image, shall be 
 of equal size, which condition can be ascertained by the application of a pair of dividers. 
 Remove the objective and measure the distance from the ground glass to the paper: one 
 fourth of this distance is the required value of OP. This method usually requires a 
 long camera, or an extension piece in front to hold the lens. 
 
 Whenever the focal distance is determined by Method I. or II., the positions of the 
 parts of the camera, especially that of the back or front according to which one of these 
 is made to traverse, should be indicated by marks carefully made upon the metal mount- 
 ings, so that this adjustment can be readily made at any time. 
 
 By Method III., the position of the optic centre can be indicated upon the 
 objective-tube by first replacing the latter, and then setting off from the paper one half 
 the distance between paper and ground-glass, or 2OP. It can also be indicated in 
 Methods I. and II. by means which will suggest themselves. 
 
 14. The Determination of the Horizon, or HH' . First find the middle point of the 
 ground-glass, which is readily done by drawing its two diagonals, and through it draw 
 two right lines parallel to a horizontal and a vertical edge respectively of the ground- 
 glass frame. These edges should be perpendicular to each other ; but if, through faulty 
 construction of the frame, they are not so, then set up the camera and level it, and, 
 with the aid of a carpenter's level, locate the horizontal line which passes through this 
 middle point, and draw it ; and also draw a vertical line through the same point. 
 
 The camera being set up and levelled, focus on a distant point, previously ascer- 
 tained with a surveyor's level to be on a level with this horizontal line, and move the 
 objective-slide up or down until the image of the distant point is cut by the horizontal. 
 The objective, or objective-slide, is now in its normal position, and H H' will evidently 
 pass through the middle point of a view taken under these conditions. 
 
METHOD BY PLANE PERSPECTIVES. 
 
 It is apparent that the amount of sky or of foreground in the view can be changed 
 by raising or lowering the objective, and that this condition usually enables the surveyor 
 to adjust his camera so as to include all the features required, whether the station be 
 elevated or depressed with reference to the surrounding features; but in moving the 
 objective-slide from its normal position, H H' will no longer pass through the middle 
 point of the view, and some means are necessary for determining its true position. 
 
 A simple device for determining HH' under these conditions is as follows: The 
 right-hand part of Fig. 15 represents a vertical section through the optic axis, G the 
 
 K 
 
 H 
 
 h" 
 
 P' 
 
 -P 
 
 / 
 
 H' 
 
 pro 
 
 FIG. 15. 
 
 ground-glass, D the objective-slide ; and K represents the plate-holder, or, in case of a 
 small plate, the kit (a frame fitting within the plate-holder) used to hold it. (See also 
 H, Fig. 9.) When D is in its normal position and an exposure made, H H' is marked 
 upon the negative, and therefore upon the print taken from it, by means of the 
 needle-points / projecting from the middle points of the vertical edges of the holder 
 or kit, and immediately in front of the sensitive plate. 
 The points p' similarly mark the position of the middle 
 vertical. The author has made use of fine cambric nee- 
 dles, reduced to about one third of their length, and 
 heated, before insertion, to prevent breaking when the plate 
 was pressed against them. For needles, metal triangular 
 pieces might be substituted, the plate resting upon them. 
 The true positions for the needle-points may evidently be 
 found in a manner similar to that described for drawing the 
 horizontal and vertical upon the ground-glass, or by other 
 simple means. It is apparent that when D is raised or 
 lowered, HH' will rise or fall an equal distance, and will 
 be found at, say, Jih' or Ji'k'" . To measure this vertical 
 interval, Hh or H/i", construct on the front of the camera 
 a scale of equal parts, as shown at c in Fig. 16, and an 
 index, which may be an edge of the strap r, on the slide will 
 then mark the division denoting the required interval. The index should mark o when 
 the slide is in its normal position, and the divisions above and below be considered 
 as positive and negative respectively. By this construction HH' may be fixed upon 
 any negative or print by setting off vertically in the proper direction, from each of the 
 marks produced by the needle-points /, the distance measured by this scale, and joining 
 the outer extremities by a right line. 
 
 FIG. 16. 
 
1 6 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 It is apparent that any truly-horizontal line of a print or negative may serve for the 
 measurement of horizontal angles ; but the needle-points usually mark HH' through the 
 middle point, and since paper prints are subject to distortion by chemical manipulation 
 (see also par. 32), this position is likely to give more exact measurements. (For another 
 method of determining HH', see close of par. 20.) 
 
 15. Measurement of the Field of View. The extent of the field of view of the camera 
 is an important element in surveying, since it determines the amount of the visible landscape 
 that can be included in the photograph. It is measured, horizontally, by the angle included 
 by right lines drawn from the optic centre to the extremities of HH'; and, vertically, 
 by right lines from the optic centre to the extremities of the middle vertical of the 
 ground-glass. The horizontal field of view is therefore twice the angle of which the 
 
 i H H ' 
 tangent is - np , and the vertical field is twice the angle of which the tangent is 
 
 i middle vertical 
 
 ~ I e --> " HH 9-6 inches and OP - 16 inches, then 
 
 p 
 
 i 9". 6 9 
 
 1 6" 32 
 
 
 which practically is the tangent of 17; .'. the horizontal field of view = 34. 
 
 This element may be determined graphically by constructing the angle on paper, 
 and then measuring it with a protractor. 
 
 This is evidently the field of view for the particular camera to which the objective is 
 attached, and a like construction gives it for any sized plate. To measure the field of view 
 of the objective ; use, instead of \HH', a radius of the circle of light formed on the ground- 
 glass, when the objective is attached to a camera larger than that to which it is suited. 
 
 Owing to the defect?, due to development, nearly always existing along the edges of 
 a plate, it is advisable in the above measurement to make an allowance of 2 or 3 for 
 any plate. It is seldom that the vertical field is insufficient. 
 
 When haste is an important element, a wide-angle objective, of which the focal dis- 
 tance is much less and the field therefore much wider, may be used to great advantage. 
 In Dallmeyer's 8xiO rapid rectilinear, OP is 13", affording a field of about 38; while in 
 his 8X10 wide-angle rectilinear, OP is 7", and the field is about 54; therefore with the 
 former, 10 plates would be required for a complete tour of the horizon ; but with the 
 latter, only 7 plates are needed. 
 
 1 6. Test of an Objective for Distortion. To ascertain beyond a doubt that the objec- 
 tive is free from distortion throughout the entire field of view assigned to it, a modifica- 
 tion or extension of the method used for measuring OP (par. 13) may be employed. 
 Thus, in Fig. 17, a horizontal projection, place additional vertical staves at S'', S'" .... 
 in SS' prolonged, the number and location being such that their images /', s'" .... are 
 well distributed along any line of G, one of them being near each of the vertical edges 
 of the latter, and s being on the middle vertical. 
 
 Measure, horizontally, the distance from 5 to each staff ; expose a plate, and, on the 
 developed negative, project s, s' . . . . into HH'; from OP and the tangents ss'", ss"' .... 
 determine the angles sOs'", sOs {v . . . . ; then compare these angles with SOS'", SOS iv . . . , 
 
METHOD BY PLANE PERSPECTIVES. 17 
 
 computed from the radius OS and tangents SS'", SS {V .... The corresponding angles 
 of the two sets should be equal. Or simple proportions may be used ; thus, ss'" should 
 be to SS'" :: Os, or OP, : OS . . . . ; or an angle-measurer may be substituted as in measur- 
 ing or. 
 
 To be very accurate, revolve the objective 90 and repeat the operation. 
 
 S" 
 
 FIG. 17. 
 
 Another method is as follows : Subdivide a rectangle, constructed on a flat surface, 
 into equal squares; make the dispositions prescribed in III., par. 13, for paper and camera; 
 and compare the different sides of the squares with their images. When a value for OP, 
 less than that given in III., par. 13, is employed, it is only necessary that these images 
 should be proportional reductions of the original squares. For convenience, the sides of 
 the rectangle should be proportioned to the edges of the ground-glass ; and, in focussing, 
 their images should lie near these edges. 
 
 17. Exposure, Development and Printing. Before proceeding to the Field-work, a 
 brief description of the operations which result in a *' view" photograph suited to survey- 
 ing purposes may be found serviceable. For fuller details of manipulation, reference may 
 be made to Manuals of Photography. 
 
 (a] The Exposure. The camera is placed firmly upon its tripod, levelled, and pointed 
 toward the subject ; the back or front, according to which of these is made to traverse, 
 is then adjusted to the mark corresponding to the focal distance (see close of par. 13); the 
 objective-slide is so disposed that the image will occupy the most favorable position upon 
 the ground-glass, and the index number of the slide is noted. The proper stop is then 
 placed in the objective ; the latter is capped to exclude the light, and the plate-holder, 
 containing the sensitive plate, is inserted. The levelling should now be verified. The 
 shield is then withdrawn, the cap is removed for a period corresponding to the sensitive- 
 ness of the plate, and atmospheric and various other influences soon observed in practice ; 
 the shield is now replaced. With a particularly rapid dry plate (sensitiveness 25) and a 
 bright day, try the use of the smallest stop and an exposure of about half a second. (See 
 also par. 23.) 
 
 (d) The Development. A printed circular, giving the process of development best 
 suited to each of the many kinds of dry plates manufactured, usually accompanies each 
 box of plates ; but the following is a simple and good one : 
 
PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 In the dark-room, remove the plate from the holder and wash it for half a minute 
 under the tap ; then develop with the following solutions : 
 
 No. i, a filtered sat. sol. of oxalate of potassa -j- a few drops of oxalic acid. 
 No. 2, a filtered sat. sol. of sulphate of iron --)- a few drops of sulphuric acid (C. P.). 
 To four parts of No. I add one part of No. 2, first diluting each separately with 
 half its bulk of water ; place the plate, film up, in a shallow porcelain or japanned tray, 
 and cover it quickly with this developer ; keep the liquid in motion by gentle rocking, 
 and develop until some of the details are visible on the glass side ; then wash the plate 
 a minute under the tap, and place it, film up, in the fixing-bath, composed of hyposul- 
 phite of soda i part -f- alum T ^ part -J- water 5 parts ; when clear, that is when the white 
 film has disappeared, leave it a few minutes longer in this bath, and then place it in 
 running water, where it should remain at least one hour (five hours are better). The 
 resulting negative is then permitted to dry spontaneously. A few drops of a 15-gr. solution 
 of bromide of potassium in water, added to the developer, will sometimes result in giving 
 more of the finer details; and distant mountains may be brought out with greater clear- 
 ness by brushing them once or twice, using a camel's-hair brush, with a 5-gr. solution of 
 this bromide during the development. A clearing solution is sometimes needed after 
 fixing ; but is oftener needed with pyro than with iron developers. 
 
 . (c) The Printing. Albumenized paper is lightly rubbed smooth with a Canton flannel 
 pad ; then sensitized by floating it about 2 minutes on a silver bath of 50 gr. to the oz., care 
 being taken not to let the solution touch the back of the paper. It is then, after thorough 
 drying, exposed in a wooden box to the fumes of strong ammonia, for from 30 to 40 min- 
 utes the shorter period in warm weather. The negative having been placed, glass face out- 
 ward, in a printing-frame, Fig. 18, a piece of sensitized paper of suitable size is laid carefully 
 
 on the film ; the back, b, is pressed down and secured, and the 
 frame is exposed facing the sun, or bright sky, until the lines 
 of the print are slightly bronzed, which condition may be ob- 
 served by means of the hinged sections of the back. When 
 a sufficient number of prints have been made, they are 
 washed in running water for about 5 minutes; then put 
 in a bath of salt i part -|- water 50 parts, until the lines turn 
 red; they are then "toned" with chloride of gold, i gr. to 
 each sheet of paper, made by adding this to a solution 
 of borax i part -f- water 80 parts a sufficient quantity to contain the prints ; they are 
 left in this bath and kept continually in motion until the red lines turn black ; then 
 washed a few minutes, and placed in the fixing-bath, same as for plates, and left 10 
 minutes. They are then placed in a strong solution of salt and water for 10 minutes, 
 and finally are washed for about 5 hours in running water. After drying, trimming and 
 mounting, they become the ordinary landscape photographs. 
 
 Other and simpler processes for printing, such as the " blue-process," using a negative 
 to print from, may be resorted to; and in most cases afford results sufficiently good for 
 the plotting. 
 
 FIG. 18. 
 
METHOD BY PLANE PERSPECTIVES. 19 
 
 THE FIELD-WORK 
 
 1 8. General Considerations. The general principles of ordinary surveying apply in the 
 field-work; the main conditions to be fulfilled in order to ensure satisfactory results 
 being the selection of suitable points of observation, or stations, and the proper distribu- 
 tion of the stations to obtain favorable intersections and to economize time. 
 
 The three following methods of working are described : 
 
 I. With the Camera, when the triangulation has been established. 
 II. With the Camera and Hand-compass. 
 III. With the Camera alone. 
 
 19. Selection and Distribution of Stations. The photographs obtained should embrace, 
 as far as possible, all of the features to be represented upon the finished map. This is of 
 easy accomplishment when commanding points exist, and easier than might at first be 
 supposed when the tract to be represented is comparatively level. What is considered 
 covered ground in ordinary surveying presents no greater difficulty here ; e.g., in the case 
 of a heavily wooded section of the tract, in working with transit or compass, salient 
 points only of the outlines are usually exactly located from the commanding stations, the 
 rest being filled in by eye; while in photographic work, with usually no additional labor 
 in the field, every visible curve and detail of these outlines may be plotted from the 
 photographs should it be desirable so to do. 
 
 As in ordinary surveying, meanders with prismatic compass and note-book are occa- 
 sionally needed, more especially to prevent multiplication of photographs in cases of views 
 being obscured by the interposition of abrupt salients or spurs; and, as in the case above 
 given, to represent roads or paths through the covered ground. 
 
 The intersection of lines very oblique to each other does not fix a point with clearness; 
 
 FIG. 19. 
 
 the most favorable intersections, in the case of two photographs of the same section, will be 
 afforded when the photographs are taken from stations, so disposed with reference to each 
 other, that, in the exposures, the optic axes produced would intersect at a right angle ; 
 between these limits there is a wide range for selection, and on this selection, or choice 
 of stations, the successful result greatly depends. For an ordinary tract of, say, 3x4 
 miles in extent, three stations disposed, as shown in Fig. 19, at the vertices A, B and C 
 
20 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 of an approximately equilateral triangle, would doubtless give the best result. If the 
 borders do not contain commanding points, three interior stations similarly disposed might 
 prove equally good; but, on account of the resulting wider field at the different stations, 
 more plates would usually be needed. 
 
 An important element in this selection, when exact work is required, is that check 
 lines shall be afforded for all important points; in other words, and as above illustrated 
 that each of these points shall be found upon one photograph from each of at least three 
 stations. 
 
 As to economy of time, and that of plates also may be added, this rests principally 
 the judgment exercised in the grouping of the stations. It is unnecessary that any 
 >mt to be represented should appear oftener than as above stated ; and practice shows 
 that the best arrangement of the stations is in groups of three, each one of a group facing 
 the exposures at that station being confined to, the same section of the tract An 
 application of this principle is afforded in selecting for stations the vertices of triangles 
 m a tnangulation system; a complete tour of the horizon would then usually embrace 
 several triangles. 
 
 20. First Method-With the Triangulation Previously Established.-^^ method of 
 development, from a measured base, of a system of triangles or quadrilaterals, of which 
 the vertices are exact positions of prominent points, is too well known to require a de 
 rnption here; , t is only necessary that this system should be adapted to the present 
 purpose,-that the positions of the stations should meet the requirements stated in par 19 
 The stations having been established, and the photographer furnished with a tracing 
 the triangulation sheet in which they are alphabetically marked, the work is begun 
 preferably at one which has an exceptionally good command ; for this may lessen the 
 number of exposures which might otherwise be required from one or more adjacent sta- 
 ns, without diminishing the number of checks required for good work. A tour of the 
 horizon, or so much of it as may be needed, is then made, working systematically here 
 as at all other stations, from left to right. The exposures are recorded in their order of 
 completion F ', F a ', V a \ . . . , the sub-letter referring to the station. Since the plate 
 holder shields are numbered in the natural order ,, 2, 3, . . . ; if the contained plates 
 exposed accordingly, mistakes will be obviated, means will be afforded in the plotting 
 as will be hereinafter shown, for properly locating the different views, and the work can 
 be plotted by those not present at the field-operations. 
 
 The following-described method of working and form of record were employed with 
 good results by the author : 
 
 The camera is set up, pointing in the general direction of the left-hand view ; ad- 
 justed to the focal distance; levelled approximately; then carefully pointed so as to in- 
 lude the objects on the extreme left ; the objective-slide is then adjusted so that the 
 field shall mclude the important features, and its index number is recorded The 
 plate-holder is now inserted; the camera levelled accurately; the exposure is made, and 
 earing of the optic axis as indicated by the compass is also recorded. 
 The camera is then turned to the right for the next view, and in this operation care 
 to include in the second view the objects on the extreme right of the field in 
 rt; this , s necessary for reasons given in par. 15, and because this double represen- 
 . serves a purpose in the plotting (see par. 26); a second exposure is then made 
 same precautions as before observed, and so on for the remaining views 
 
METHOD BY PLANE PERSPECTIVES. 
 The form of record employed is here given. 
 
 21 
 
 
 VIEW: 
 
 
 
 No. 
 
 Index 
 Number. 
 
 Bearing. 
 
 A 
 
 I 
 
 
 
 
 
 
 
 2 
 
 O 
 
 227 
 
 
 B 
 
 I 
 
 2 
 
 8 4 
 
 
 The index number of the objective-slide is frequently the same for all views taken 
 at any one station. 
 
 In this instance, a detached hand-compass was used ; as soon as the plate-holder was 
 withdrawn, the bearing of a point of the image of any prominent object, cut by the mid- 
 dle vertical of the ground-glass, was taken, and a description of the point was recorded 
 either verbally or by a rough sketch (see Fig. 20), sufficiently intelligible to be recog- 
 nized in the developed negative. Since this vertical extended throughout the field, no 
 difficulty arose in finding some object projected upon it and well disposed for the purpose. 
 It is apparent that- the bearing indicated by an attached compass immediately after the 
 exposure, and before the camera is 
 disturbed by withdrawing the plate- 
 holder, is likely to be more exact. 
 
 From a consideration of the fore- 
 going, it is seen that with suitable 
 appliances the camera could be auto- 
 
 \\ X 
 
 \ \ 
 \ 
 \ 
 \ 
 
 V 
 
 X 
 
 \ 
 
 \ 
 
 
 x v 
 
 ^ 
 
 
 
 FIG. 20. 
 
 FIG. 21. 
 
 matically checked in its revolutions, for the different fields in succession ; or, a hori- 
 zontal limb might be added and the arc of revolution measured by it, thus requiring 
 but one bearing for a set of views from any station. 
 
 A judicious use of the hand-compass may frequently serve to diminish the number 
 of views otherwise required. Thus, in Fig. 21, the buildings are located by means of the 
 view from station A, supplemented by the compass bearing of the front line taken from 
 any point of it, as C, which has been fixed by interpolation, or otherwise, from stations 
 
22 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 already determined. This line evidently intersects the lines of direction from A in required 
 points of the buildings ; similar applications of the compass will suggest themselves. 
 
 The needle-points//', Fig. 15, could be dispensed with and the horizon, or HH' of 
 a print, be otherwise determined as follows : At the station from which any view is 
 taken, measure with a hand-clinometer the angle of elevation or depression of a promi- 
 nent point, situated near the right and left vertical, respectively, of the field of view a 
 third point would serve as a check, and its vertical angle should therefore be measured 
 take also from this station and from at least one other station the compass-bearings of 
 these three points, so that their plan may be constructed. Set off, in the proper direc- 
 tion from the image of each point on the print, a vertical equal in length to the natural 
 tangent of the corresponding angle X distance from station to its horizontal pro- 
 jection ; then the right line joining the outer extremities of the verticals will be the 
 required horizon. It is apparent that, by a simple graphical application of the "three- 
 point problem," the three compass-bearings taken as here described might also serve to 
 orient the projected horizons on the plot. 
 
 21. Second Method With Camera and Hand-compass. There being no previous tri- 
 
 FJG. 22. 
 
 angulation in this case, the stations are determined by intersecting compass bearings 
 taken as the work progresses, the details of the photographic work conforming to those of 
 Method 1. 
 
 Thus, as illustrated in Fig. 22, beginning at A, a tour of the horizon, or so much 
 thereof as may be needed, is made ; and, if no natural signal exists by means of which the 
 camera's position at A may be subsequently recognized from at least one other station, as B 
 

 METHOD BY PLANE PERSPECTIVES. 2$ 
 
 or C, an artificial signal is set up ; this may be a white or black flag according to the 
 nature of the background as viewed from the second station, whether earth or sky respec- 
 tively. Proceeding to B, another tour is made and the bearing of A taken. If a suit- 
 able position for the camera at C or D can be discerned from B, its bearing is also 
 taken ; and, as a rule, on reaching any station, the bearing of every other station to be 
 subsequently occupied, or which has already been occupied, when its position can be discerned, 
 should be taken and recorded. 
 
 Interpolation by compass-bearings is of much use here ; e.g., referring to the preced- 
 ing figure, C is fixed if from it the bearings of A and B are taken, since these bearings if 
 plotted at A and B and produced must intersect at C. 
 
 The bearings evidently serve to locate the stations in their proper relative positions, 
 the only element lacking to plot to any required scale being the exact length of some 
 line represented in the view. This element may be supplied, as in ordinary triangula- 
 tion, by the measurement of a side as AB; or a shorter distance as ab may be meas- 
 ured and the points A and B fixed by intersections ; in either case the measured dis- 
 tance is plotted according to the desired scale, and is the base of development for the 
 plotting. 
 
 Plotting to scale may also be effected as illustrated in Fig. 23. Assume two points, 
 as A and B, in their true directions 
 with reference to each other, but with 
 the intervening distance taken at 
 pleasure ; then, starting from these 
 points, plot the extremities a and b of 
 a distance which has been measured 
 on the ground ; also plot some con- 
 tained point, as C, used as a station. 
 Construct the triangle ABC; and 
 from a set off toward b, ab' = the 
 measured distance reduced to the 
 required scale ; then from A set off 
 ab' X AB, 
 
 B 
 
 FIG. 23. 
 
 , and through B' draw a line parallel to BC; B' and C' thus determined 
 
 are the required positions of B and C for the given scale, and the triangle AB'C' forms 
 a basis of development for the plotting. 
 
 22. Third Method With Camera only. In this case, it is best to start from a meas- 
 ured base. With an attached compass, the bearings of OP for the different views serve 
 to orient the views on the plotting sheet. The case in which a compass is used for 
 orienting only, and that in which it is dispensed with altogether, require no further de- 
 scription than that given for the plotting in " Special Cases of Orientation," par. 26. In 
 regard to the stations, careful attention should be paid to occupying those points only 
 which may be clearly recognized in the views taken from adjacent stations ; and in this 
 respect it is best to have at all stations visible signals, natural or artificial, which present 
 a strong contrast to their surroundings. With ordinary care this method is quite accurate, 
 and may often prove the only one available; but, on account of the independent data 
 afforded, the addition of compass-bearings, of at least the stations, is advisable. 
 
24 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 23. Best Ti,nefor ,he Fi.ldvork.-f* to the season of year: for wooded tracts, the 
 best tune ,s undoubtedly the Spring or Fall, when the leaves are off the trees and the 
 range of v,ew is thereby very much increased ; the Winter season also often affords favorable 
 opportunities. For tracts not wooded, or with scattered .trees only, there is but little 
 
 The dry plates now made can be used in all seasons with equally good effect 
 As to the time of day: the main rule to follow in visiting the different stations is 
 . so arrange the route, that at any station the camera may be pointed toward the sun 
 as httle as poss.ble. The most favorable direction of the sun's rays is from the right or 
 ft, and rear, of the instrument; in some cases, however, as in that of hilly or undu 
 ung ground, it is of advantage to have them in a direction nearly perpendicular to the 
 c axis, thus giving the elevations and depressions greater prominence by the addition 
 the,r visible shades and shadows. In this respect, a little observation of the relative 
 rees of clearness with which various portions of a landscape are seen, under different 
 Jegrees of inclination of the sun's rays, will prove of value. Bouguer's table of the 
 tensmes of solar rays corresponding to different altitudes of the sun is very instruc 
 t,ve here, an. s given below ; ,0,000 represents the intensity in a perfectly transparent 
 
 SUN'S ALTITUDE. 
 
 INTENSITY. 
 
 SUN'S ALTITUDE. 
 
 INTENSITY. 
 
 
 
 6 
 
 o 
 
 
 
 
 2O 
 
 5474 
 
 I 
 
 7 
 
 2 5 
 
 6136 
 
 2 
 
 192 
 
 30 
 
 6613 
 
 3 
 
 454 
 
 40 
 
 7237 
 
 4 
 
 802 
 
 5 
 
 7624 
 
 5 
 
 1201 
 
 70 
 
 8016 
 
 10 
 
 3M9 
 
 90 
 
 8123 
 
 '5 
 
 4534 
 
 
 
 Since the actm.c effect of solar rays is directly proportioned to the length of time 
 during wh,ch they act, the variations above given must be mainly due to the d.ffeen 
 d,stances traversed in the atmospheric envelope; for the same reason, and because of the 
 f the atmosphere, exposures made in mountainous or elevated tracts should 
 
 lira T:; a : at the r level - According to this tabie ' *- -<*. " 
 
 . at 8 A.M. and 4 P.M. should be of the same duration ; but, in practice it is 
 found that, owing to the comparatively greater amount of moisiure t the er 1 o 
 
 o ;rar q d uent t partial r absorption f the uitra - vi iet ** <"< d 4i2 
 
 eLtof ,'ht "fl TH f ' S , aIS ^^ ''" la " d -P-P>* g ~Pt>y that the actinic 
 light reflected from wh.te clouds is even more powerful than that of a perfectly 
 clear sky; brmgmg out the details of deep valleys with great clearness 
 
 after a ?*"* " *"* **" ^^ ' reqUent ' y "" in Winter, and usually 
 r a storm during all seasons. ' 
 
 On account of the extreme rap.dity of the field-work as compared with that of any 
 
METHOD BY PLANE PERSPECTIVES -5 
 
 of the other methods of surveying, it is seldom that the surveyor may not take advan- 
 tage of the most favorable conditions and circumstances. 
 
 THE PLOTTING. 
 
 24. General Considerations. The plotting of a survey consists of the two following 
 distinct operations: 
 
 I. Constructing the plan, or the horizontal projections of the different points and 
 lines ; 
 
 II. Determining the heights of points with reference to the datum plane , and, as 
 already described in par. 6, the result is the complete determination of these elements. 
 
 It seems hardly necessary to state here that the principles defined and illustrated 
 in Section I., and there confined to a perspective drawing, apply directly to a photo- 
 graphic representation. Thus, in Fig. 24, V and V represent respectively the relative 
 positions, with reference to 
 the point of sight, of a per- 
 spective drawing and a pho- 
 tographic representation ; they 
 are vertical and parallel, equi- 
 distant from O, and the right 
 line POP' pierces each at its 
 middle point ; and since 
 any point as D is projected FIG. 24. 
 
 by the same visual ray in d and d ', which are equidistant from the middle points of 
 V and V , the two views are similar. Another consideration ; since the maps are sel- 
 dom constructed to a scale greater than ^VoiT' an< ^ because OP seldom exceeds 15 
 inches, ^ of an inch to this scale, it makes no sensible difference in the plotting 
 whether in the field-measurements a station-point is directly beneath the optic centre 
 of the lens, or beneath the ground-glass ; to be strictly exact, however, it is apparent 
 that the camera should be so constructed as to revolve about a vertical axis through 
 the optic centre ; and that in setting it up in the field the optic centre should be 
 plumbed over the station-point. 
 
 25. To Construct the Plan. Assume two views, VJ and V b \ Fig. 25, taken from 
 the extremities of a given line, AB, and embracing the same objects Having plotted 
 AB; from each extremity, with OP as a radius, describe circumferences as shown 
 Draw the radii A V u and BV b \ making with the line of the magnetic meridian 
 angles (20 and 280 in the figure), equal respectively to the bearings of OP for the 
 views; then the tangents hti , at the extremities of these radii, are the required projec 
 tions of HH ' . This operation is termed the Orientation of the views. 
 
 All the points which are to appear in the plot having been projected on the 
 respective horizons of the views (par. 5), and then transferred by means of dividers 
 or otherwise, to the corresponding projections hh' , any point is plotted by simply draw- 
 ing right lines from the plotted stations through the transferred projections, and pro- 
 ducing them to intersection ; thus c, the intersection of Ac and Bc'\ and d, of Ad' and 
 
26 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 Bd' t are the required projections of the actual points C and D. If a view from a third 
 station also contains these points, a check is afforded upon the accuracy of the plot. 
 
 FIG. 25. 
 
 The plan of the entire survey is plotted in a similar manner; by first orienting all 
 the views at their respective stations, and then fixing the required points by intersec- 
 tions as above. 
 
 26. Special Cases of Orientation. I. Reference has been made to the necessity of 
 
 having the views overlap slightly. This con- 
 dition may be utilized, not only as a check in 
 ^ the plotting, but also to orient the views when 
 
 ^ - compass-bearings of OP have been omitted. 
 
 Thus, in Fig. 26, let o' and o" be the representa- 
 tions of the same object on the consecutive views 
 Va and F a 2 , and suppose the compass-bearing 
 of A Va is unknown ; to orient the second view, 
 in other words to find the tangent-point of V a : 
 at d construct a tangent to the given arc, and 
 on it set off dd' equal to the horizontal distance 
 measured, on the second view, from the middle ver- 
 tical to the image of O , join d' and A by a right 
 line ; then will V* be the tangent-point required. 
 
 It is readily seen that in the other case, when the bearing A V* is known, the check 
 referred to is afforded by o' and o" being on the same line of direction. 
 
 FIG. 26. 
 

 METHOD BY PLANE PERSPECTIVES. 
 
 27 
 
 II. When the compass-bearing of some line of direction other than the optic axis 
 is given, as that of AO, Fig. 27; -graphically, plot at A the bearing of O as indicated; 
 construct the tangent at p, and make pp' equal to the horizontal distance, measured 
 on the print, from the projection of o to the middle vertical of the print ; then 
 Ap' will intersect the given arc at the tangent-point required : or, trigonomctrically, 
 since the horizontal distance pp' , or Vo', is the tangent of VAo' to the radius 
 
 OP = A V; tan. VAo' = 
 
 , 
 
 and VAo' can then be obtained from a table of natural 
 
 tangents ; therefore first plot the bearing of O ; then protract VAo, and the side A V 
 will intersect the given arc at the tangent-point required. 
 
 III. In the case of confined space on the plotting-sheet: when the plotted station is 
 near the border of the sheet, it may happen that the usual orientation would cause the 
 work to fall outside of the sheet. This inconvenience can be avoided either by propor- 
 tional reduction, or by reversing the position of the transferred horizon ; thus, in Fig. 28, 
 
 Jiorder 
 
 FIG. 27. 
 
 FIG. 28. 
 
 AV is the direction of OP, and HH' falls without the border. With Av = $ or f . , . . 
 of A V, as a radius, describe the interior concentric arc ; at v, construct the tangent kh', 
 its length bearing the same ratio to HH' that Av does to AV: then with sector, pro- 
 portional dividers, or otherwise, transfer the different horizontal distances, proportionally 
 reduced, from the view to hh' : the required intersections are then found as heretofore 
 described. 
 
 Or, by reversing the positions of the horizons; thus, at V , the intersection of A V 
 produced with the circumference, construct the tangent H" H'" . It is plain from inspection 
 of the figure that in the transfer, the horizontal distances are now to be set off from V 
 
 o 
 
 in the reverse direction, e.g., o' on the view falling at o'" the line of direction remain- 
 ing unchanged. When necessary, the horizon can be both reduced proportionally and 
 oriented in reverse on an inner circumference. 
 
 IV. Suppose the camera to have been used independently of the compass. Thus, 
 
28 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 in Fig. 29, if the view from A contains both B and C, and that from B both A and 
 C: draw a right line AB representing the horizontal distance from A to B according to 
 the scale of the map. With radius OP describe arcs as shown. Project the images a, 
 b and c on the horizons of their respective views. To orient the views: A, b' , a' and B 
 must be contained in AB\ and C should serve as a check for accuracy. At t, the inter- 
 section of one of the arcs with AB, construct the tangent tg, in length the horizontal 
 distance from P (par. 4) to 6', measured on the print; draw Ag\ then will the tangent at 
 p be the projected horizon, /i/i', of the view from A. ti'ti" is similarly constructed; and 
 the subsequent plotting from these views is performed as already described for other cases. 
 C being thus fixed, and the view from it embracing A or B, the plotting is extended as 
 
 FIG. 29. 
 
 above described for the stations A and B; and so on throughout the survey. It is ob- 
 served that stations to be occupied must be so located as to be readily fixed by 
 intersections from those already occupied. Numerous opportunities for checking will 
 evidently be afforded. (See also par. 22.) 
 
 27. To Determine the Heights, or References. The general expression for the vertical 
 distances of points from a datum plane is (par. 6) H = D tang. @ ; therefore, the 
 plan having been constructed, in order to obtain the reference of any point from a view, 
 it is only necessary to measure on the plot the distance from the station to the point 
 and multiply it by tan. @. 
 
 A reference may be determined arithmetically, trigonometrically, or mechanically. 
 
 Arithmetically : Referring to par. 6 and Fig. 8, tan. @ = yp ; therefore measure a'a, or 
 
 the vertical distance of the point from HH' \ divide this by Oa' and multiply the quo- 
 tient by the plotted distance OA' ; the product is the required reference. Trigo- 
 nometrically : a'a" , Fig. 6, being equal to a'a measured on the view, make the con- 
 struction indicated in this figure; measure a" Oa with a protractor; find the tan. of 
 
METHOD BY PLANE PERSPECTIVES. 
 
 29 
 
 this angle from a table of natural tangents, and multiply it by OA' . The mechanical 
 determination is described in par. 29. 
 
 28. Mechanical Measurement of Horizontal and Vertical Angles. To avoid linear con- 
 structions upon the negative or print, the author has devised a tangent-glass, which may be 
 constructed as follows: select a flat, clear pane of glass, a trifle larger than the ground- 
 glass of the camera; coat one face with white wax reduced by heat to a thin liquid; 
 after the wax is poured on, it may be made to cover evenly by gently moving the 
 glass back and forth over a lamp-flame. When cold, rule with a needle-point two right 
 lines HH' and VV, Fig. 30, exactly perpendicular to each other, intersecting at the 
 
 12 
 
 14- 
 
 h 
 
 FIG. 30. 
 
 middle point P, and approximately parallel to the edges of the glass. From a table of 
 nat. tangents, compute the tangents, to the radius OP, first of , then of i, i, and 
 so on, increasing by for each computation, up to the number of degrees required to 
 include half the vertical field of the camera, full degrees only are shown in the figure. 
 Set off these tangents, Pv, Pv', Pv* . . . . , on VV in each direction from P; through 
 their extremities, v, v', v* .. . . , draw lines parallel to HH' and mark them as shown. 
 Make a like construction of lines parallel to VV. The ruled face is then exposed to 
 the fumes of hydrofluoric acid, produced by adding sulphuric acid to fluor-spar in a leaden 
 
30 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 dish. The dish may be made from sheet-lead, by bending up the edges to form sides 
 about I inch in height, and should be a trifle smaller than the glass so that the latter 
 may rest upon the edges and prevent escape of the fumes. When etched, the wax is 
 removed and the tangent-glass is ready for use. 
 
 To ascertain a horizontal angle, it is only necessary to place the glass so that HH' 
 shall cover the horizon of the print, and VV the middle vertical ; and then read off 
 the required angle. This position of the glass is evidently assured by means of the 
 images of the needle-points (par. 14) ; also by location of a central point (par 20). 
 
 With the usual focal distance, the sides of the small rectangles are of sufficient 
 length to admit of estimating to & of arc, which for a distance of i mile represents 
 about 9 feet; but with the aid of a microscope, and a scale of hundredths of inches, 
 angles of i' are easily observed. For rough work, as in topographical sketching, the glass 
 may be replaced by tracing-cloth, the frame, Fig. 34, then coming in handily. 
 
 To ascertain a vertical angle: Referring to Fig. 31, HH' represents the middle hori- 
 
 r\ 
 
 
 \ 
 
 H 
 
 
 
 H' 
 
 yi I/ 
 
 FIG. 31. 
 
 zontal, and VV the middle vertical of the tangent-glass, ab is a circular arc in the 
 plane HOH', this arc evidently representing the intersection with this plane, of a 
 cylinder of which the vertical axis passes through O, and of which VV is the tangent 
 element in the plane of the picture. From consideration of the figure, it is apparent that 
 the only vertical angles which can be measured directly with the glass, on the view, are 
 those of points contained in the plane of O and VV ; and that to read the vertical angles 
 of other points, it is first necessary to find their tangents, in terms of the radius OP 
 and the azimuths ; which may be effected as follows : 
 
 Suppose the tangent-glass laid upon the view as described for the measurement of 
 horizontal angles, and o and / to be points of the view on the same horizontal. De- 
 note the angle pOP by @, and the angle oOo', which is evidently less than pOP, by @'; 
 also let z denote the azimuth POo' . Then to obtain the required value for tan. @': 
 
 Pp OP 
 tan. @ = -^-p ; tan. @ =^5-7 = -^; but Oo' = = OP sec. z ; 
 
 PP. 
 
 'OP 
 
 PP 
 
 o'o 
 ~Oo' 
 
 cos. z 
 
 tan. &' 
 
 tan. 
 
 OP sec. z ' sec. z 
 
 and since - = cos., tan. 
 sec. 
 
 = tan. 
 
 cos. 2. 
 
METHOD BY PLANE PERSPECTIVES. 31 
 
 Therefore on stiff paper or card-board (or the glass might be of sufficient extent for 
 the purpose) make the construction shown in Fig. 32, of which the part subdivided into 
 rectangles is a duplicate of the upper half of Fig. 30. From P, on the middle vertical, 
 set off PO = the focal distance; draw AB parallel to HH' ; and produce the verticals 
 (not produced in figure) to meet AB. Let o be any point of the view, and o" the intersection 
 with AB of the vertical passing through it : place a straight-edge, or scale of equal parts, 
 on /'and o", and from P set off along the edge, indicated by the broken line, Po'" = o'o: 
 
 
 
 
 \ 
 
 \ 
 
 1 
 
 \ 
 \ 
 
 \ 
 
 
 
 
 
 
 I 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 ^ 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 1 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 <0 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 21 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 f-" 
 
 
 
 r 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 ^ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 \ 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 0' 
 
 
 
 
 
 
 
 \ 
 
 P 
 
 
 
 
 
 
 
 
 
 
 
 
 
 H' 
 
 00 
 
 FIG. 32. 
 
 the required vertical angle of o will then be indicated by the horizontal passing through 
 
 o" ' . For, Pr is the cos. of the angle o'"Pr; and, since cos. = ,- ^- , the base or 
 
 hypothenuse 
 
 tan. @' = tan. @ cos. z, as above. This scale may be termed the vertical angle scale; 
 and, for a short focal distance, may be conveniently etched upon the same glass with, 
 and as an extension of, the tangent scale. 
 
 29. Mechanical Measurement of HetgJits. The vertical angles determined, another 
 scale which may be called the scale of heights can be used for the lineal measurements. 
 Let CD, Fig. 33, represent the scale of distances used in the construction of the plan. 
 From the o extremity as a centre, and with a convenient radius, describe a circular 
 arc, which subdivide into half degrees (whole degrees in the figure), and draw radials 
 as shown. To measure the height of any point, as O in the preceding figure: meas- 
 ure its horizontal distance on the plot; suppose this to be 1765 feet; at d, the 1765' 
 division of the scale, measure the vertical distance dd' to the radial indicating the 
 vertical angle, in this case the 3.6o' radial; which distance, reduced to the scale of 
 the map, is the height required. The map scale itself may be used for this purpose, 
 or a scale of equal parts can be substituted. For a map scale of 317 W gV incn divi- 
 
 o" 
 
 3* 
 
 & 
 
PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 sions measure to within 9 feet, and estimation to half of this amount is easily had by 
 eye ; but the application of y^-inch divisions, and the aid of a magnifying-glass, are 
 advisable for more exact work. 
 
 D 
 
 FIG. 33. 
 
 Great exactness may be obtained by doubling the length of the scale of heights, 
 and using correspondingly double the distances measured on the plot. 
 
 When it is considered that, once made, these scales serve for all views taken with 
 the same lens, the slight labor expended in their construction is certainly of little 
 moment as compared with the facility afforded by their use. It would be advantageous 
 to have the last one described engraved on metal. 
 
 30. Reduction of References to a Datum Plane The Contouring. In the determination 
 of heights heretofore described, the plane of reference is the horizon of the station from 
 which a view is taken ; but, as in all topographical work, the measurements thus made 
 for the different stations must all be reduced to a single or common datum plane. 
 As previously stated, this is assumed as passing through or below the lowest point of 
 the tract. The reduction is made in the usual manner, by adding to the measurements 
 for any view the reference of the station at which that view was taken. To facili- 
 tate this operation, a form of record similar to the following will prove convenient : 
 
 FORM OF RECORD FOR REDUCTION OF HEIGHTS TO COMMON DATUM. 
 
 VIEW. 
 
 POINT. 
 
 REFERENCE. 
 
 REFERENCE OF STATION. 
 
 FINAL REFERENCE. 
 
 REMARKS. 
 
 
 
 Feet. 
 
 Feet. 
 
 Feet. 
 
 
 V 1 
 ' a. 
 
 I 
 
 -f- 102 
 
 + 460 
 
 + 562 
 
 
 
 2 
 
 + 9 
 
 
 55 
 
 
 
 3 
 
 - 25 
 
 
 435 
 
 
 * 
 
 * 
 
 * 
 
 * 
 
 * 
 
 
 No difficulty arises in determining the reference in the fourth column ; each station 
 will appear on at least one of the views, and its reference is then obtained the same 
 as for any other point ; if it appear on two, a check is afforded. In the case of re- 
 ciprocal views, the difference in elevation of the stations is readily ascertained from the 
 
METHOD BY PLANE PERSPECTIVES. 33 
 
 relative positions of the HH'% on them; and when the same point is represented on 
 views from two stations, the difference of its references, as measured on them, is evidently 
 the difference in height of the stations. 
 
 As to the contouring, or plotting of contours: the reduced or final references hav- 
 ing been noted beside the plotted points, the views are examined, and the confor- 
 mation of ground carefully observed ; the equidistant points are then marked on the 
 plot, and the contours can be drawn with nearly if not quite as much precision as is 
 usually attained in the " irregular methods" of contouring with other instruments. Addi- 
 tional points are supplied where needed for this purpose, and aid is derived from the 
 condition, that all points cut by HH' on any view, have the same reference as the 
 station from which that view is taken. From the fact that lines of direction, from stations 
 to points of profiles, are tangent in plan to the corresponding contours, additional aid is 
 afforded, and is frequently made use of in this part of the plotting. 
 
 31. Suggestions in Regard to Plotting. The plotting of a triangulation is fully 
 described in text-books of topographical drawing. The triangulation is plotted first ; then 
 come the orientation of the views at plotted stations and the construction of the 
 plan ; and, finally, the determination of the heights and the contouring. It will be 
 found advantageous to fasten together in their proper order a set of views taken from 
 each station, as the general view of the tract thus afforded gives a better knowledge 
 of the work in hand, and the different points are more readily recognized ; blue-prints 
 serve this purpose. Each view should be marked as already described, V a \ V . 
 V . . . so as to be quickly identified ; and, as soon as oriented, the transferred 
 horizon is to be similarly marked. It is well to number, on the views, all the points 
 to be plotted, using like numbers for the same points both on the different views 
 and for their projections on the transferred horizons. In the plotting by intersections, 
 silk threads may be used if desired, instead of a straight-edge : each thread is fast- 
 ened at one extremity to a needle^ thrust into a plotted station ; the threads are 
 then stretched over corresponding transferred projections and by their intersections fix 
 the points. The references of points should be marked as soon as determined, beside 
 the plotted points ; and also recorded for reduction to a common datum (par. 30) ; for 
 marking the final references, it is best to use a colored pencil. If possible, all im- 
 portant points should be checked by a third intersection ; in fact, to secure a faithful 
 representation, all the precautions that are observed when other methods of surveying 
 are employed, should be taken. 
 
 32. Preparation of Prints Advantage in Use of Negatives. When the plotting is to 
 be done from prints, or regular photographs, these should be carefully and equally damp- 
 ened between sheets of blotting-paper, and then pasted, without rubbing, to stiff card- 
 board. As already stated, if the tangent-glass is used, no lines need be drawn upon 
 them ; otherwise, the horizons are drawn ; and the points to be plotted, numbered as 
 described in par. 31, are projected by verticals into their respective horizons. The 
 horizons and any other desired lines may be printed from the negatives, by previously 
 marking them on the latter with a fine needle-point. 
 
 Owing to distortion of the paper, frequently produced in the chemical manipulations, 
 prints are less reliable than the glass negatives ; therefore for exact work the latter should 
 
34 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 be used. Sometimes, owing to unfavorable conditions of weather, or to improperly timed 
 exposures, negatives do not show details with sufficient clearness for plotting purposes. This 
 fault can ordinarily be remedied by using transparencies made from these negatives, pref- 
 erably by the wet-plate process, and which will usually give fine definition. Better prints 
 are often obtained from obscure dry-plate negatives by first making transparencies as above, 
 and then, from the latter, making wet-plate negatives to print from. If the plotting is 
 rapidly done, the paper prints, if excluded from light when not in use, may be em- 
 ployed without subjecting them to the process of toning, etc., and the consequent 
 liability to distortion. In any of these operations, increased clearness in definition of sky- 
 lines, hill-profiles and of other features, may be obtained, to almost any degree, by em- 
 ploying the usual process of retouching. Enlargements or reductions to any desired scale 
 are readily effected by the ordinary photographic process. In applying the tangent-glass, 
 it is more convenient to place the negative, film down, upon its etched face, and to 
 rest both upon a strong plate of glass secured to an open frame, so disposed that 
 the light may be reflected from a white surface upward through the glass to the 
 
 eye. Fig. 34 represents a frame of 
 this kind ; placed in front of a window, 
 and tilted at any desired angle, a sheet 
 of white paper beneath serves as a 
 reflector, and all but the reflected light 
 is easily excluded by an opaque curtain 
 conveniently disposed about the frame 
 and draughtsman. With this arrange- 
 ment, the details are clearly visible, 
 and ruler and triangle may be applied 
 without inconvenience. The negatives 
 also should be labelled so as to be readily identified. 
 
 To prepare prints for accurate plotting, proceed as follows: Construct a duplicate of 
 the tangent-glass lines on the film of an exposed, blank, developed and fixed dry plate, 
 using a needle-point and marking prominently the extremities of HH' and VV; print 
 these lines on the sheets of sensitized paper intended for the views; and, in the sub- 
 sequent printing, be careful to adjust these sheets so that the extremities of HH' and 
 VV will rest exactly upon the images of the needle-points on the station negatives. A 
 variation from this operation, and one which may possibly give better results, is to expose 
 each dry plate, for an instant, beneath the ruled negative, before exposing for the land- 
 scape-views ; the tangent-lines will then appear in the latter. Care should be taken to 
 have both sensitive plate and ruled negative rest evenly on the base and against one 
 end of the plate-holder; in ruling the negative, HH' should be at the exact height of 
 the needle-points, and marks made on the sensitive plate, in its first exposure, to show 
 which edges of it to place up and against the end for the view. 
 
 33. Choice of Scale Accuracy Attainable. As a result of experience it has been 
 found that the scale of j-faf, or of T ^, is the most convenient for photographic work. 
 A smaller scale would naturally be used for reconnaissances, in which the observations 
 are made with great rapidity and therefore as a rule with less precision. 
 
 FIG. 34. 
 
METHOD BY PLANE PERSPECTIVES. 35 
 
 As to the accuracy attainable : in regard to the plan ; admitting an error of 
 of an inch in the observation with a magnify ing-glass of points on prints, this on 
 a circumference of 2O-inch radius (corresponding to a focal distance of 20 inches) 
 represents an angle of about 45", and the maximum error in the location of a point 
 2700 yards distant would therefore be but 20 inches, which, reduced to the scale of 
 would of course be inappreciable. For the heights; assuming a point 1000 yards 
 
 / OO/l 
 
 distant and the same focal distance, the error would be -- ' - = 7.2 inches, 
 
 20 
 
 which corresponds to an angular error of about 30", and, in observing with ordinary 
 instruments, this degree of accuracy would seldom be excelled. 
 
 The facts set forth in this paragraph are results of the observations made at a 
 time when photographic instruments were in a less perfect condition than at present. 
 In the author's experience during the past year, in the outline photographic sur- 
 vey of a tract of about 12 square miles, using the camera represented in Fig. 9, a Dall- 
 meyer rapid rectilinear objective, with OP 15 ".68, and plotting from the negatives 
 with the aid oi the tangent glass, points of the plan checked exactly and in plotting 
 the levelling, the difference in heights of points at various distances corresponded to 
 the measurements made by careful work with a surveyor's level. The scale of the plot 
 in this case was ro^To an d the field-work for the entire tract required but half a day. 
 
 It may be said that the accuracy attainable is sufficient for all ordinary purposes. 
 
 34. Explanation of Plate /, a Photographic Map (see end of book], Plate I (Plate 
 XXIV, in "Topographical Drawing and Sketching"), from the Memorial de l'Officier du 
 Genie, represents a photographic map of an area of 4500 hectares, in English units 
 the rectangle is about 3.1 X 4-4 miles. The field-work was first plotted to the scale 
 of s-ffVu"' and afterwards reduced by photo-lithography to its present scale, -g-g-J-oTr- The 
 small triangles, A, mark the vertices of the triangulation ; these and the other points 
 marked by a small circle, O, are the photographic stations, the minor triangulation, 
 or traversing, by which the Qs were fixed, is omitted. The base was measured in 
 the valley E. of Sainte-Marie-aux-Mines, and served, as shown, to fix points I and 
 2, and the directions of points 4, 5, 16 and 17: the side 1-2 was then used to 
 fix the points 3, 4, 10, ..... The contours have an equidistance of 5 metres ; 
 the field-work occupied 10 days; the number of photographic stations was 31 ; of 
 prints, 52 ; and of points determined, 1400. This survey was conducted by M. 
 Javary in 1869. 
 
SECTION III. 
 
 CYLINDRIC PERSPECTIVES. INSTRUMENTS, FIELD-WORK AND PLOTTING. 
 
 35. The Topographic Cylindrograph. This ingenious instrument, a type of the 
 cylindro-perspective class of cameras, is the recent invention of M. Moessard, formerly 
 professor of topography at St. Cyr, now commandant of engineers and professor 
 "a 1'Ecole Superieure de Guerre." 
 
 Views obtained with it are cylindric perspectives; the radius, the focal distance, 
 is a constant quantity for vertical as well as horizontal angles ; and the number of 
 views required for a complete tour of the horizon is but two and a fraction (20). 
 Fig- 35 is a general view of the apparatus. 
 
 36. The Camera. The camera consists of a semicircular wooden lid A, Fig. 36, 
 and base , which are horizontally disposed : and a vertical, rectangular, wooden frame 
 
 F. These pieces are fastened together with hinges 
 and hooks as shown, so that they may be readily 
 folded for transportation. When opened, a brass 
 standard s, attached to the lid in rear, rests upon 
 the base, where it is fastened in position by a 
 button, and thus serves to keep these parts rigidly 
 parallel ; this may also be folded against the lid 
 
 FIG. 35. 
 
 FIG. 36. 
 
 FIG. 37. 
 
 for transportation. Strips b, of brass, project vertically 0.4 of an inch beyond the edges. 
 The projections, e, Fig. 37, on the upper surface of the base, are guards or guides for 
 the flexible plate-holder (see Fig. 40), which, when in position for an exposure, forms the 
 cylindrical part of the camera. Projecting pieces, / and c, in rear of the base, serve as sup- 
 ports for the circular level, and for the compass, which is graduated from o to 360 in the 
 usual direction, N., E., S. and W., and which has its o-i8o line parallel to the middle 
 radius of the lid, with the o point to the front. In the latest form of the instrument, 
 two tube levels at right angles, and the compass, 4.7 inches in diameter, are let into the 
 
C YLIND RIC PERSPECTI VES. 
 
 37 
 
 upper surface of the lid. A plate in the lower surface of the base serves to attach the 
 camera to the tripod-head. 
 
 37. The Objective. The objective-carrier, shown in Fig. 38, is so attached by a 
 vertical spindle to the middle of the frame F (Fig. 36), that the optic axis of the objec- 
 tive is horizontal and the optic centre, or, in the case of a single lens, the rear nodal 
 point, is in the axis of the camera. Fig. 38 shows the arrangement ; the objective o is 
 rigidly fastened to the rear plate /, and enters freely an opening in the front plate /, 
 the plates being held together by adjusting screws and spiral springs as shown. Fig. 35 
 shows the manner in which the objective may be made to revolve horizontally, the 
 alidade, attached to the upper rectangular extremity of the spindle / (Fig. 38), serving as 
 a handle. The other extremity of the alidade traverses a circular scale of degrees, which is 
 attached to or engraved upon the lid, and which has its centre in the axis of rotation ; the 
 alidade is also furnished with a spring of slight tension, a click, projecting beneath and 
 engaging in notches (Fig. 35) cut in the lid ; which device serves to regulate the rate of 
 motion of the alidade, by assigning so many clicks per second, and thereby to time 
 the exposure. In front, light is excluded from the interior of the camera by a rubber 
 cloth, attached to the rear plate of the objective and the inner edges of the vertical 
 frame, and of such dimensions as to permit perfect freedom of motion to the objective. 
 
 FIG. 39. 
 
 Fiu 40. 
 
 To confine the deviated rays to the area effectively covered by the objective, there 
 are two wings or screens, w, Fig. 39, of tin blackened, attached to the rear plate of the 
 objective-carrier and projecting into the camera; each turns freely about a vertical axis 
 a, and, by means of the notches n in the rear projecting piece, may be made to occupy 
 three different positions, thus increasing or diminishing the breadth of field : the pro- 
 jecting piece and screens may be folded against the plate for transportation. The 
 screens should be of such size as to turn freely in the camera during the rotation of 
 the objective. The folding vanes (Fig. 35) with cross-hairs, attached to the alidade, serve 
 to determine at any instant that portion of the landscape which is being " taken " when 
 the screens are parallel. 
 
 Any rectilinear objective, composed of either a single or double combination lens, will 
 serve, provided its focal distance is equal to or a very little less than the radius of the 
 camera. 
 
 38. The Plate-holder. The plate-holder, Fig. 40, is a rectangular frame, /, of some solid, 
 flexible material, backed with rubber cloth, b t stretched upon and glued to the frame, the 
 sensitized paper, which is used instead of glass plates, when in position, resting against it. 
 
PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 The shield s, of flexible card-board blackened, fits into grooves in the edges of the 
 holder, and is manipulated in the usual manner. The milled head, shown at the left in 
 Fig. 35, serves to release a hinged brass clamp, or cleat, when the holder is to be inserted, 
 and to secure the holder after insertion, so as to entirely exclude the light. Several 
 plate-holders are used, and are easily packed for transportation. 
 
 39. Scale of Azimuths and Slopes. Fig. 41 represents the means by which a scale of 
 azimuths and of slopes is impressed or marked upon the negative. Two notched, curved 
 
 brass strips, h, having their centres in 
 the axis of rotation, are attached verti- 
 cally to the lid and base respectively, and 
 immediately in front of the plate-holder ; 
 they project slightly beyond the frame 
 of the latter, so that the notches may 
 be represented on the negative by the ex- 
 posure. The strips v are similarly at- 
 tached to the edges of the vertical frame. 
 The notches in h correspond to divisions 
 of i "grade," or of i, as the case may 
 
 be, to a radius = the distance from the plate-holder to the axis of rotation of the ob- 
 jective; and the notches in v, to T ^-g part of this distance. Projecting points at / 
 mark the horizon, or HH'. (Note. On account of the blurred images thus produced, it 
 would seem better to attach these strips made of very thin metal to the inner edges of 
 the holder, so that during exposure they would rest in contact with the paper.) 
 
 40. Adjustment of the Optic Centre. In order that the image of every point should be 
 clearly defined and immovable during rotation of the objective, it is necessary that the 
 optic centre should be in the axis of rotation, in or near which line it is supposed to 
 have been originally fixed by the instrument-maker. To test this condition : a narrow plate 
 of ground-glass, ruled with fine vertical lines, is placed in the usual manner to receive the 
 image of a distant point ; if the image is displaced in the direction of the rotation, the 
 rear plate of the objective-carrier must be advanced by means of the adjusting screws; if 
 in a contrary direction, the plate is moved to the rear. In this adjustment, to preserve 
 the parallelism of the plates, it is necessary that the screws should be turned equally, 
 which condition is assured by means of indices on the screw-heads. 
 
 41. The Manipulation. To make an exposure: the camera, with objective capped, is 
 set up and approximately levelled ; the lid is raised ; the plate-holder, containing the sen- 
 sitized paper, is inserted and clamped; the screens are favorably disposed, and the lid 
 is closed. The camera is then levelled ; the alidade is adjusted to the 85 division of 
 the scale; the vanes are raised, and, by revolving the camera, the vertical cross-hairs are 
 aligned upon the object to occupy the centre of the view. The shield is then withdrawn; 
 the alidade is adjusted to the o of the scale; the objective is uncapped and revolved by 
 means of the alidade, until the latter has reached the 180 division, the velocity of rota- 
 tion being determined as in ordinary landscape photography; and the cap and shield are 
 then replaced. The time of exposure is also determined in some degree by the positions 
 of the screens, being shorter when these are placed in the outer notches (Fig. 39). 
 
UN:; 
 
 CYLINDRIC PERSPECTIVES. 
 
 39 
 
 42. Device for Orienting the Views. A semicircular limb, having its centre in the axis 
 of rotation and its radius less than that of the camera, is attached to the upper surface 
 of the base, and is graduated into degrees or parts thereof, from 90 to 270, correspond- 
 ing in position to those on half the compass-limb. Two blocks are made to slide along 
 this limb, each carrying a vertical spindle about i inches in length; one spindle is ter- 
 minated at its upper extremity by an arrow-head, and the other by a crescent. Just be- 
 fore closing the lid for an exposure, one of these spindles is adjusted to the division cor- 
 responding to that marked by the compass-needle; and the other, to the division differing 
 from it by 90. By this obstruction of the deviated rays, their positions are marked upon 
 the negative, the arrow-head indicating the N. or S. point, and the crescent the E. or W. 
 point, thus furnishing the basis for the scale of azimuths. The division indicated by the 
 compass-needle is also recorded in the note-book. 
 
 To illustrate: For the first exposure (see Fig. 42), the compass indication is 55; the 
 
 First Exposure. 
 
 Second Exposure. 
 
 Third Exposure. 
 
 FIG. 42. 
 
 o-i8o line being parallel to the optic axis, with the o to the front, the direction of 
 the middle point of the view is N. 55 W.; consequently to mark the N. point, the 
 arrow-head is adjusted to the 55 -J- 180 = 235 division, and the crescent to the 235 
 90 = 145 division of the scale to mark the W. point. For the second exposure ; the 
 field being 170, and the order in which the views are taken being from left to 
 right: when the camera is revolved 170, the compass indication is 245; the arrow-head 
 is adjusted to this division and will mark the S. point on the negative, while the cres- 
 cent, at the 245 90 = 155 division, marks the E. point. The two views cover 
 170 X 2 = 340; a third view of 20 is therefore required to complete a tour of the 
 horizon. The exact supplement is obtained by revolving the camera until the needle in- 
 dicates 245 20 = 225, when the adjusted arrow-head will mark the S. point ; and by 
 withdrawing the shield a sufficient extent, the E. point may be marked by the crescent 
 placed at 225 90 = 135. For reasons heretofore given, it is advisable to have the 
 views overlap, say. 10, in which case the arcs of revolution of the camera would be 
 correspondingly diminished. (Note. The additions and subtractions above required could 
 in great measure be avoided by reversing the order oj the numbers on the base scale.) 
 43. The Field-work and Plotting. The field-work corresponds in detail to that de- 
 
PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 scribed in paragraphs 18 to 23 for plane-perspective cameras; any exceptions due to the 
 form of the instrument readily suggesting themselves. 
 
 Fig. 43 represents a print ready for plotting. HH' is the horizon ; SS and WW 
 are verticals drawn through the S. and W. points respectively ; and z is the azimuth of 
 a point o, 65 from S., or 115 from N., S. being usually taken as the origin. The verti- 
 cal angle of o is read off, as a slope-angle, on the side scale ; and is y-^j-, yf^, 
 y^, , according to the number of divisions from H' to //. 
 
 In measuring the azimuths, use the mean of the distances SZ and S'Z' ; and simi- 
 larly for the vertical angles, use the mean of Hh and H ' h '. 
 
 The stations plotted, a circle is described with each plotted station as a centre, 
 and with any convenient radius , the azimuths of the different points, determined as above, 
 are protracted, and the points are located on the plot by intersections of right lines drawn 
 
 h 
 H 
 
 S' 
 
 Z' 
 
 FIG. 43. 
 
 from the plotted stations through the corresponding azimuthal divisions. It is apparent 
 that when chords, sines, tangents, or ordinary protractors are used in protracting the 
 azimuths, the greater their radii the more accurate graphically will be the results. 
 
 As to the heights ; the reference of any point is given by means of the side scale, 
 
 H'h' + Hh 
 e.g.: Suppose = 3.5 divisions; then, the reference required is .035 X the 
 
 OT 
 
 plotted horizontal distance from the station to the point. 
 
 44. The advantages claimed in the use of this instrument are : 
 
 I. Since the scales are already impressed in the exposure, measurements may be made 
 directly upon either the negative or print ; the only construction required being that of 
 the right lines, as hh' and ZZ' (Fig. 43), through the images of the required points ; but 
 on account of distortions, which might take place in printing, it is advisable to draw these 
 lines on the negatives. 
 
 II. No construction of the focal distance is required ; nor is it necessary to consider 
 it in the field-work, because the apparatus is effective only when the optic centre is in the 
 axis of rotation ; and, when adjusted, the focal distance is exactly equal to the radius of 
 the cylinder. 
 
 III. No distortion exists if the objective is rectilinear, owing to the screens limiting 
 the field to that effectively covered by the lens ; if it does exist, the images will not be 
 clearly defined, therefore when these are clearly defined there is positively no distortion. 
 In vertical planes, however, there may be a slight distortion near the edges of the field ; 
 but this is of slight importance, since the higher points are in the sky portion of the 
 
CYL1NDRJC PERSPECTIVES. 41 
 
 view, and the lower points are so near the station that any small error in a correspond- 
 ing slope-angle may be neglected. 
 
 IV. Eccentricity of position in the plates "during the exposures i, 2 and 3 at any 
 station presents no inconvenience, because each view is oriented independently of the 
 others, on the cardinal points ; and, when required, the compass-readings, which are always 
 recorded, furnish the elements necessary to orient the three views at the plotted station. 
 The three-point problem may also be utilized in locating a station on the plot. (See also 
 close of par. 52.) 
 
 To give the photographic view a natural appearance, or to represent the details as 
 they would appear at the station, the " cylindroscope " may be used. It is simply a 
 wooden semi-cylinder, of dimensions equal to those of the cylindrical part of the camera ; 
 the photographs are placed on its interior surface and observed from the middle point 
 of its axis. 
 
SECTION IV. 
 
 o 
 
 D 
 
 RADIAL PERSPECTIVES. INSTRUMENTS, FIELD-WORK AND PLOTTING. 
 
 45. Photographic Plane-tables. If at any station a photographic view of the sur- 
 rounding tract be represented upon a horizontal surface, every point of the representa- 
 tion occupying its true position ; then will the horizontal angle of any two points be 
 measured by the angle included by the right lines, or radii, drawn from the plotted 
 
 station through the images of these 
 points. This is the principle of 
 construction of the following-de- 
 scribed instruments. As to rela- 
 tive heights of points, means for 
 their determination similar to those 
 described for plane and cylindric 
 perspectives are employed. 
 
 46. Chevallier's Camera. In 
 1858, this instrument was submitted 
 by M. Auguste Chevallier to the 
 consideration of the " Societe d'En- 
 couragement," and favorably re- 
 ceived ; and since that date the in- 
 ventor has succeeded in bringing 
 it to a still higher state of perfec- 
 tion. In its present improved form, 
 and as manufactured by the optician 
 Duboscq of Paris, it consists of a 
 circular dark-chamber, A, Fig. 44, 
 resting upon a triangular support, B, 
 which is furnished with levelling- 
 screws that bear upon the tripod- 
 head, as shown. These attachments 
 are similar in construction to those 
 of a theodolite, thus admitting of 
 a revolution of the chamber in 
 azimuth, a tangent-screw, T, serv- 
 ing for close adjustments. A 
 
 plate-holder, D, shown inserted, contains the sensitive plate. The cover, or upper 
 plate of the chamber, which carries the objective system, is movable about its centre, its 
 edge resting in double grooves in the upper edge of the cylindrical rim ; its serrated edge is 
 geared to two pinions, which may be engaged at pleasure ; to one of these pinions motion 
 
 FIG. 44. 
 
RADIAL PERSPECTIVES. 
 
 may be communicated by means of a simple clock-work movement, C, thus ensuring 
 uniform rotation ; while the other, furnished with a projecting milled head, m, serves to 
 revolve the plate by hand to any desired position. A graduated circle is fastened to 
 the edge of the chamber immediately below the edge of the plate, and diametrically 
 opposite openings in the latter permit the reading of arcs or angles. A small circular 
 level serves to ensure the horizontality of the plate ; at the centre of rotation and 
 projecting through the milled head m', is a sharp metal point, furnished with a spiral 
 spring, which is thrust downward to mark this centre on the sensitive plate, an oper- 
 ation, however, which may be dispensed with, as will be shown ; m' also serves another 
 purpose, to be described. 
 
 The objective system consists of the box E and tube F, the axis of the latter 
 and that of the dark-chamber being in the same plane ; the tube contains the lens, 
 and the box has at its rear extremity a triangular glass prism, with its edges horizon- 
 tally disposed. Rays from external objects entering the tube at O are, after devia- 
 ation by the lens, reflected vertically downwards through a circular opening in the 
 plate, and are focussed on the sensitive plate ; this opening, as shown in the figure, 
 being between the centre and edge of the plate. 
 
 A circular disk with sectoral openings of different dimensions is suspended imme- 
 diately above the sen- 
 A sitive plate, its centre 
 being in the axis of 
 B rotation of the instru- 
 ment ; and, by means 
 of indices on the milled 
 
 head m', any desired sector may be brought 
 beneath the opening through which the re- 
 flected rays pass, thus limiting the field and 
 preventing it from extending beyond the cen- 
 tre of rotation. Thus, in Fig. 45, X is the 
 axis of rotation of the instrument, / and P 
 the lens and prism respectively, r the sen- 
 sitive plate, and 5 the screen, S' represent- 
 ing the latter in horizontal projection, and 
 with different sectoral openings, h and h' are 
 cross-hairs, which by their obstruction of the 
 light are represented upon the negative ; 
 h is coincident with a radius of the screen ; 
 and h' is perpendicular to h at the point 
 in which the ray, coincident with the optic 
 axis of the lens, intersects it after reflection 
 by the prism : therefore, when operating by 
 
 FIG. 45. 
 
 " fixed sectors " (par. 47), all points, of which the images are located on k', are on a 
 level with the optic axis at the station where the exposure is made. Simply to mark 
 the trace of the " principal plane," that containing the axes of rotation and of the ob- 
 jective, the smaller sectors are used; and the smallest, about i, in operating by "con- 
 tinuous movement " (par. 48), in which case h is suppressed. 
 
44 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 It is apparent that to facilitate plotting, the distance of the image of h' from the 
 centre of rotation of the sensitive plate, should be exactly equal to the focal distance of 
 the objective, and this condition is assured by the instrument-maker. 
 
 In addition to the parts above enumerated, a small telescope with cross-hairs, and a 
 vertical limb, are attached, as shown, to the upper surface of the box ; and are used for 
 pointing, and to measure vertical angles of points that may lie without the field of the 
 objective, the latter case however rarely occurring. When the instrument is set up, the optic 
 axes of the telescope and of the camera-objective are in the same vertical plane. An 
 attached compass affords the means for orienting the views in the subsequent plotting, 
 and two methods for this purpose are prescribed ; one consists in rotating the plate, 
 after an exposure is made, until a covered slit in the base of the compass-box, and 
 which extends through the plate, is in the vertical plane of the needle ; this cover is then 
 removed for an instant, and an image of the needle is formed on the sensitive plate, 
 thus giving the required meridian line: as the compass is located near the edge of the 
 plate, the image of the needle will not interfere with other details. The other method 
 consists in placing a staff vertical at some distance in front, and in the field of view ; its 
 image in the negative, and its bearing obtained by means of telescope and compass, afford 
 the necessary data. The instrument is permanently focussed by the maker for distant 
 views; and the usual form of cap is used in making exposures. In regard to size, it may 
 be stated that excellent work has been done with a camera constructed for sensitive 
 plates affording circles of but 4 inches in diameter. 
 
 47. Operating by Fixed Sectors. This is its least important use. Suppose the cam- 
 era set up at any station, pointed at a distant signal, and an exposure made ; an image 
 of the signal and its surrounding features is obtained, and the image of h will cut that 
 of the signal. If now the plate be rotated, the chamber being clamped, and an ex- 
 posure made toward a second signal, it is evident that the angle included by the two 
 images of h on the developed negative, is the true horizontal angle of these signals as 
 measured at the station. The image of h' will in each view intersect points of all 
 objects having the same altitude as the optic axis. By using a sectoral opening of 
 about 10, a view is obtained in which the maximum error due to distortion falls within 
 the limit permissible in the graphical construction of a plan. It is unnecessary to 
 point exactly at any signal, because with rough pointing the image will always be found 
 in the vicinity of h, and all the images of h in the different sectors must have their true 
 relative positions; and in this property rests the value of the instrument for rapid work. 
 
 48. Operating by Continuous Movement. In this mode of working lie the great ad- 
 vantages derived from its use, viz.: absolute accuracy in the measurement of horizontal 
 angles, and the great rapidity with which the field-work may be performed. 
 
 In the preceding operation, a narrow sector had to be used to avoid distortion, and 
 to complete a tour of the horizon many views were required. If with this sector a 
 continuous movement were given to the plate, sector and plate revolving together, images 
 of objects would be superposed and become indistinguishable ; but by reducing it to a 
 very narrow slit, coincident in position with k, the sensitive plate will receive at each 
 instant the images of those points only that are situated in the principal plane (par. 46) ; 
 there will be no superposition throughout a revolution ; and, from a view thus obtained, 
 the true horizontal angle of anv two points is given by the radials that intersect 
 the images of these points, 
 
 \ 
 
RADIAL PERSPECTIVES. 
 
 45 
 
 If, in reference to the required plan, certain sectors of the landscape are unimpor 
 tant, it is only necessary to cap the objective while traversing them : but with the rapid 
 plates now in use, no appreciable time is lost by making the entire tour. 
 
 As already stated, the cross-hair k is suppressed, while ti ', reduced in length practi- 
 cally to a point, leaves its image upon the sensitive plate in the form of a circumference 
 of a circle, described with the centre of rotation as a centre, thus indicating the true 
 horizon of the station. 
 
 It is apparent that with an inverting objective, and the relative positions of objec- 
 tive and prism shown in Fig. 45, the sky will occupy the outer portion of the picture. 
 Such a representation is called "nadiral," in contradistinction to one in which the sky 
 
 FIG. 46. 
 
 occupies a central position, which is termed " zenithal :" the latter is evidently produced 
 by placing the inverting objective between the prism and sensitive plate, as in the other 
 form of this instrument. Although these representations possess the same value for sur- 
 veying purposes, the former presents the more natural appearance. In either case, a radial 
 drawn upon the sensitive plate represents the vertical of any point, the image of which it 
 intersects. 
 
 49. The Field-work and Construction of the Plan. As in other methods of surveying, 
 a base, as AB, Fig. 46, is measured, and from each extremity a tour of the horizon is 
 made ; this is repeated at a third station, as C, which is contained in views from A and B ; 
 and so on throughout the tract. The selection of stations is governed by the conditions 
 
4 6 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 already stated in par. 19. It is apparent, from the usual multiplication of representations 
 of any point, that numerous opportunities for verification are afforded for the plotting. 
 It is hardly necessary to state that, in setting up the instrument, the centre of rotation 
 should be plumbed over the station-point, and the axis of rotation made truly vertical. 
 
 In constructing the plan, either negatives or prints may be used directly as pro- 
 tractors. On the negatives, the landscape is represented as if rotated horizontally 180 ; 
 due allowance should therefore be made for the position of the N. point. On the 
 prints, in addition to this, when the sensitive plate is exposed film up, the positions of ob- 
 jects are reversed ; and the remedy lies in either placing the film down, or in making the 
 prints transparent with oil or other material. 
 
 Fig. 46 fully illustrates the construction of the plan; the base is plotted to the 
 required scale; the views from the extremities are placed with their centres at these 
 plotted points, and are rotated until their meridian lines are parallel to the direction 
 assumed for the meridian upon the sheet; right lines are then drawn from the centres 
 to corresponding points and produced to intersection, thus locating required points as in 
 plane-table surveying. A third view is then similarly oriented and used, and so on 
 throughout the work. 
 
 50. Determination of the Heights. Since the image of the circumference described 
 by the cross-hair h' is the horizon of the picture, it serves as a datum line to which all 
 points are referred: the reference of any point is therefore given by the formula 
 
 h 
 
 x = md -f. ; 
 
 in which / is the focal distance, - - the scale of the map, d the distance from the sta- 
 
 m 
 
 tion to the point, measured on the plan, and h the vertical distance of the point from 
 the horizon, measured on the view. Since the maps, or at least the first sheets of the 
 
 m 
 
 respective maps, may all be plotted to the same scale, -p may be considered as a con- 
 stant quantity; a convenient coordinate table may therefore be prepared, giving the values 
 of x for values of d and /z, differing by, say, 100 feet and 10 feet respectively, thus 
 facilitating the plotting. The references to a common datum plane are obtained in the 
 usual manner. 
 
 51. Instrumental Error, Advantages Claimed. The errors are those observed in all 
 optical instruments employing lenses. Distortions in a horizontal direction are destroyed ; 
 while those in a vertical direction, if existing, may be detected by comparison of "fixed 
 sector" views with those obtained by "continuous movement," and then corrected. The 
 only cause of confusion is the slight angular deviation of a vertical ray after passing 
 through a small sector, thus causing it to spread to the right and left of its true posi- 
 tion a distance equal to half the breadth of the sector; but this lack of definition is 
 obviated by using a sectoral opening not greater than i. 
 
 As illustrating its accuracy, M. Jouart states that, in his own practice with an instru- 
 ment of this kind having a focal distance of 7.5 inches, and in plotting to a scale of 
 yaViT' satisfactory results were obtained up to distances exceeding I mile. In a recent 
 Parisian publication, M. Gossin also states that its use is attended with excellent results. 
 
RADIAL PERSPECTIVES. 
 
 47 
 
 The range of the instrument evidently depends entirely upon the power of the objec- 
 tive used. 
 
 The main advantages claimed for it, as compared with ordinary surveying instruments, 
 are the certainty with which a tour may be closed, and errors of arc-reading thus ob- 
 viated ; the great rapidity of execution ; its simplicity of manipulation, which enables a 
 novice to produce practically perfect views; and, as with all photographic surveying in- 
 struments, the production of complete views of the topographical features. Captain Han- 
 not, chief of the photographic service of the Belgian war department, states, in regard 
 to this instrument, that in its 
 construction it embodies in a 
 most happy manner all the im- 
 provements of which a camera 
 is capable;' and that, as com- 
 pared with the ordinary plane- 
 table, it possesses a great num- 
 ber of advantages in its favor. 
 With the very sensitive dry 
 plates now generally available, 
 and the very narrow sectoral 
 opening made permissible by 
 their use, joined to careful con- 
 struction and the employment 
 of a good objective, this in- 
 strument must prove of valu- 
 able assistance in surveying, 
 for both civil and military pur- 
 poses. 
 
 52. Mangins Camera. This 
 instrument, like that just de- 
 scribed, is of French invention. 
 It produces, without rotation 
 of any part, an entire tour of 
 the horizon upon a single plate. 
 The theory of its construction 
 is based upon the idea due 
 to Capt. Prudent that virtual 
 images produced by a spherical convex reflector, might be photographed so as to represent 
 the original objects in their true proportions. 
 
 I. The Metallic Reflector. Col. Mangin solved the problem, by first constructing a 
 reflector of which the form of the generating curve was dependent upon the condition 
 that the virtual images should be rigorously exact for its median zone. This reflector.. 
 R, Fig. 47, is a silver-coated surface of revolution, having its axis, X, vertical, in order that 
 it may receive the incident rays from all points of the horizon. By total reflection, the 
 pencils are made to converge to a point, O, of this axis; where an objective, having its 
 
 FIG. 47. 
 
48 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 optic axis coincident with that of the reflector, serves to produce upon the sensitive 
 plate placed on the base, B, of the chamber, the conjugate of an image formed by the 
 reflector. 
 
 For photographic purposes, each point of the virtual image should be true and clearly 
 defined, and the pencil of rays emanating from it rigorously conical, in order to produce 
 on B a true image of the original point. Neither a spherical nor a conical reflector would 
 serve this purpose, when the incident and reflected rays make considerable angles with 
 each other; but, by making the generatrix an arc of a parabola, with its axis, a, horizon- 
 tal, the incident rays in the horizon of the station are reflected to O as desired. In the 
 manufacture of the reflector, the arc, c, of an osculatory circle, tangent at the middle point 
 
 FIG. 48. 
 
 of the parabolic arc, was substituted for the latter, and the construction is allied to that 
 of the annular light-house lens. 
 
 As the distance, measured on the reflector, from the median zone, or middle horizon- 
 tal circle, increases, the definition decreases ; but it is found by experiment that rays, inci- 
 dent upon any part, which make angles of 15 with the horizon, are sufficiently well de- 
 fined for practical purposes; the vertical field of the instrument is therefore 30. 
 
 The distance from O to B, and the selection of an objective to place at O, are gov- 
 erned by the rules observed in making enlargements by photography. For example : Let 
 d denote the distance from O to the median plane of the reflector; then, to produce on 
 
RADIAL PERSPECTIVES. 
 B an image twice the diameter of the virtual image ; OB must be equal to 2d, and an 
 objective is used of which the focal distance f is derived from the expression -7. = ~j -\-~~j 
 
 II. The Glass Reflector. The metallic reflector is easily tarnished by chemical reaction, 
 and is therefore hardly suited to photographic work; to remedy this, Col. Mangin con- 
 structed one of glass, of which the form is such as to correct the effect of refraction. 
 The outer surface, S, Fig. 48, which receives the incident rays, is torus-shaped ; the upper 
 or reflecting surface, S', is of the same form as the metallic reflector; and the lower sur- 
 face, S", is spherical, its centre, O y being the focus of the reflected rays: the objective at 
 O is for the purpose described in (I). This reflector has another advantage, since reflected 
 rays are not obstructed by the mountings which are required for the metallic reflector. 
 
 With this camera, the diedral angles formed by vertical planes intersecting the station 
 and exterior points, are truly represented on the picture by the sectors included by ra- 
 dials drawn to the images of the points, the centre being the image of a minute open- 
 ing, which admits the vertical ray R in the axis of the reflector ; and all points, in any 
 vertical plane of the station, have their images in the same radius. The horizon of the 
 picture, ////', may be defined, either by the image of a circumference engraved on the 
 reflector, and which is concident with the line of con- 
 tact of a vertical cylinder ; or independently, in which 
 case any distant point in the horizon of the station is 
 noted, and, with the axial point as a centre, a circum- 
 ference passing through the image of this point is then 
 described on the negative or print, checks are afforded - 
 by observing two or more points. 
 
 The operation of plotting, from the negatives or 
 prints, is evidently similar to that described for Che- 
 vallier's camera. 
 
 Fig. 49 represents the camera complete. In another form of this instrument, a tri- 
 angular prism is employed to reflect the rays, after deviation by the objective, to the 
 sensitive plate vertically disposed, this arrangement making the camera more compact. 
 
 FIG. 49. 
 
SECTION V. 
 
 THE CAMERA WITHOUT A LENS. 
 
 53. It is well known that, if rays of light from exterior objects enter a dark chamber 
 through a very small aperture in one of its sides, images of these objects are formed on 
 the side opposite the aperture, and that the images will be sharply defined if the aper- 
 ture is circular and its edge is free from flaws. 
 
 The apparatus required is either an ordinary camera, or a box especially constructed 
 for the purpose. For general purposes its length when extended should be at least 12 inches, 
 and its lateral dimensions such as to admit an ordinary plate-holder containing plates of 
 the required size. Means already described are resorted to for the setting up and levelling. 
 The position of the aperture should correspond in every respect to that of the intersection 
 of the optic axis of an objective with the front face of a camera ; therefore, to make 
 the arrangement complete, the aperture should be constructed in an ordinary objective- 
 slide, of which the centre, or flange for a lens-tube, is filled with some opaque material. 
 Apertures of different sizes, corresponding to different fields of view and sizes of images, 
 are required ; it is therefore more convenient for manipulation to construct them as 
 hereinafter described, and attach the plates to a single metal strip which may be held 
 and moved in grooves against or across a central opening in the slide, so that any desired 
 aperture may be readily made to face the opening; or a revolving disk containing the 
 apertures may replace the strip. A superposed sliding strip, or another revolving disk, will 
 serve as a cap in making exposures. 
 
 To admit oblique rays, either the aperture must be conical, or else constructed in 
 very thin metal. In the former case, a conical cavity, of which the angle at the vertex 
 should be at least 90, is drilled nearly through the metal, and the perforation is com- 
 pleted with a needle-point; in the latter, the thin metal, platinum, tin-foil, or gold leaf, is 
 placed on a flat, smooth, hard-wood surface and pierced with a needle, care being taken 
 in this as in the former case to make the aperture circular and of the required size ; the 
 latter is easily determined with the aid of a magnifying-glass and a finely divided scale 
 of equal parts, a needle gauged and suitably marked will prove useful in this operation. 
 When thin or leaf metal is used, it is of course attached to annular pieces of thicker, or 
 less pliable material, before being fastened to the strip or disk; and the latter is suitably 
 pierced so as not to obstruct the rays. As a precaution against reflections in the camera, 
 metal parts facing the interior should be blackened. 
 
 To obtain well-defined images the sizes of apertures, and the distances from apertures 
 to the sensitive plate, which may be termed aperture distances, should vary according to the 
 following rules derived from experiment : 
 
 For an aperture distance of 3.14 inches, the size of aperture is 0.012 of an inch ; 
 " " " 11.80 " , " " " 0.02 " " ; 
 
 and between these distances, the sizes vary proportionally from those given. 
 
 It is apparent from a consideration of the limiting rays of any field of view, or of 
 any object, that the shorter the aperture distance, the wider will be the field embraced in 
 a photograph, and the smaller will be the images. A short aperture distance entails less 
 
THE CAMERA WITHOUT A LENS. 
 
 exposure than a long one, because of the greater illumination ; and a 90 field is found to 
 be about the limit of equal illumination. 
 
 Assuming, for surveying purposes, 90 as the desired horizontal field or angle of view, 
 then in order that any plate shall exactly contain the view, the aperture distance should 
 be one half the horizontal dimensions of the plate ; but for reasons heretofore stated a 
 marginal allowance is advisable, therefore it is best to make the distance a trifle less 
 say 4.75 inches for an 8xio, and 3.75 inches for a 5x8. 
 
 In the use of this camera, images on a ground-glass are so faint as to be dis- 
 tinguished with difficulty, except in the brightest light ; therefore, to dispose the camera 
 for any view, the following simple device is employed : For the horizontal field ; mark, by 
 a projecting pin or otherwise, a point on the front upper edge of the camera and exactly 
 in the vertical of the aperture ; and mark two other points on the rear upper edge, one 
 near and opposite to each extremity of the upper edge of the sensitive plate ; then the 
 prolongations of the sides of the angle, formed by joining the front point with the other 
 two, will evidently serve, by sighting along them, to adjust the aperture distance so 
 that any desired field will be contained by the plate. A similar device on a side of the 
 camera serves to determine the vertical field. 
 
 A tour of the horizon requires but four separate views of 90 each ; and each view 
 may be limited to its proper quadrant as follows : At the first exposure, sight a distant 
 point in the prolongation of one of the lateral faces of the camera ; then, assuming the 
 camera to be rectangular in plan, for the second disposition, revolve the camera until this 
 point is intersected by the plane of a longitudinal face ; and similarly for the two remaining 
 views. 
 
 With sensitive dry plates, the period of exposure for landscapes is about 10 seconds in 
 sunlight, and from 25 to 35 seconds on a cloudy day. With a Stanley dry plate, sensitive- 
 ness 25, aperture 0.013 of an inch in platinum leaf, aperture distance 4.5 inches, and 
 exposure 10 seconds, the author obtained an excellent negative, which at ordinary visual 
 range could hardly be distinguished from one sharply focussed with a lens. The following 
 table from successful experiments will prove useful: 
 
 SUBJECT. 
 
 PLATE. 
 
 APERTURE. 
 
 Ap. DISTANCE. 
 
 EXPOSURE. 
 
 REMARKS. 
 
 
 
 Inches. 
 
 Inches. 
 
 
 
 Landscape 
 
 Wet plate 
 
 O.O2 
 
 II. 81 
 
 10' to 15' 
 
 Cloudy weather. 
 
 
 
 Dry plates, sen- 
 sitiveness 25.. 
 
 0.12 
 
 3-M 
 
 10" 
 
 In sunlight. 
 
 
 
 
 
 0. 12 
 
 3- T 4 
 
 25" to 35" 
 
 Cloudy weather. 
 
 In studio 
 
 
 
 0.02 
 
 12.0 
 
 i' 
 
 Subject 10 ft. dis- 
 
 
 
 
 
 
 tant, well lighted. 
 
 There are certain merits due to this method, viz. : that, when the camera is properly 
 set up and levelled, the images formed are free from distortion, and it may be recom- 
 mended for accuracy and simplicity. Its usefulness must of course be determined by local 
 circumstances and conditions, such as the degree of definition required, sensitiveness of 
 the plates, and the kind of lens available for other methods of working. The principle 
 may be readily applied to the production of plane, cylindric, or radial perspectives. 
 
SECTION VI. 
 
 TELESCOPIC AND BALLOON PHOTOGRAPHY. 
 
 54. Telescopic or Long-range Photography. -This has for an object the photographic 
 representation of features beyond the range of the ordinary camera-objective. The 
 means required consist of a telescope attached to the front of the camera, and which 
 has its optic axis in the prolongation of that of the camera-objective ; the latter ob- 
 jective may be dispensed with. Fig. 50 shows an arrangement of camera and telescope 
 for the purpose. Fig. 51 represents M. Lacombe's device for attaching the telescope; 
 after inserting the eye-piece a, the tube-ring b, which has a thread cut on its projecting 
 cylinder, is screwed into the outer extremity c of the objective-tube; a diaphragm, d, 
 
 
 
 FIG. 50. 
 
 FIG. 51. 
 
 with large opening preventing the eye-piece from injuring the objective. Fig. 50 repre- 
 sents the other arrangements and the manner of focussing. 
 
 Another French operator, M. Mathieu, dispenses with the metal ring b, using instead 
 a cylinder of stiff red cloth, which excludes actinic rays. While making the exposure, he 
 envelopes the apparatus with a thick black cloth, in order to thoroughly exclude all light 
 except that which enters through the telescope. There are two conditions indispensable 
 to success : one, that all light, except that proceeding from the subject to be photo- 
 graphed, must be excluded from the apparatus ; and the other, that the exposures must 
 be timed with great care. 
 
 Figs. 52 and 53 are sketches from " La Nature " of M. Mathieu's work, the feature 
 C in the former being magnified as shown in the latter, by the means just described. As 
 to the details in this case: The camera used was a 5x7, and the lens a No. 2 Darlot. 
 The time of exposure with an o.o8-inch stop, an ordinary dry plate, and a range of 0.75 
 mile, was 90 seconds. Good negatives were obtained at a distance of 3.75 miles. 
 
 This method is much like that practised in taking lunar photographs, and which is 
 described in some astronomical treatises. Its application for surveying purposes would 
 prove of value for representing inaccessible objects, such as the peaks and precipitous 
 
TELESCOPIC AND BALLOON PHOTOGRAPHY. 53 
 
 slopes of mountains ; and, in military operations, when the detailed description of a dis- 
 
 / 
 
 ^T-jf-y f >- 
 V >.%.* 
 
 r-'""-"" ."'"' ^yp^^'^'^ ''"""'' 
 
 "^M::^^^^j^>t^^^^ 
 
 ^ ?SP 
 
 mtfe 
 
 r 4 ^n~\ - rr ""t ^IQ* 
 
 tn3QXi;,ii3u^ r " 
 
 FIG. 52. 
 
 tant redoubt, or other fortification or position is needed. 
 
 55. Balloon Photography Sketch of Progress to 1880. Visual command afforded 
 from great heights on a clear day is nowhere so perfectly attained, as from the cage or 
 car of a balloon a few thousand feet above the earth's surface; there is no substructure 
 to interfere, and the country is spread out 
 like a map beneath. The idea of taking 
 advantage of such favorable conditions for 
 photographic surveying seems to have 
 originated, in 1855, with M. Andraud ; but 
 it was not put into practical form until 
 1858, when M. Nadar, who, having no 
 knowledge of a previous conception of the 
 idea, resolved to do no less than make a 
 survey of entire France, by means of pho- 
 tography and a captive balloon. His plan 
 was to ascend to a height of 1000 metres 
 above points previously determined ; this 
 height would command an area of 
 1,000,000 square metres, or 100 hectares 
 
 (about 250 acres) ; and, since at least ten FIG> ^ 
 
 stations could be occupied daily, wea- 
 ther permitting, 1000 hectares would be the day's work. For military purposes, during 
 a campaign, communication with headquarters was to be maintained by means of a box, 
 
54 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 sliding along one of the cords which held the balloon, and by which the prints contain- 
 ing views of the enemy's position, etc., were to be dispatched every fifteen minutes. 
 Patents were secured both in France and foreign countries, and he set to work developing 
 his plan. The swaying of the balloon, lack of sufficiently rapid plates, and the escape of 
 gas from the envelope, made the manipulation so difficult that the photographs obtained 
 were of little value. Subsequently, however, in 1868, he succeeded in taking many useful 
 balloon photographs which, considering the means then available, were as perfect as 
 could be expected. 
 
 In the War of the Rebellion, captive balloons were frequently used to obtain in- 
 formation as to the surrounding country, the enemy's position, etc. In May, 1862, the 
 Union army before Richmond employed one, and succeeded in photographing on a 
 single plate all the country between Richmond and Manchester on the west, and the 
 Chickahominy on the east ; the rivers, smaller streams, railroads, marshes, pine-woods, etc., 
 were all represented, as well as the dispositions of troops. Two prints were made from 
 the negatives thus obtained ; one of which was retained by the army commander, and the 
 other by the aeronaut. Rectangles were ruled on them, the same number and disposition 
 on each ; reference-letters were attached ; and, since in the subsequent ascents telegraphic 
 communication was maintained between headquarters and the balloon, the prints enabled 
 the aeronaut to give information of all important events that transpired in any rectangle ; 
 which information in several instances proved very valuable. 
 
 At this time, similar and successful use of balloons was made on the Mississippi near 
 Cairo. These, results, added to those attained during the war of 1866 in Bohemia, led 
 to the formation in France of the French Society of Aerial Navigation, in existence at 
 the present date, and including men of wealth and scientific attainment : the participa- 
 tion of its member in the Franco-Prussian War, particularly during the siege of Paris, 
 when both free and captive balloons were employed, is well known. The faults of 
 management and the failures that occasionally took place, and these were more especially 
 noticeable on the German side, were mainly due to inexperience, imperfect apparatus 
 and lack of instruction; in this connection, it may be stated that the Germans, in 1887, 
 added to their army an aeronautic section of fifty men. 
 
 The Meudon School of Aerostation was formed in 1871 for the purpose of instructing 
 soldiers in the duties of aeronauts. A commission, of- which Col. Laussedat was the most 
 active member, was appointed the same date to study this subject, and it devoted es- 
 pecial attention to balloon photographic surveying. 
 
 In 1878, M. Dagron undertook to carry out the scheme proposed by M. Nadar ; but 
 the slow action of wet plates, joined to oscillations of the balloon, resulted in con- 
 fused negatives ; and it was only on a very calm day that he finally succeeded in ob- 
 taining a fair negative of part of the city of Paris, in which most of the prominent 
 features could be recognized. 
 
 In the following year, dry plates and the instantaneous shutter were used by M. 
 Triboulet ; but after making an exposure in a free balloon at a favorable altitude of 500 
 metres, a storm caused a descent into the Seine, and custom-house officers opened his 
 plate-holders to examine for contraband articles. 
 
 In 1879, on account of services rendered in Afghanistan and Zululand, the English 
 
V 
 
 TELESCOPIC AND BALLOON PHOTOGRAPHY. 
 
 55 
 
 war department made ballooning apparatus part of their permanent military outfit, and 
 provided for the instruction of troops in its use. 
 
 56. Recent Experiments. In June, 1880, M. Desmarets, with rapid dry plates and an 
 electric instantaneous shutter, obtained two good negatives. The first represented a 
 portion of the village of Mesnil-Esnard near Rouen, including an area 900 metres 
 square, to a scale of about j-yVo 5 the altitude of the balloon was about noo metres, the 
 velocity of ascent 6 to 8 metres, and the weather misty. Although lacking in clearness, 
 buildings, roads, trees, and piles of stones intended for paving purposes, were defined. 
 The camera was so disposed that the objective-tube projected through an opening in the 
 base of the car. The other negative was made at an altitude of 1300 metres, and em- 
 braced the country between Rouen and Quillebceuf ; unfortunately clouds intervened in 
 many places ; but, by an enlargement of the original, M. Carette unexpectedly brought 
 out many details, which before were unobservable with a microscope. In the second op- 
 eration, the camera was secured to the side of the car, altitudes were measured with a 
 very sensitive aneroid barometer, and the objective used was a Derogy 8X10, with a focal 
 distance of 11.5 inches; and, since the time of exposure was about ^ of a second, and 
 the velocity of translation of the balloon about 6 
 
 to 7 metres per second, the angle of displacement 
 about 8" was insensible. This experiment at- 
 tracted much attention and therefore gave the sub- 
 ject greater prominence. 
 
 In 1883, Mr. Shadbolt of England succeeded in 
 making with dry plates good negatives from ex- 
 posures at altitudes varying from 500 to 1000 metres. 
 In all the prints, the topographical features were dis- 
 tinguishable ; but in one obtained at 650 metres 
 above Stamford Hill, in the northern part of London, 
 the features were particularly well defined ; in fact, 
 with the aid of a microscope, the smallest objects 
 could be easily recognized. Experiments of this 
 nature are now being made at the school- of bal 
 looning at Chatham. 
 
 57. Special Apparatus for Balloon PJiotography. 
 In the instances given, the aeronaut occupied the car 
 and manipulated his camera in the usual way; but, 
 in 1884, M. Triboulet devised another method of 
 working. He constructed a hexagonal willow basket 
 to contain the photographic apparatus, and suspended 
 it beneath the car of a captive balloon. The latter 
 was unaccompanied by an aeronaut, electric com- 
 munication with the earth, by means of a light cable, FIG. 54. 
 serving to make the exposures. As shown in Fig. 54, 
 
 the interior of the basket' was subdivided into seven dark chambers ; six of the objectives 
 were disposed horizontally to obtain a tour of the horizon, while the seventh projected 
 
PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 through the base of the basket and gave the view beneath. Sensitive dry plates about 
 8XIO inches in dimension were used, the plate-holders occupying the positions/; and the 
 images on the plates overlapped 0.4 of an inch to facilitate joining the prints. The in- 
 stantaneous shutter, drop-pattern, was arranged as shown in Fig. 55 ; the drop a, held 
 in the grooved strips b, was retained in position by the stop c attached to a metal spring 
 d\ on closing the circuit the electro-magnet e withdrew the stop, and the drop was quickly 
 closed by means of the rubber bands /. The photographs obtained in this instance 
 were fairly good. 
 
 At the close of 1884, M. Casse made use of a very small captive balloon; and, 
 to avoid the weight due to the use of an electric cable, substituted slow-match, which 
 being of a length corresponding to the period of ascent, released the stop (c, Fig. 55) 
 at the proper elevation. Clock-work has also been employed for this purpose. Experi- 
 
 
 
 L 
 
 
 a c 
 
 'OH 
 
 
 o 
 
 - 
 
 
 XX 
 
 
 b 
 
 /\ 
 
 b 
 
 
 f \ 
 
 \ 
 
 f / 
 
 
 \ 
 
 FIG. 55. 
 
 FIG. 56. 
 
 ments of this nature and attended with similar results were made at Chatham during 
 the same year. 
 
 58. Notably Successful Experiments. A notable instance is afforded in the experience 
 of MM. Tissandier and Ducom at Paris in 1885. The ascent was made at 1.40 P.M., in 
 a partially cloudy atmosphere, and with a wind-velocity of about 12 miles per hour. The 
 photographic apparatus consisted of a 5 X 8-inch camera, Mackenstein tourist pattern, at- 
 tached to the car as shown in Fig. 56 ; and a Francois rapid rectilinear objective No. 4, 
 having a 22-inch focus and an opening of 1.4 inches, stopped to I inch. The drop- 
 shutter used gave an exposure of -^ of a second, and the gelatino-bromide plates were 
 specially manufactured for the purpose. Exposures were made at altitudes varying from 
 600 to iioo metres; several were very successful, the best appearing to be that at 
 600 metres above the isle of Saint-Louis, and in which, with the aid of a microscope, the 
 coils of rope contained in the passing boats are plainly visible a tracing from the pho- 
 tograph, in the author's possession, is given in Fig. 57. The most sharply-defined views 
 were obtained when the sun's rays were so oblique that slight shadows weie cast. The 
 motion of translation did not affect the sharpness, but it was found best to spring the 
 shutter near the beginning or end of an oscillation. 
 
TELESCOPIC AND BALLOON PHOTOGRAPHY. 
 
 57 
 
 In July of the same year, M. Pinard, of Nantes, obtained good 7X9 negatives 
 from exposures made at altitudes varying from 400 to 1700 metres. He used a 10- 
 inch focus Steinheil antiplanat, with a stop of y 1 ^, and the drop-shutter gave an expo- 
 sure of -gJ-Q- of a second. 
 
 The latest and best results appear to be from the experiments of Captains C. and 
 P. Renard and M. Georget in 1885, and of MM. Tissandier and P. Nadar in 1886. Of 
 the many good photographs made by the former party, that one obtained at 10.17 
 A.M., at an altitude of 720 metres, deserves especial mention for fineness of detail. The 
 latter party of aeronauts made an ascent lasting 6 hours, and obtained 30 photographs, 
 including some from an altitude of 1200 metres; various dispositions of the camera were 
 
 FIG. 57. 
 
 made in order to obtain both plan and oblique views. The time of exposure was 
 of a second. 
 
 59. Photographic Outfit and Method of Working. The following is a brief descrip- 
 tion of Mr. Shadbolt's preparations for balloon photography : " My list of requisites com- 
 prises a 5X7i-inch camera, a good supply of double plate-holders, lenses, shutters, etc., an 
 aneroid barometer with altitude-scale registering up to 15,000 feet, a pocket-compass and 
 maps of the country over which I am to travel, and a copy of Murray's threepenny 
 time-tables, small edition, which comes in handily on landing. If circumstances permit, I 
 prefer to fix my apparatus before starting ; the case containing the slides, etc., being 
 attached to the . side-ropes of the car by means of a strap, so as to be close at hand, 
 and the camera fixed in its place on the edge of the basket by means of any simple 
 device. . . . The barometer I fasten to the rigging close by the camera, so as to 
 enable me to record the altitude as each exposure is made." 
 
5 8 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 60. Use of Small Balloons, Captive and Free. In 1881, Capt. Elesdale, R.E., pro- 
 posed to use a small balloon, either captive or free, having sufficient lifting power to carry 
 a very light camera to any desired height ; the exposure to be made at the proper time 
 either by electric apparatus controlled from the ground, or, automatically, by clock-work- 
 attachment. In his subsequent experiments, various devices were employed to ensure 
 a proper direction of the optic axis ; for plan views, the axis was kept vertical by at- 
 taching the camera, pointing downward, to a rectangular frame, suspended horizontally 
 from a ring beneath the balloon by means of four cords or steel chains of equal 
 length ; for inclined views, the optic axis was to be pointed in any desired direction 
 by attaching the camera to one end of a bamboo rod, the retaining or ground cord 
 being secured to the other end, while the rod was suspended at the required angle by 
 two cords of different lengths; a light sail, formed by filling in the triangle of the rod 
 and cords with silk, was suggested as a means for keeping the camera steady in the 
 direction of the wind. In case of no wind, a multiple camera similar to that used 
 by M. Triboulet (see par. 57), was to be employed. 
 
 For military purposes, as in the case of an investment, the besieger could resort to 
 either one of the two following processes for obtaining comprehensive views of the 
 enemy's position : 
 
 a. A line of connected balloons is to be floated across the position, and the views 
 are to be taken in transit as follows : Having selected a suitable point to windward, 
 its location being accurately ascertained by the preliminary use of one or more pilot- 
 balloons, balloon No. I with camera attached is sent up, its retaining-cord of silk, or 
 best hemp, being paid out freely from a windlass running on friction-rollers, so as to 
 offer as slight a resistance as possible. No. I having reached its equilibrium level, say 
 2000 feet, No. 2 is instantly toggled on and sent up ; then No. 3, and so on through- 
 out the series of five or more ; the interval, or section of cord between balloons, evi- 
 dently depending upon the lifting power. No check of ascent is to be permitted, or 
 the series will tend to pivot around the starting-point and come to the ground. The 
 automatic apparatus is set at the proper time for making an exposure, from observa- 
 tion of the rate of travel of the pilot-balloons ; or, for the later balloons of the series, 
 from information communicated by signals from an assistant, stationed in a direction 
 perpendicular to the path, and who would observe the leading balloons. 
 
 The system is brought to the ground beyond the enemy's position by attaching 
 a very light grapnel, with a suspension cord 50 to 100 feet in length, to the last bal- 
 loon ; and by cutting a small hole near the crown of the latter to permit the gradual 
 escape of its gas, which will begin to be effective when the balloon has reached its 
 equilibrium level. 
 
 b. In the other process, each balloon is to be sent up independently, the escape of 
 gas being so provided for, as above, that the balloon shall descend at a suitable distance 
 and be held by its grapnel. 
 
 In a special case, as in the close investment of a fortress when it is necessary to 
 ascertain if an inner line of defence is being constructed by the besieged, two methods 
 are prescribed, the choice depending upon the direction of the wind. First, when the 
 
TELESCOPIC AND BALLOON PHOTOGRAPHY. 59 
 
 wind is favorable and blowing towards the enemy: the small balloon is inflated in the 
 advanced trench, say at 200 yards from the work; it is sent up, with camera attached 
 to a rod, as above described ; the view is ta"ken at, say, 80 yards from the work, when, 
 by the release of a weight suspended from the balloon by a cord, which is automati- 
 cally parted by the burning of a time-fuze, the balloon rapidly ascends out of reach 
 of the enemy's bullets; the windlass is then quickly carried to the rear trenches, the 
 cord being paid out freely, and the balloon is hauled down at a safe distance. Pro- 
 vided the camera is not hit, a few bullets through the envelope do no harm. In 
 the second case, when the wind is from the opposite direction: a solid shot, to which 
 a short length of chain connected with a fine hempen line is attached, is fired with 
 the proper powder-charge to a point near the crest of the glacis ; the balloon, fastened to 
 the line and also to its retaining cord, is then sent up; the line is burned through at the 
 proper time by a time-fuze, the balloon rises rapidly and is hauled down as before. 
 
 To ensure steadiness of ascent and to prevent oscillation of the captive balloons, 
 Capt. Elesdale prescribes that " all vertical strain except that due to the dead weight of 
 the cord itself should be removed ; and all horizontal strain should be avoided if possible, 
 and when this is not possible it should be changed into a small, steady and uniform 
 retarding force, instead of an unyielding pull, until the picture is taken." In case of 
 the balloon rising to still air above a wind-current, to keep it in position " the drift- 
 ing lower portion of cord must be fed with more cord from the ground as fast as it 
 will run out." 
 
 As a result of his experiments up to August, 1883, Capt. Elesdale remarks "that 
 with a proper and thoroughly worked out apparatus, constructed by the aid of opticians 
 and mathematical-instrument makers, so as to admit of instrumental accuracy in all the 
 processes, and for pictures taken on ordinary ground (not representing any violent slopes 
 or extreme differences of level) we may expect to attain by balloon photography more 
 accurate plans than are usually obtainable by ordinary surveying." 
 
 61. Observations on Balloon Photographic Surveying. The facts given in paragraphs 55 
 to 58 show that good photographs, giving either vertical, oblique or panoramic views 
 of a country, may be obtained from balloons ; and the first question now presented appears 
 to be whether, aside from their value as pictorial representations, they may be used 
 as data for the construction of exact or true maps. 
 
 I. With Captive Balloons. Taking first the case of a single view: When a balloon 
 in mid-air is stationary, as in the case of a captive balloon in still weather, a panoramic 
 view obtained corresponds to cases described in the preceding sections ; with a difference 
 in its favor, however, that the point of observation is exceptionally commanding. The 
 vertical or plan view that obtained by pointing vertically through the base of the car 
 affords under favorable illumination a perfect means for the measurement of horizontal 
 angles ; the middle point of the view being the radial or angular point. A vertical view 
 of a flat tract would be a most perfect and detailed map or plan of itself, and could be 
 enlarged or reduced as desired to conform to any scale, the plans of buildings, which, 
 by their positions in the outer part of the view, would present one or more faces, being 
 given by the bases ; but in the case of a mountainous or hilly tract, neither the eleva- 
 
6o 
 
 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 tions nor horizontal distance would, as a rule, be truly represented. Thus, as shown in 
 Fig. 58 : at the altitude PB, the peak D is projected by a visual ray in D' instead of 
 D", representing an error of D" D' in its horizontal distance from P. Both the distance 
 PD" and the elevation D" D may however be readily determined as follows : On the 
 
 print, measure the angle @, which is given by its 
 tangent, PD 1 ', PB being measured by the aneroid ; 
 from the station P measure the angle of elevation 
 @' ; plot the triangle PDB, in accordance with the 
 scale of the map; then D" D and PD" , measured 
 by this scale, are respectively the elevation and 
 horizontal distance required. For oblique views, the 
 plotting would be more complex the measured 
 angles requiring the correction due to the angular dis- 
 placement of the optic axis from a vertical. It has 
 been suggested that compensation for obliquity of 
 the plane of the picture, due to either uniformly 
 sloping ground, the optic axis being vertical ; or to 
 an oblique position of the axis, in the case of flat 
 country ; may be effected, as in copying by pho- 
 tography, by a simple adjustment of the original 
 photograph, or of the sensitive plate, in a secondary 
 photographic process. 
 
 When the captive balloon is not stationary, and 
 even when oscillating under a strong wind, it is shown, from experiments made at 
 Berlin in 1885, that the car itself can be maintained with its base horizontal. The cords 
 of the net, instead of converging in the usual manner to a ring beneath, were at- 
 tached at intervals to a horizontal bar. For the balloon used, a sphere of 50,000 cu. ft. 
 capacity, filled with hydrogen, and carrying eight persons, the bar was 2 inches in diam- 
 eter and 33 feet in length, and the car was suspended from its middle point ; two 
 light cables 0.4 inch in diameter were fastened one to each extremity of the bar, and 
 were joined 65 feet beneath it to the main cable. Suppose the balloon to be moved out 
 of the vertical of the station ; then, if the sensitive plate is horizontal, the view itself 
 will show the intersection of the optic axis with the earth ; this point might also be 
 fixed by bearings of known points taken from the car, or by transit measurements from 
 the earth, the instant of exposure in the latter case being communicated by signal. 
 
 If a series of views are thus taken above known points of an extensive tract, 
 they may be used in plotting as so many protractors, by fastening their centres with 
 needle-points to the plotted stations ; then, by orienting them and conforming to the 
 plane-table method of intersections, the required points are readily located. A scale for 
 any single view is easily constructed from the ratio of any represented distance to the 
 corresponding actual distance. For the general plot, since the method of intersections 
 is independent of the diameters of the views, differences of altitude would not affect 
 the construction of a plan, it being only necessary to fix the stations to scale. 
 
 From an altitude of 2000 feet excellent photographs have been obtained ; and each 
 
 FIG. 58 
 
TELESCOPIC AND BALLOON PHOTOGRAPHY. 61 
 
 of these, taken with a Hermagis 90 lens, would represent an area about of a mile 
 square ; while from a 3250-^. altitude, at which good results are noted, the area would 
 be about i^- miles square. 
 
 II. With Free Balloons. But few experiments in photographic surveying with free bal- 
 loons have up to the present date been recorded ; but the proposition seems by no 
 means infeasible, since the dirigibility of balloons has become practicable. In the experi- 
 ments of Captains Templar, Lee and Elesdale, at Woolwich in 1879, the differences in 
 direction of the air-currents at different heights were utilized to travel in any desired 
 direction ; a small pilot-balloon of about 200 cu. ft. capacity being made to ascend or 
 descend a distance of 2000 feet from the car, to show the directions of the air-currents. 
 At Meudon, in 1884, Capt. Renard, with a balloon propelled by means of an electric 
 motor, the screw being at the bow, and steered with a rudder-sail, travelled at will in a 
 9-mile wind, at the rate of 14 miles an hour, returning after a trip of 47 minutes to the 
 point of departure. It is evident that, in either of these cases, vertical views taken at 
 proper intervals would have furnished a map of the entire country passed over. More 
 recently, Captains Renard and Krebs have perfected an electric motor which ensures a 
 continuous voyage of four or five hours' duration. 
 
 For the purpose of measuring altitudes with precision in balloon surveying, M. Casella 
 has recently constructed a very sensitive aneroid barometer which indicates differences in 
 height of one foot. 
 
 With very sensitive plates and rapid shutters, the photographic operations are of easy 
 performance, and photographs are obtained that lack nothing in clearness and accurate 
 representation. Much assistance is afforded to military art in the rapidity and faithfulness 
 with which the enemy's defences and positions may be depicted ; and to the geographer 
 the means are supplied for obtaining descriptions of isolated tracts and of those inac- 
 cessible by ordinary means. Panoramic and plan views well combined give results of 
 great interest, and present to the aeronaut and photographer new and important applica- 
 tions of their arts. 
 
SECTION VII. 
 
 VARIOUS APPLICATIONS OF PHOTOGRAPHIC SURVEYING, AND SOME OF ITS 
 
 ADVANTAGES. 
 
 62. General Considerations. It has been shown in the preceding pages that with 
 the camera as a surveying instrument the field-work may be performed with great rapidity, 
 and with an economy of men and material exceeding that due to other means ; and that, 
 in the plotting, the plan results from, an application of the very simple method of in- 
 tersections ; while the representation of heights and surface-forms is accomplished either 
 mechanically, or by the use of tabular values which may be easily and rapidly applied. 
 As to the contouring, it is apparent that in some cases, owing to the surface-covering of 
 trees and the interposition of projecting spurs or salients, the lesser inequalities are not 
 given in the prints ; but this deficiency can be supplied, as in other methods, by field- 
 sketching ; and, in respect to this part of the work, it may be said of this method, as of 
 ordinary topographical surveying, that the production of a complete map requires a true 
 appreciation of forms of ground. With carefully-constructed instruments, and most of 
 the standard cameras and rectilinear lenses now made are suitable, the results exceed in 
 precision those ordinarily obtained with either compass or plane-table; and as to thorough- 
 ness of detailed description, there" is of course nothing to exceed a photographic repre- 
 sentation. 
 
 63. To Farm-surveys and Engineering Purposes. -For farm-surveys, fields, and small 
 tracts in general, a few exposures in many cases three will suffice from well-selected 
 points furnish the data for a complete map. 
 
 For engineering purposes, aside from its adaptation to the topographical survey of 
 projected routes of railroads, canals, and other subjects which have already been referred 
 to, this means is especially useful for conveying information as to the condition of impor- 
 tant constructions. Photographs obtained at different stages in the progress of the work 
 in hand afford descriptions which, for thoroughness, written communications are inadequate 
 to give ; these faithful records, representing the site, foundations, intermediate conditions 
 of the structure ; with date, number of men at work, state of the weather, etc., noted on 
 the back of each print, are very valuable for future reference. 
 
 64. To Geographical and ArcJiceological Descriptions. As an aid to the geographer this 
 means is of great value. The difficulties due to optical illusion which, in ordinary topo- 
 graphical sketching, mountainous or hilly tracts present, are obviated ; from the peaks, 
 panoramic views are comprehensive and far-reaching ; and exposures from the less elevated 
 points furnish details of the deeper valleys, and of those tracts which from the peaks are 
 either shut out or obscure. The choice of illumination, properly exercised, gives a defini- 
 tion to surface-lines and points, that actually appears sharper in a print than in nature ; 
 
VARIOUS APPLICATIONS OF PHOTOGRAPHIC SURVEYING. 63 
 
 therefore, as compared with ordinary observation, a more accurate map-delineation results. 
 Both geological .and botanical information are included in the print, the rock-structure, 
 peculiar formations, deposits, the forest and individual growths, and much else that 
 assists to a thorough knowledge of the country. 
 
 Profile views of a range of mountains or hills, taken either from peaks or favorable 
 points of a valley route, are excellent data for plotting. The directions of stations from 
 each other are ascertained by compass-bearings ; while the aneroid would prove of valuable 
 assistance in the levelling, and the odometer would measure the distances between stations 
 of the valley route. Coast-lines also may be accurately described from views taken off 
 shore. For the representation of inaccessible objects, tracts, summits, declivities, mo- 
 rasses, or craters, the method is unrivalled. 
 
 To the archaeologist its use is of inestimable value ; intricate lines of figure of 
 buildings, monuments, or earth constructions, are faithfully produced, and their measure- 
 ment is thus made easy of accomplishment. The photographic survey of Persepolis 
 and of the Mosque of Mesdjid-a-djunia, by Dr. Stolze, are recent instances of its suc- 
 cessful application. 
 
 As addenda to plotted maps, the photographs, for minute information, will often 
 serve the same purpose as actual inspection. 
 
 65. To Military Purposes, The rapidity with which ground may be represented 
 offers special advantages to the military service. As an instance of rapid work : In 
 1874, M. Javary made in one day, in the valley of 1'Arly, a photographic recon- 
 naissance extending 14 miles, and embracing the features within a distance of from 
 one to two miles on either side of the route followed. This work was performed with 
 two cameras of the ordinary type, a compass and a stadia. The two last instruments 
 were kept on the route, while a photographer, with stadia-rod and camera, marched on 
 each side. At a given signal, the distance to either photographic station was measured 
 by the stadia, and its bearing taken ; the photographer took the bearings necessary to 
 orient his views ; and means for checking the orientation were supplied by bearings, 
 taken from the stadia-stations, of prominent points that would appear in the views. 
 
 On another occasion, M. Javary made, in six hours, a photographic survey of the 
 ground included in a zone three fourths of a mile in width, in front of a fortified place ; 
 and, after 8 hours expended in the plotting, presented the commanding officer of the 
 attacking force with an exact and complete map, including the levelling ; and' this in rainy 
 weather and with wet plates. He states that, with four operators, the exact survey of any 
 fortified place can be made and plotted within 24 hours. 
 
 As an instance of its value in defensive operations, views of the surrounding country 
 may be taken from the different salients or other commanding points of a work ; the dis- 
 tances to the enemy's batteries, etc., ascertained from the plotting, can then be noted 
 on the representations of the objects, and the prints distributed to the chiefs of pieces, 
 who are thus enabled in an instant to get the range of any contained point. 
 
 Aside from the merit of this method as a means of military surveying, the rep- 
 resentation by^ photographs of all the details possesses a special advantage in camp or 
 field ; it may happen that a knowledge of some feature of ground, which in ordinary 
 
64 PHOTOGRAPHY APPLIED TO SURVEYING. 
 
 topographical sketching might be omitted, would prove of great value ; again, the natural- 
 ness of a photographic representation might convey to some the information not afforded 
 them by a topographical map. 
 
 Since balloon photography claims such an important part in warfare ; the element of 
 danger to the aeronaut, from the enemy's fire, is of considerable moment. During the 
 siege of Paris, the Germans made use of a large rifle so mounted, and furnished with 
 stock and butt, that it could be fired from the shoulder ; this was transported by wagon 
 to points over which the French balloons chanced to pass, and some of the aeronauts tes- 
 tified to hearing the bullets whiz by them when at altitudes of from 800 to 1000 
 yards. In November, 1870, one balloon, " La Daguerre," lost so much gas from perfora- 
 tion by the balls from the ordinary infantry rifle, that it had to descend and was captured 
 by the Prussians. From experiments subsequently made by the French war department 
 it was found that, at an altitude of 1300 feet, a balloon 13 feet in diameter was unharmed 
 by the best sharpshooters with a chassepot rifle ; but from the testimony above given, their 
 failure in attaining it must have been due to ignorance of the true form of the trajectory 
 when the line of fire is inclined to the horizon. It is found, theoretically, that with a 
 chassepot rifle, model of 1874, the extreme range in firing upward, at angles between 80 
 and 90 with the horizon, is nearly 2100 yards, while the effective range as against 
 an ordinary balloon is about 800 yards ; for angles between 60 and 70, the effective 
 range is increased to about 950 yards, and between 40 and 50 to about 1000 yards. 
 (For an elaborate discussion of this subject, see " Tir Centre les Ballons," by Dufaux, 1886.) 
 
 The " Jahresberichte " enumerates the following conditions to be fulfilled, in order 
 that balloons may be generally adaptable to military service : 
 I. To be readily transportable, either inflated or empty. 
 
 II. That the envelopes shall be resisting and impermeable to the contained gas, 
 in order that they may lose as little as possible of their ascensional force. 
 
 III. The means to be supplied for filling them rapidly, in any place whatever, and 
 of replacing the gas lost. 
 
 IV. For free balloons: power to move in any direction, of landing promptly and with 
 security, also of rising promptly. 
 
 V. For captive balloons : means for preventing rotation about a vertical axis, and for 
 causing them to rise in good condition notwithstanding the wind. 
 
 Experiments very lately made in France and England have been successful as to the 
 first three conditions ; as may be observed in the preceding pages (Section VI.), the fourth 
 and fifth are in a fair way of fulfillment ; and it certainly seems, after having within a 
 comparatively short period surmounted so many obstacles, that the efforts made to perfect 
 this valuable aid to scientific investigation, for civil as well as military purposes, will soon 
 meet with a favorable termination. 
 
 66. Some of the Advantages Derived from its Use. In photographic surveying, an en- 
 tire tour of the horizon, or any desired part thereof, is obtained in a few minutes by a 
 series of very simple operations, involving no multiplied or fatiguing observations ; no 
 drawings, sketches, or lines of direction difficult to trace in the field ; no delicate pointings 
 or minute readings, and no prolonged computations. There is no fear of having forgotten, 
 or omitted, some important point ; because every visible detail is represented, and, when 
 
VARIOUS APPLICATIONS OF PHOTOGRAPHIC SURVEYING. 65 
 
 negatives are used in plotting, with a precision that is rarely exceeded ; there is also no 
 occasion, as in other methods, for rejecting erroneous or doubtful observations. 
 
 The office-work presents no difficulties ; the constructions are simple and rational ; and 
 errors of destination, pointing or orientation, are made impossible by the numerous verifi- 
 cations which serve at each step to check results already attained. All the work done in 
 the field is of value, and there are no regrets for not having made all useful observations 
 at any station. 
 
 The map finished, the photographs, as addenda, present as already stated a view as 
 perfect and comprehensive as if the observer had personally traversed and inspected the 
 tract or region represented. 
 
 The only inconvenience that seems to present itself, is that of acquiring a certain 
 degree of skill in the chemical manipulations; but this part of the work is now reduced 
 to such simplicity that but little practice is necessary for the purpose ; besides, in surveys 
 of considerable extent, a professional photographer would usually be retained. 
 
 Finally, in the prosecution of a survey, the stations may often be so selected that in 
 the views artistic effect and necessary data are joined, and the resulting photographs would 
 thus be made as interesting in the former respect as they are useful in the latter. 
 
INDEX. 
 
 PAR. PAGE 
 
 33. Accuracy Attainable (see also pars, i, 51, 52, 61, 
 
 62, and 65), 34 
 
 64. Addenda to Maps, Use of Photographs as, ... 62 
 
 40. Adjustment of the Optic Centre, Cylindrograph, . 38 
 
 32. Advantage in the Use of Negatives, 33 
 
 44. Advantages claimed for Cylindrograph, .... 40 
 64. derived from Applications of Photo- 
 graphic Surveying, 62 
 
 28. Angle Scale, Vertical, 31 
 
 57. Apparatus for Balloon Photography, Special, . . 55 
 
 APPLICATIONS OF PHOTOGRAPHIC SURVEYING, . 62 
 
 63. To Farm-surveys and Civil Engineering, ... 62 
 
 64. To Geographical and Archaeological Description, 
 
 and as Addenda to Maps, 62 
 
 65. To Military Purposes, 63 
 
 66. Some of the Advantages derived from, ... 64 
 
 3. Azimuth and a Distance, Location of Points by, . 5 
 39. Azimuth and Slope Scale, for Cylindrograph, . . 38 
 
 BALLOON PHOTOGRAPHY, 52 
 
 55. Sketch of Progress in, to 1880, 53 
 
 56. Recent Experiments in, 55 
 
 57. Special Apparatus for, 55 
 
 58. Notably Successful Experiments in 56 
 
 59. Photographic Outfit for, and Method of Working, 57 
 
 60. Use of Small Balloons, Captive and Free, . . 58 
 
 61. Observations on Balloon Phot. Surveying, . . 59 
 61. Balloon Phot. Surveying, Observations on, . . 59 
 60. Balloons, Small, Captive and Free, Use of, . . 58 
 23. Best Time for Field-work 24 
 
 Camera, Early Forms of, see " Historical Sketch." 
 
 7. " , for Plane Perspectives, 9 
 
 36. " , " Cylindric " , 36 
 
 46. " , " Radial " , 42 
 
 5 2 - i " " , Mangin's, ... 47 
 
 53. CAMERA WITHOUT A LENS, 50 
 
 Captive Balloons, see "Balloon Photography." 
 
 46. Chevallier's Camera, 42 
 
 10. Changing-boxes, n 
 
 33. Choice of Scale, 34 
 
 63. Civil Engineering, Application to, 62 
 
 9. Compass and Level, for Use with Camera, ... to 
 
 30. Contouring from Photographs, . , 32 
 
 35. Cylindrograph, Topographic- 36 
 
 30. Datum Plane, Reduction of References to, ... 32 
 
 4. Definitions of Terms used in Perspectives, ... 5 
 17. Development of Photographic Plates, .... 17 
 
 5. Direction of a Point, To Determine from a Per- 
 
 spective Representation, 5 
 
 13. Distance, Focal, Measurement of, 13 
 
 16. Distortion, Test of an Objective for 16 
 
 19. Distribution, and Selection, of Stations 19 
 
 PAR. PAGB 
 
 4. Drawings, Perspective, Relation of Photographs to, 5 
 
 63. Engineering Purposes, Application to 62 
 
 51. Error, Instrumental, of Cylindrograph 46 
 
 56-58. Experiments in Balloon Photography, . . .55-6 
 
 17. " Exposure" of Photographic Plates, 17 
 
 63. Farm-surveys, Application to, 62 
 
 15. Field of View, Measurement of, 16 
 
 FIELD-WORK : 
 
 24. General, Conditions, Methods, 25 
 
 19. Selection and Distribution of Stations, .... 19 
 
 20. Method I. With Triangulation Established, . 20 
 .21. " II. " Camera and Hand-compass, . 22 
 
 22. " III. " Camera only, 23 
 
 23. Best Time for, 24 
 
 43. In Case of Cylindric Perspectives 39 
 
 47. " of Radial " , 44 
 
 47. Fixed Sectors, Operating by, 44 
 
 17. " Fixing" of Photographic Plates, 17 
 
 13. Focal Distance, Measurement of, 13 
 
 20. Form of Record Used in Field-work (Method I.), . 21 
 Free Balloons, see " Balloon Photography." 
 GENERAL PRINCIPLES, 4 
 
 2. Range and Object of the Method, 4 
 
 3. Location of Points 4 
 
 4. Relation of Photographs to Perspective Draw- 
 
 ings, Definitions of Terms 5 
 
 5. 6. To Determine the Direction and Position of a 
 
 Point from a Perspective, 5,7 
 
 64. Geographical Description, Application to, ... 62 
 
 52. Glass Reflector, Mangin's, 49 
 
 21. Hand-compass, Method with Camera and, ... 22 
 
 27. Heights, or References, To Determine, .... 28 
 29. " , Mechanical Measurement and Scale of, . 31 
 50. " , To Determine, in Radial Perspectives, . 46 
 43. " , " " , in Cylindric " , . 39 
 61. " , in Balloon Photography, 59 
 
 14. HH', the Horizon, To Determine 14 
 
 r. HISTORICAL SKETCH, i 
 
 14. Horizon, or HH', To Determine, 14 
 
 Lens, see "Objective." 
 
 9. Level, for Camera 10 
 
 Levelling, see "Heights, or References." 
 
 3. Location of Points 4 
 
 54. LONG RANGE PHOTOGRAPHY 52 
 
 52. Mangin's Camera, 47 
 
 34. Map, Photographic, Explanation of, 35 
 
 64. Maps, Use of Photographs as Addenda to, . . . 62 
 
 15. Measurement of Field of View, 16 
 
 13. " of Focal Distance, 13 
 
 28. Mechanical Measurement of Angles 29 
 
 29. " " of Heights 31 
 
68 
 
 INDEX. 
 
 PAR. PAGE 
 
 52. Metallic Reflector, Mangin's . . 47 
 
 65. Military Purposes, Application to, 63 
 
 Minor Surveys, see " Farm-surveys." 
 
 35. Moessard's Camera, 36 
 
 48. Movement, Continuous, Operating by, .... 44 
 
 10. Negative-box, n 
 
 32. Negatives, Advantage in Use of 33 
 
 17. " , Development of, 17 
 
 2. Object of the Method 4 
 
 37. Objective, and its Carrier, Cylindrograph, ... 37 
 8. " , Description of, Kind Required, . . . 10 
 
 16. " , Test for Distortion 16 
 
 " , For other Kinds of, see Text under 
 
 " Camera." 
 
 14. Objective Slide, and Objective-slide Scale, ... 14 
 
 61. Observations on Balloon Photography, .... 59 
 
 13. OP Focal Distance, Measurement of, .... 13 
 
 48. Operating by Continuous Movement, 44 
 
 47. " " Fixed Sectors, 44 
 
 17. Operation of Taking a Photograph, 17 
 
 Optic Axis, see " Tests and Measurements." 
 
 13. Optic Centre, To Find Position of, 13 
 
 25. Orientation of Views, Definition of, 25 
 
 42. " " , Device for, Cylindrograph, 39 
 
 26. " " , Special Cases of, .... 26 
 59. Outfit, Photographic, for Balloon Photography. . 57 
 
 7-10." , " , General 9" 11 
 
 i. Panoramic Apparatus, Early Forms of, .... I 
 4. Perspective Drawings, Definition of Terms, . . 6 
 i. " , Early Use of, in Map Con- 
 struction, i 
 
 4. " " , Relation of Photographs to, 5 
 
 PERSPECTIVES, CYLINDRIC, METHOD BY, ... 9 
 
 " , PLANE, , ... 36 
 
 , RADIAL, " , ... 42 
 
 1 7. Photographs for Surveying Purposes, How to Make, 1 7 
 
 34. Photographic Map (Plate I.), Explanation of, . . 35 
 
 4. Photographs, Relation of, to Perspective Drawings, 5 
 
 25. Plan, Construction of the, 25 
 
 PLANE PERSPECTIVES, METHOD BY, 7 
 
 45. Plane-tables, Photographic, 42 
 
 38. Plate-holder for Cylindrograph, 37 
 
 7. " , Ordinary, 9 
 
 10. Plates, Sensitive, or Photographic, n 
 
 PLOTTING: 
 
 24. General Considerations, 25 
 
 25. Construction of the Plan, 25 
 
 26. Special Cases of Orientation, 26 
 
 27. To Determine Heights or References of Points, 28 
 
 28. Mechanical Measurement of Horizontal and Ver- 
 
 tical Angles, Tangent-glass and Vertical Angle 
 Scale 29-31 
 
 29. Mechanical Measurement of Heights, Scale of 
 
 Heights, 31 
 
 30. Reduction of References to a Datum Plane, The 
 
 Contouring, 32 
 
 31. Suggestions in Regard to 33 
 
 PLOTTING ( Continued:) 
 
 32. Preparation of Prints for, Advantage in Use of 
 
 Negatives, Transparencies, 33 
 
 33. Choice of Scale, Accuracy Attainable, .... 34 
 
 34. Example of a Photographic Map 35 
 
 43. From Cylindric Perspectives, 40 
 
 49. " Radial , 46 
 
 61. " Balloon Photographs, 59 
 
 8. Pneumatic Shutter, Reference to, 10 
 
 5. Point, To Determine Direction of, from Perspec- 
 
 tives, 5 
 
 6. " , To Determine Position of, from Perspectives, 7 
 
 3. Points, Location of, 4 
 
 27. " , To Determine References of, 28 
 
 32. Preparation of Prints for Plotting, ..... 33 
 
 PRINCIPLES, GENERAL, 4 
 
 17. Printing Frame and Printing from Negatives, . . 17 
 
 32. Prints, Preparation of, for Plotting 33 
 
 RADIAL PERSPECTIVES, METHOD BY 42 
 
 2. Range of the Method, 4 
 
 20. Record, Form of, for Field-work 20 
 
 30. " , " " Reduction of Heights to Com- 
 mon Datum, 32 
 
 27. References of Points, Determination of, .... 28 
 
 30. , Reduction of, to Datum Plane, 32 
 52. Reflectors, Glass and Metallic (Mangin's), ... 47 
 12. Register, To Test for 12 
 
 4. Relation of Photographs to Perspective Drawings. 5 
 10. Roller Plate-holder, Reference to, n 
 
 28. Scale, Vertical Angle 31 
 
 29. " of Heights 31 
 
 39. " , Azimuth- and Slope-, Cylindrograph, . . 38 
 
 35. " , Choice of, 36 
 
 47. Sectors, Fixed, Operating by, 44 
 
 19. Selection and Distribution of Stations 19 
 
 10. Sensitive Plates and Changing-boxes, ir 
 
 60. Small Balloons, Captive and Free, Use of, . . . 58 
 
 66. Some of the Advantages, in Applications of Subject, 64 
 
 57. Special Apparatus for Balloon Photography, . . 55 
 
 26. " Cases of Orientation, 26 
 
 19. Stations, Selection and Distribution of 19 
 
 31. Suggestions in Regard to Plotting, 33 
 
 63. Surveys, Farm-, Application to 62 
 
 28. Tangent-glass, or -Scale, 29 
 
 i. Telemetrograph, Reference to the i 
 
 TELESCOPIC PHOTOGRAPHY 52 
 
 4. Terms Used in Perspective, Definitions of, ... 5 
 
 12. Test for Register, 12 
 
 16. Tests and Measurements Required 16 
 
 23. Time for Field-work, The Best 24 
 
 17. Toning of Photographs 17 
 
 35. Topographic-Cylindrograph, 36 
 
 32. Transparencies, Use of, in Plotting 34 
 
 20. Triangulation, Reference to 20 
 
 28. Vertical Angle Scale 3 1 
 
 6, 27-8. Vertical Angles, Measurement of, . . 7, 28-31 
 
 28. " " , Mechanical, 30 
 

 . o, 
 
 -a. 1 3 
 

 
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GENERAL LIBRARY - U.C. BERKELEY 
 
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