LIBRARY OF THE UNIVERSITY OF CALIFORNIA. > > ^CX/-v^ Class LTMENT, COAST AND GEODETIC SURVEY. HENRY S. PRITCHETT, SUPERINTENDENT. SURVEYING. PLANE TABLE MANUAL. By U. B. WAINWRIGrHT, Assistant. APPENDIX NO. 8-REPORT FOR 1897-98. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1899. TREASURY DEPARTMENT, U. S. COAST AND GEODETIC SURVEY. HENRY S. PRITCHETT, SUPERINTENDENT. SURVEYING. A PLANE TABLE MANUAL. By r>. B. WAIN WRIGHT, Assistant. APPENDIX No. 8-REPORT FOR 1897-98. WASHINGTON: GOVERNMENT PRINTING OFFICE. 1899. fen APPENDIX No. 81897-98. A PLANE TABLE MANUAL. By D. B. WAINWRIGHT, Assistant. 870 409 TABLE OF CONTENTS. (a) PRELIMINARY STATEMENT. Page. Definitions 415 Topographical map 415 Projection 415 Scale 415 Datum plane 415 Relief 415 Control 415 (b) INSTRUMENTS AND ADJUSTMENTS. Plane table 416 Description . . . 416 The board 416 Movements 416 Tripod 417 Mountain plane table 417 The alidade 417 Description 417 Declinatoire 418 Metal clamps 418 Adjustments of the alidade 418 Fiducial edge of rule 418 Level attached to rule 418 Parallax 419 Line of collimation 419 Axis of revolution 419 Middle horizontal line 419 Stadia rod 420 Description 420 Graduation 420 Inclined sights 421 Table for reduction of hypothenuse to base 422 Micrometer eyepiece 425 Plane-table sheet , 425 Characteristics 425 Distortion 425 Scale 426 Projections 426 Selecting limits 426 Polyconic projection 428 Method of drawing 428 Rectangular 429 Accessories 429 Weights 429 411 412 COAST AND GEODETIC SURVEY REPORT, 1897-98. (c) FIELD WORK. Page. Organization of party 4 2 9 Tableman 429 . Rodsmen 43 Aid 43 Preliminary reconnoissance 43 Signal poles 43 Graphic triangulation 43 Amount of control 433 Three-point problem 433 Advantages 433 Definitions 434 Lehman's method 434 Rule i : 434 Demonstration 434 Classification 434 Rules 2, 3, and 4 434 Procedure 435 Examples (table deflected to the right) 435 Examples (table deflected to the left) 435 Repetition 435 Orienting by estimation 436 Bessel's method by inscribed quadrilateral 436 Tracing cloth protractor 437 Two-point problem 437 Deflection of long lines 438 Distortion errors 439 Example 439 Position by compromise 440 Rules 440 Application v 440 Height of instrument 441 Procedure 441 Table of heights (in feet) 441 Example of use of table 442 Formula for determining heights 442 Table of heights (in meters) .*.... 444 Table of factors for computing differences in elevation (in feet) 446 Table of corrections for curvature and refraction (in feet) 447 Table of factors for computing differences in elevation (in meters) 448 Table of corrections for curvature and refraction (in meters) 449 Table; comparison of feet and meters 449 Relief 449 Hill shading r 449 Contours 450 Profile 450 Advantages and disadvantages of contours and hill shading 450 Contour interval 451 Datum plane 451 Reference signal 45 1 Regular and irregular methods of contouring 45 1 Station routine 452 Number of elevations to be determined 452 Contour sketching . 452 TABLE OF CONTENTS. 413 Page. Typical contour groups 453 Order of development of contours 453 Filling in 453 Traverse lines .... 453 Main traverse 454 Subordinate traverse 454 Determinations for hydrography 454 Low-water mark 455 High-water and storm-water line 455 Determination of inaccessible points 455 Large scale surveys , 455 District of Columbia survey ! 455 Rapid surveys 456 .Military reconnoissance with plane table 456 With compass and notebook 457 Photogrammetry 457 Camera and plane table in Alaska 457 Survey in advance of triangulation 458 Office work 459 or THE UNIVERSITY or APPENDIX No. 81897-98. A PLANE TABLE MANUAL By D. B. WAINWRIGHT, Assistant. a. PRELIMINARY STATEMENT. A topographical map is the delineation upon a plane surface, by means of conven- tional signs, of the natural and artificial features of a locality. Every point of the drawing corresponds to some geographical position, according to some method adopted for representing the surface of the spheroid on a plane, which is called the projection. Since it is a representation in miniature, the distance between any two points on the map is a certain definite fraction of the distance between the same points in nature. This ratio is called the scale. Each point, besides being projected on a horizontal plane, has its height, either within, above, or below a level surface, in some way indicated. The level surface adopted for the map is called the datum plane, and the representation of the variations in the vertical element, the modeling of the country, is called the relief. CONTROL. All topographical surveys of importance are based upon a system of triangulation. A sufficient number of points, whose geographical positions have been determined in this manner, properly distributed over the area to be surveyed, forms a strong frame- work for controlling the accurate location of the various details. , a The edition of Appendix No. 13, Report of 1880, "A treatise on the plane table," by E. Herges- heinier, Assistant, being exhausted, a new edition has been prepared by the direction of the Superin- tendent, in which the material has been rearranged, some portions rewritten, and some additions made. 415 416 COAST AND GEODETIC SURVEY REPORT, 1897-98. b. INSTRUMENTS AND ADJUSTMENTS. THE PLANE TABLE. The principal instrument in use by the United States Coast and Geodetic Survey for mapping these details is the plane table. For this purpose it is a universal instru- ment. All the necessary operations for producing a map are executed with it in the field and directly from the country as a model. Other instruments are employed as auxiliaries to it under certain conditions, as will be seen later on under the head of " Field practice," but in general it fulfills all require- ments alone. Description (PI. i). The plane table is composed of a well-seasoned drawing board about 30 inches in length, 24 in width, three-quarters of an inch thick, with beveled or rounded edges. It is commonly made of several pieces of white pine, tongued and grooved together, with the grain running in different directions to prevent warping. It is supported upon three strong brass arms, to which it is attached by screws passing through them and entering the underside of the board, the three holes for the reception of the screws being guarded by brass bushings and situated equidistant from each other and from the center of the table. By means of these screws the board can be removed at will. The movements (Pis. i and 2) of the tables in use by the Coast and Geodetic Survey are made from several different models, but as the principal features are the same in all designs the description of one type will to a large extent suffice for all. The arms to which the board is fastened rest upon the sloping upper face of a rather flat hollow cone of brass, to which they are permanently fixed. Upon its lower edge or periphery this cone is fashioned into a horizontally projecting rim, the inferior face of which is as nearly as possible a perfect plane, and this in its turn rests upon a corresponding rim of somewhat greater diameter projecting slightly beyond it. This second rim forms the upper and outer flange of a circular metal disk in the form of a very shallow cylinder. The inferior face or plane of the upper flange or rim has, at its contact with the superior face of the lower, a horizontal rotary movement about a common center which is also the center of the instrument, and the two are held together by means of a solid conical axis of brass extending upward from the center of the inner face of the lower disk. A socket of similar shape fits exactly over this axis, projecting downward from the inner side of the apex of the conical or upper disk. The two plates are held together by means of a mill-headed screw, capping the cone from the outside, and which can be loosened or removed at pleasure. A tangent screw and clamp fastened to the edge of the upper rim permit, when loose, the revolution of the table about its center, and, when clamped to the lower limb, hold the table firm while the tangent screw gives a more delicate movement. Three equidistant vertical projections of brass grooved on the underside, and cast in one piece with the under face of the lower disk, extending from the periphery towar4 the center, rest upon the points of three large screws which come through a heavy wooden block below. This block, which is the top of the stand and is approximate in form to an equilateral triangle, is 2^ inches thick when made of wood. Coast arul Geodetic Sur-vey Report, 1897-98 Appendix 8. No. 2. APPENDIX NO. 8. PLANE TABLE MANUAL. 417 The three screws last mentioned have large milled heads, are quite stout, and play through the block below by means of brass female screws let into it. They are the leveling screws of the instrument and are equidistant from its center. Upon the underside and center of the metal lower disk is a socket containing a ball with a brass arm, which projects through the center of the block from beneath. The lower end of the arm is threaded, and upon it plays a female screw with a large milled head, which can be relaxed or tightened at pleasure. The screw clamps the whole upper part of the instrument to the stand; it is loosened only before leveling and kept securely clamped at all other times. The block, made either of wood or brass, is supported upon three legs, and with them forms the tripod or stand of the instrument, the legs being of such a length as to bring the table to a convenient height for working, and so arranged as to be taken off at will, or closed so that their brass shod and pointed ends can be brought together or moved outward, as may be required. They are made on the open or skeleton pattern, and each is securely attached to a segment of the tripod head by a long brass bolt. MOUNTAIN PLANE TABLE. (PL 3-) A small plane table, with a board measuring only 14 by 17 inches, is employed in reconnaissance, mountain work, or as an auxiliary to one of the standard size. All the various parts are reduced in size to correspond with the board, and the construction of the movement simplified. THE ALIDADE. The type of alidade in general use (PI. i) consists of a brass or steel rule (12 inches long by 2^2 inches wide) nickel plated underneath, from and perpendicular to which rises a brass column (3 inches high), surmounted by Y's, receiving the transverse axis of the telescope, to one end of which axis is firmly attached a graduated arc of 30, each sicte of a central o , an accompanying vernier being attached to the lower part of the Y support. The arc moves with the telescope as it is raised or depressed, and it is used in the measurement of the vertical angles for height. A clamp and a tangent screw placed on the other side of the telescope, opposite the arc, controls its vertical movement. The telescope is fitted accurately near its center of gravity within a closely fitting cylinder, to which is solidly attached the transverse axis. The telescope revolves within the cylinder 180, stops being fitted for that range. This affords an easy mode of adjusting the cross webs to the axis of revolution, and for correction with a striding level of the errors of level and collimation and revolution of the telescope. Upon the tube of the telescope are turned two shoulders, on which rest a striding spirit level, which can be readily reversed or removed at pleasure. The eye piece carries the usual reticule with screw r s for the collimation adjustment, and to this is attached a glass diaphragm, having one vertical and three horizontal lines engraved upon it. One of the horizontal lines crosses the middle of the diaphragm, the other two are placed equidistant from it, one above and one below. The interval between them remains a constant cnord for the measurement of distance upon a graduated staff or rod. In some cases short auxiliary lines have been added dividing the interval into still smaller chords. S. Doc. 48 27 418 COAST AND GEODETIC SURVEY REPORT, 1897-98. Several of the alidades are furnished with a micrometer eyepiece so attached that the thread is horizontal, and has a vertical movement for measuring the angular distance of a fixed length on a rod which remains a constant chord. To the rule of the alidade are attached two spirit levels, one in the longitudinal direction of the rule and the other at right angles to it. The latest Coast and Geodetic Survey alidade (PL 4) differs from the preceding type in having the vertical arc so placed that the vernier can be read without the observer moving away from the eye end of the telescope. This is effected by placing the arc in the quadrant nearest the eyepiece and the graduation of the arc upon its outer periphery. The weight of this alidade has also been reduced by using aluminum bronze in its construction, and by providing it with a skeleton rule. A dedinatoire (shown in Pis. i and 3) accompanies the alidade and is carried in the same packing box. It consists of a rectangular brass box 7 inches long by 2 wide, with an arc at each end graduated to 15 on each side of the o. It contains a needle long enough to extend from arc to arc, and resting on a pivot midway the box. The sides running lengthwise the box are parallel to a line connecting the zero marks of the two graduations. The metal clamps, for holding the projection on the board, are of two kinds: The V-shaped for the ends, and the side clamps, the latter-being made of thin metal strips about 12 inches in length, with two or more springs to grip the under side of the board.- THE ADJUSTMENTS OF THE ALIDADE. Adjustments. From the nature of the service in some sections of the country the plane table is often necessarily subjected to rough usage, and there is a constant liability to a disturbance of the adjustments; still, in careful hands, a well-made instrument may be used under very unfavorable conditions for a long time without being perceptibly affected. One should not fail, however, to make occasional examinations, and while at work, if any difficulty be encountered which can not otherwise be accounted for, it should lead directly to a scrutiny of the adjustments. 1. The fiducial edge of the rule. This should be a true, straight edge. Place the rule upon a smooth surface and draw a line along the edge, marking also the lines at the ends of the rule. Reverse the rule and place the opposite ends upon the marked points and again draw the line. If the two lines coincide no adjustment is necessary; if not, the edge must be made true. There is one deviation from a straight line, which, by a very rare possibility, the edge of the ruler might assume, and yet not be shown by the above test; it is when a part is convex and a part similarly situated at the other end concave, in exactly the same degree and proportion. In this case, on reversal, a line drawn along the edge of the rule would be coincident with the other, though not a true right line; this can be tested by an exact straight edge. 2. The levels attached to the rule. Place the instrument in the middle of the table and bring the bubble of either level to the center by means of the leveling screws of the table; draw lines along the edge and ends of the rule upon the board to show its exact position, then reverse 180. If the bubble remains central it is in adjustment; if not, correct it one-half by means of the leveling screws of the table, and the other half by APPENDIX NO. 8. PLANE TABLE MANUAL. 419 the adjusting screws attached to the level. This should be repeated until the bubble keeps its central position whichever way the rule may be placed upon the table. This presupposes the plane of the board to be true. The other level should now be examined and adjusted in a like manner. Great care should be exercised in manipulation lest the table be disturbed. 3. Parallax. Move the eyeglass until the cross hairs are perfectly distinct, and then direct the telescope to some distant well-defined object. If the contact remains perfect when the position of the eye is changed in any way, there is no parallax; but if it does not, then the focus of the object glass must be changed until there is no displace- ment of the contact. When this is the case the cross hairs are in the common focus of the object and eyeglasses. It may occur that the true focus of the cross hairs is not obtained at first, in which case a readjustment is necessary, in order to see both them and the object with equal distinctness and without parallax. 4. The line of collimation perpendicular to the axis of revolution of the telescope. Point the intersection of the vertical and the middle horizontal lines of the diaphragm on some well-defined distant object; revolve the telescope in its collar 180 and again observe the object. If the intersection covers it, the adjustment is perfect; if not, one half the error must be corrected by moving the diaphragm, by means of the adjusting screws, and the other half with the tangent screw of the table. This operation should be repeated until the adjustment is complete. 5. Axis of revolution. Since the bearings of the axis are unchangeable, the axis of revolution is assumed to remain parallel to the plane of the rule. 6. Middle horizontal line of diaphragm to the plane of the horizon. (i) Adjust the striding-level by reversing it end for end and correcting its error half the difference by its own adjustment, half by the tangent-screw of the telescope. (2) Point the telescope to a target, and note the reading or make a mark where the wire points when the bubble is in the middle. (3) Revolve the telescope about itself, put the level on again, and note the reading or mark the place where the telescope now points when the bubble is in the middle. (4) The mean of the two pointings is the true level line, upon which the wire is .to be adjusted, which may be done in this way: Point the wire to the mean of the two observations by the tangent-screw; then, by means of the adjusting-screws, bring it to point on the lower reading, if the second reading has been high, and vice versa. If now brought back to the mean by the tangent-screw, the bubble should be in its place; and, when the telescope is turned back into its first position, the adjustment should verify. (5) If it is now desired to make the vernier read zero on the vertical arc, the table must be carefully leveled; and in order to do this more perfectly than can be done with the levels on the ruler it may be done by observing the striding-level; the telescope remaining clamped, the striding-level should read the same in every position of the alidade when the table is perfectly level. (In general, this will be found too delicate a test, as the table is not sufficiently even for so sensitive a level to be employed.) The table being leveled, move the telescope with the tangent-screw until the bubble is in the middle, and then set the vernier to read zero; the screw-holes in it are oblong, so that it admits of being pushed either way. (6) It is easy to have the adjustments near enough to serve for running curves of equal elevation, but in determining the heights of stations it is best to make the obser- 420 COAST AND GEODETIC SURVEY REPORT, 1897-98. vations complete, with reversals, both of level and of telescope, taking the mean of the observations, by which the errors of adjustment are eliminated. This, in fact, is always done with the theodolite, and should be equally done with the alidade when precision is required. The following may serve as an example: TELESCOPE DIRECT. Level direct, reading + o i' Level reversed, reading o' Mean + o o''5 Station, reading +- 2 17' Elevation (difference) 2 i6'"5 TELESCOPE INVERTED. Level direct, reading o 2' Level reversed, reading - \' Mean o i x Station.. . + 2 12' Elevation (difference) 2 Mean 2 15' It will be seen, from analyzing this observation, that the level was one-half minute out of adjustment, the horizontal wire one and one-half minutes, and that revolving the telescope about itself changed its relation to the index on the vernier by i'. The mean is free from all errors of adjustment. The stadia rod* (PI. 3), as used in the Coast and Geodetic Survey, is simply a scale of equal parts painted upon a wooden rod about 10 feet long, 5 inches wide, and i ^ inches thick, so graduated that the number of divisions upon it, as seen between the upper and lower horizontal wires of the telescope, is equal to the number of units in the distance between the instrument and the rod held at right angles to the line of sight. Graduation. In all cases the rod should be graduated experimentally for the par- ticular instrument, and, if the best results are to be obtained, to suit the eye of the observer. In practice the alidade is mounted on a stand, and its centre (corresponding to the standard) is plumbed over one end of a hundred-metre base, measured on level ground. A line, representing the zero of the graduation, having been drawn about 5 inches from * It has been decided to adopt the word "stadia" instead of "telemeter" to accord with the almost universal usage of civil engineers and surveyors. In spite of its faulty derivation the former word is now imbedded in the literature relating to measurements made according to this method, while the latter is gradually but surely disappearing. For further details of the theory of stadia measurements see: Elemente der Vermessungs-Kunde, Bauernfeind, 1873, p. 322; Handbuch der Ver- messungs-Kunde, Jordan, 1888, p. 554; Theory and Practice of Surveying, Johnson, 1898, p. 238; Gillespie's Higher Surveying, Staley, 1897, p. 311; Experimental Study of Field Methods, Smith, Bulletin of University of Wisconsin, Engineering series, Vol. I, No. 5. APPENDIX NO. 8. PLANK TABLE MANUAL. 421 one end of the rod, the latter is held vertical at the other end of the base, zero mark upward. The observer at the alidade then makes the upper horizontal line of the diaphragm coincide with the zero and directs the rodsman by signals where to draw a line which coincides with the lower horizontal line. This intercepted space on the rod is now subdivided into the smaller parts of the adopted scale and the graduation con- tinued to within a short distance of the bottom. This graduation is represented by the equation where 5^ '99959 99946 99932 99915 99908 1-99917 1-99891 1-99863 1-99831 1-99801 2-99875 2-99838 2-99794 2-99746 2-99693 3-99834 3-99783 3 '997 25 3'99659 3 '99590 4'99793 4-99729 4'99657 4'99572 4-99488 5-99752 5-99676 5-99589 5'99489 5 '99386 6-99711 6-99622 6-99520 6-99406 6-99284 7-99669 7-99568 7-99452 7-99323 7-99182 8-99628 8-99514 8-99384 8-99239 8-99080 200 / 99878 1-99756 2 '99635 i 3 '995 13 4'9939* 5-99269 6-99147 7-99025 8-98904 io" 20' 30' 40' 50' 99857 99834 99810 99784 99756 1-99714 1-99669 1-99620 1-99568 1-99511 2'9957i 2'99503 2-99429 2'9935i 2-99267 3-99428 3 '99337 3 '99239 3'99i35 3-99023 4-99285 4-99171 4-99049 4-98918 4-98778 5-99142 5-99006 5-98859 5-98702 5 '98534 6-99000 6-98840 6-98669 6-98485 6-98290 7*98857 7-98675 7-98479 7-98268 7-98046 8-98714 8-98509 8-98289 8-98053 8-97802 3 oo / 99726 1-99452 2-99178 3-98904 4-98630 5'98357 6-98083 7-97809 8-97635 10' 20' 30' 40' 5o / 99695 99662 99627 99591 '99553 1-99390 1-99324 i '99255 1-99182 1-99106 2-99085 2-98986 2-98882 2-98773 2-98659 3-98780 3-98648 3'98509 3 '98364 3-98212 4-98474 4-98309 4-98136 4'97955 4^7765 5-98169 5'97972 5 '97764 5-97546 5'973i8 6-97865 6-97634 6-97391 6-97137 6-96871 7*9756o 7-97296 7-97019 7-96728 7-96424 8-97255 8-96958 8-96646 8-96319 8-95978 4 oo' 99513 1-99027 2-98540 3 '98054 4'97567 5-97081 6-96595 7-96108 8-95621 10' 2O X 3^ 40' 50' 99472 99429 99384 99338 99290 1-98944 1-98858 i "98769 i -98676 1-98580 2-98416 2-98287 2-98153 2-98014 2-97870 3-97888 3-97716 3-97537 3-97352 3-97160 4-97360 4'97i45 4-96922 4-96690 4-96450 5-96832 5'96574 5-96306 5-96028 5 '95 740 6-96304 6-96003 6-95691 6-95366 6-95030 7*95776 7*95432 7*95075 7-94704 t 7-94320 8-95249 8-94862 8-94460 8-94043 8-93611 5 oo' 99240 1-98481 2-97721 3-96961 4-96202 t 5 '95443 6-94683 7-93923 8-93164 10' 2O X 30' 40' So 7 99189 99136 99081 99025 98967 1-98378 2-97567 1-98272 2-97408 1-98163 2-97244 1-98050 2-97075 1-97934 2-96901 3'96756 3'96544 3-96326 3'9t>ioo 3-95868 4'95945 4-95680 4 '95407 4 '95 1 25 4 '94835 5 '95 134 5-94816 5 '94489 5 '94 1 50 5-93802 6-94323 6-93952 6-93570 6-93I75 6-92769 7-93512 7-93088 7-92652 7-92200 7-91736 8*92702 8-92224 8-9I733 8-91225 8-90703 6 oo' 98907 1-97814 2-96722 3 '95630 4*94537 5 '93445 6-92358 7-91260 8-90167 10' 2O' 30' 40' 50' 98846 98783 98718 98652 98584 1-97692 1-97566 1-97436 1-97304 1-97169 2-96538 2-9. 6 349 2-96155 2-95956 2-95753 3'95384 3-95I32 3'94873 3-94609 3'94337 4-94230 4'939 J 5 4'9359* 4-93261 4-92921 5 '93077 5-92698 5-92310 5'9i9!3 5*9 J 5o6 6-91923 6-91481 6-91029 6-90566 6-90090 7-90769 7-90264 7-89748 7-89218 7-88674 8-89615 8-89048 8-88467 8-87870 8-87259 7oo' 98515 1-97030 2-95544 3'94059 4'9 2 574 5-91089 6-89604 7-88119 8-86634 * Furnished by J. A. Flemer, Assistant, Coast and Geodetic Survey. 424 COAST AND GEODETIC SURVEY REPORT, 1897-98. Table of coefficients for reducing readings of inclined sights on vertical rod to horizontal distance Continued. Angle of inclina- tion. Horizontal projection of : i m 2 111 3 ^^ 4 in 5 6 ill 7 m 8 m 9 m 10' 20' 30' 40' 50' 98444 98371 98296 98220 98142 I-96888 I-96742 I-96592 I -96441 I-96285 2-95331 2-95112 2-94889 2-94661 2-94427 3'93775 3 '93483 3-93I85 3-92881 3-92570 4-92218 4-91854 4-91481 4-91101 4-90712 5-90662 5-90225 5-89322 5-88855 6-89105 6-88596 6-88073 6-87542 6-86997 7-86967 7-86370 7-85762 7-85140 8-85993 8-85337 8-84667 8-83982 8-83282 8 oo' 98063 I-96I26 2-94189 3-92252 4-903I5 5-88378 6-86441 7-84504 8-82568 10' 20' 30' 40' 50' 9oo' 97982 97899 97815 97729 97642 r95964 I '95798 I-95630 i '95459 1-95284 2-93946 2-93698 2-93446 2-93188 2-92926 3-91928 3-9'598 3-91261 3-90918 3-90568 4-89910 4-89497 4-89076 4-88647 4-88209 5^7892 5-86891 5-8585I 6-85874 6-85296 6-84707 6-84106 "6-83493 7-83196 7-82522 7-81836 7-81134 8-81839 8-81096 8-80337 879565 8-78777 '97553 1-95106 2-92658 3-90211 4-87764 5-853I7 6-82870 7-80423 877975 10' 20' 3 => 16-4 16-8 I7-2 17-6 18-0 9 ooo 18-0 18-4 iS'8 19-2 19-6 20'0 20-4 20-8 21-3 21-7 22'2 10 ooo 22'2 22'6 23-0 23-5 24-0 24-4 24-9 25-4 2 5 -8 26-3 26-8 II OOO 26-8 27'3 27-8 28-3 28-8 29'3 29-8 30-3 30-8 3i-4 3^9 12 OOO 3*'9 3 2- 4 33'o 33 "5 34' i 34'6 35 '2 357 36-3 36-9 37'4 13 ooo 37'4 38-0 38-6 39'2 39-8 40-4 41 "o 41-6 42-2 42-8 43 '4 14 ooo 43'4 44-1 447 45 '3 46-0 46-6 47-2 47'9 48-5 49-2 49'8 15 ooo 49-8 50-5 5i-2 5i-9 52-5 53'2 53 "9 54'6 55'3 56-0 567 16 ooo 567 57'4 58-2 58-9 59'6 60-3 6i'o 6r8 62-5 63 '3 64-0 17 ooo 64-0 64-8 65-6 66-3 67-1 67-9 68-6 69-4 70-2 71*0 71-8 18 ooo 71-8 72-6 73 '4 74-2 75'o 75-8 76-7 77'5 78-3 79-1 80-0 19 ooo 80-0 80-8 817 82-5 ,83-4 84-2 85-1 86-0 86'9 877 88-6 * Furnished by G. R. Putnam, Assistant, Coast and Geodetic Survey. 448 COAST AND GEODETIC SURVEY REPORT, 1897-98. Table of factors for computing differences in elevation. To obtain the difference in elevation in meters multiply the horizontal distance in meters by the factor in^ this table corresponding to the observed angle of elevation or depression. The factors are given for each ten minutes, but the value of the nearest minute may be interpolated, using the column of differences for one minute. The result is still to be corrected where necessary for the effect of curvature and refraction. TABLE V. Angle o' lo' 20' 3 o' 47 0-o8 O'lO 0*11 0-13 0'15 0-17 0-19 O'22 0-24 0-27 2 000 0*27 0-29' 0-32 0-35 0-38 0*42 0'45 0-49 0-52 0-56 0*60 3 0*60 0-64 0-68 073 077 0'82 0-86 0-91 0-96 I '01 1-07 4 ooo 1-07 I'I2 1-18 1-23 I '29 I'35 1-41 1-47 I'54 i -60 1-67 5 00 1-67 174 i -80 1-87 1-94 2 -O2 2-09 2-17 2-24 2-32 2-40 6 ooo 2-40 2-48 2-56 2-65 273 2-82 2-91 3-00 3-09 3'i8 3'27 7 ooo 3-27 3-36 3 "46 3 '55 3-65 3'75 3-85 3-96 4 -06 4-16 4-27 8 ooo 4-27 4-38 4'49 4-60 471 4-82 4-93 5-05 5-i6 5-28 5 '40 9 ooo 5 '40 5'52 5-65 577 5-89 6'02 6-15 6-28 6-41 6-54 6-67 10 000 6-67 6'8o 6-94 7-08 7'22 7-36 7-50 7-64 7-78 7-93 8-07 II 000 8-07 8-22 8-37 8-52 8-67 8-82 8-98 9-I3 9-29 9-45 9-61 12 OOO 9'6i 977 9^3 10-09 IO'26 10-42 10-59 10-76 10-92 II'IO 11-27 13 ooo 11-27 ii '45 11-62 ir8o Il-gS I2'l6 12-34 12-52 12-71 12-89 13-08 14 ooo 13-08 13-26 I3-45 13 '64 I3-83 14-03 14-22 14-42 14-61 14-81 15-01 15 ooo 15-01 15-21 15-41 15-62 I5-82 16-03 16-24 16-44 16-65 16-87 17-08 16 ooo 17-08 17-30 i7'5i 1773 I7-95 18-17 i8-39 18-61 18-83 19-05 19-28 17 ooo 19-28 i9'5i 1973 19-96 20-I9 20-43 20-66 20-89 21-13 21-37 2I'6l 18 ooo 2I'6l 21-86 22'IO 22-34 22-58 22-83 23-08 23-33 23-58 23-83 24-08 19 ooo 24-08 24-34 24-60 24-85 25-II 25-37 25-63 25-89 26-15 26-42 26-68 Comparison of feet and meters. [i meter = 3. 280869 feet.] Meters. Feet. Feet. Meters. I .... 3-2808 I o'miS 2 6"s6i? 2 0-6096 Q*842S T.. . O'QI44 A IVI2"^ 4 1-2192 e 16*4042 5 I "5240 6 iQ'68so 6 1-8288 7 22*9658 7. . 2'IT, T,6 8 26*2467 8 2*4 / ";84 2Q""~i27"*; q. . 2 7432 Relief. There are two methods of representing it^by hill shading and by contours. Hill shading is generally effected by a system of lines, called hachures, drawn in the direction of the slope. When it is steep, the hachures are thick and closely spaced. On the other hand,, a gentle incline will be indicated by fine lines widely separated. S. Doc. 48 29 450 COAST AND GEODETIC SURVEY REPORT, 1897-98. Contours* or horizontal curves are the outlines of horizontal sections of ground at different elevations with designated equal intervals between their planes, delineated in their true positions relatively to each other and to the rest of the map, and conforming to the scale of the map itself; or, briefly, a contour is the curve produced by the intersec- tion of the horizontal plane with the surface of the ground. They may also be described as imaginary shore lines formed at stated or regular elevations, by water which is sup- posed to rise successively to these elevations over the face of the country. Profile. As each curve has equal vertical ordinates at all points, the elevation or profile of a hill, as well as a model in relief, can be constructed from the map, when it is accurately executed on a large scale, without further field measurements. A profile of a hill is the outline or trace formed with its surface by a vertical plane cutting the hill in any direction. PI. ii shows the profile through the line A' B' of the hill H, as represented on a topographical map. The full parallel lines upon the profile represent the successive heights or sections of the hill of 20 feet, and the broken or intermediate lines x x x those of 10 feet. A reference to the letters of the diagram is all that is necessary to a full understanding of the subject: a is the shore line or high-water line upon the map, xxx are the auxiliary lo-foot curves; f the coincidence of curves upon the chart at the perpendicular face of the hill f , upon the section. This is the only case where contours of different heights rtm into each other upon a topographic plan. D' D' are depressions in the face of the hill, represented on the profile by D D. d' is a barranca or dry broken gulley, and c' c' a water course. It will be plain that if we were to suppose the water to rise to a height of 20 feet above the high- water line, to h on the profile, the 2ofoot curve upon the map would become the shore line and the depression D' would fill up and become a pond of water; and if the water were to rise to a height of 30 feet, the dotted broken line would form a shore line, and the knoll G would become an island. Advantages and disadvantages of hill shading and contours. -In a mountainous country the method of hill shading presents a picture which expresses more forcibly to the eye the configuration of the country than a system of contours. But the objection to its sole use arises from the fact that although one ridge is perceived to be higher than another, there is ho guide for stating in terms of some linear unit this difference in ele- vation. It also obscures the symbols representing other details on the surface. A system of contours furnishes a convenient means for obtaining the heights on any part of a map, but does not adapt itself to the representation of the small but important accidents of the ground, such as gullies, ledges, rocks, etc. ; nor does it satisfactorily delineate such features as cliffs, bluffs, quarries, railroad cuts, and embankments. For these reasons the United States Coast and Geodetic Survey has adopted both methods; employing hachures for the smaller features and where the steepness of the slope would make the contour lines approach together so closely that individual lines would become indistinguishable, and relying on the contours to delineate less precipitous ground. The two systems can be seen combined when it is necessary to indicate a rocky and broken mountain face. (Pis. 12 and 13.) *For interesting articles on the diagrammatic properties of the contour line see: On Contour and Slope Lines, Cayley, London & Ed. Mag., 1859, pp. 264-268; On Hills and Dales, Clerk Maxwell, ibid, 1870, pp. 421-426; Properties of Matter, Tait, 1890, pp. 70-81. Coast caul Geodetic Survey Report 2897 38 Appendix 8 Moll iUustraJtiruj Phjf, mode of conjstruuctinq J*rafiLe from APPENDIX NO. 8. PLANE TABLE MANUAL. 451 The contour interval customarily used on the Coast and Geodetic Survey field sheets is 20 feet. When, however, the contour runs very near to some remarkable accident of ground, as a prominent spur or indentation, a slight deviation above or below its true plane is admissible to include this feature, although it is preferable to avoid doing so, if possible, by the introduction of an auxiliary curve. In abruptly mountainous and comparatively inaccessible regions, where sketching must be relied upon, loo-foot curves may suffice to develop all necessary features. Datum plane. Probably the best plane of reference for heights of points on the earth's surface is the mean level of the sea, since the mean of the rise and fall of the tides is approximately this level. In practice, however, mean high water is usually taken, as it includes all land not covered by the tide range, and is the line dividing land and water. Reference signal. It is advisable in commencing the survey of a region bordering on tide water to locate one or more signals at the assumed high-water line, carefully noting the height of the top of the flag above the same, to be used for observing angles of depression upon for heights from points occupied during the progress of the graphic triangulation. As the heights of other points are determined in the course of the survey and verified from observations from two or three other points, these in turn may be used for the same purpose. Regular and irregular methods of contouring . The two methods of surveying curves of equal elevation are known as the Regular and the Irregular methods. The Regular methods include 1 . Surveying and leveling the skeleton and its traverses. 2. Surveying and leveling the profile lines. "' 3. Surveying and leveling the base of each level section. 4. Surveying and leveling the parts of several level sections from one station. 5. The division of the terrene into squares, triangles, or, parallelograms. The profile is a traverse line on which are determined the heights of the points at which the surface changes slope. The points where this line is intersected by the suc- cessive level equidistances are with the level and rod easily determinable.' To determine the base of each level section the table is set up in position where this level intersects the profile, and using the alidade as a leveling instrument, with the tar- get fixed on the staff at the height of the optical axis of the telescope, the line is traced by locating the rod by successive steps at characteristic points of the terrene, when the target comes in the horizontal plane of the optical axis, direction and distance of the rod being determined and drawn in each case. A line drawn through these points, recognizing features between the stations, locates the curve. In this operation allow- ance should be made for curvature and refraction, when the distance becomes sufficiently great to make it a factor. When parts of several level sections are run from one station, set up the table at a point in an equidistance curve, and observe on the staff the height of the optical axis of the alidade. Set the' target on the staff above this height as many equidistauces as its length will include. The aid carries the staff below the instrument and is signaled to stop when the target comes in the horizontal plane of the optical axis, and at successive steps traverses the lower curve. The target is then lowered on the staff one equidistance and the next curve above is in the same manner traced, continuing the proceeding until the level of the instrument is reached, when the table is moved to an upper station and the proceeding continued until the summit is reached. 452 COAST AND GEODETIC SURVEY REPORT, 1897-98. By the mode of regular division of the surface into squares, triangles, or parallelo- grams, pegs are driven at regular intervals, and their heights determined by level in the way that may be most convenient, a spirit-leveling instrument being the most accurate. The Irregular method consists in determining the positions and heights of a number of characteristic points of the terrene, and in determining from these the traces of the curves. This is the method generally used in surveys embracing such areas as the sheets of the Coast and Geodetic Survey on scales of T ^oo anc * 2 ffffo fl- it has the merit that the development of the terrene proceeds with the survey of the skeleton, and does not necessitate a return to a station when once occupied. In connection with the determination of position by resection it works harmoniously and economically, since points that would be selected for position as having the best outlook are likely to be the characteristic ones of the terrene. Station routine. The topographer having determined his position on the sheet, and also the height of the instrument, he proceeds to map the natural and artificial details of the area surrounding the station. For this purpose the direction of each detail is obtained by pointing the telescope upon it, the edge of the rule cutting the station point; its distance by reading the stadia rod held there for -the purpose. This distance is then taken off the metal scale with a pair of dividers and plotted along the edge of the rule. While this is in progress, the alidade is used both as a level for the observation of objects of the same height as the instrument, and for measuring angles of elevation and depression to such of the plotted details, whose position at critical points of the contours would materially assist the topographer in tracing them. Number of elevations to be determined. No rule can be laid down as to the number of elevations that should in this manner be determined from each plane table station or for a given area. It will depend on the skill of the topographer and the modeling of the ground. The number will be adequate when he is confident of tracing by their aid the contours with an accuracy sufficient for the scale and the purpose of the survey. It would indicate careless and slovenly work if the contours were found on exam- ination to deviate frequently from their true position on the sheet by more than half an interval for a slope of less than 5 in an open country. When the slope is steeper, or in wooded regions, a greater latitude is permissible, but even here in representing the crests of ridges, prominent hill tops, and valley floors, this limit of half an interval should not be departed from for good work.* Contour sketching. The topographer will be assisted in sketching contours, where the modeling is intricate, by lightly drawing a skeleton composed of the ridge lines and thalweg lines (lowest lines of valleys) in their proper positions around the station. On the ridge lines will be found the extreme outward or convex bends of the contours, and on the thalweg lines the extreme inward or concave bends. It can be readily imagined that if each spur and. each small depression was repre- sented by its appropriate line, and on each of them were located, either by observation * For some pertinent remarks on this subject see Bulletin of the University of Wisconsin, Eng. Series, Vol. i, No. 10, Topographical Surveys. Their methods and values, J. F. Van Ornum, pp. 360-361. Coast and. Geodetic Survey Report 1897-9H. Appendix 8. No.12 Crest. Frwc cine/. TaJus of a Gr-u,ru,i,e COW. (Eagljt* Cliff, Mt.. Desert I.) . PNOTO-LITMO WASHINGTON. 2 te' G 'or 9J Coast and Geodetic Survey Report 189 7 '98 Appendix 8. No. 14 Fig Fig. 22 Fig. 23 Fig. 25 Fig. 24 i. 27 Typical Contour Groups APPENDIX NO. 8. PLANE TABLE MANUAL. 453 or estimation, points having elevations equal to some multiple of the contour interval, it would be .only necessary to connect those points having the same elevation with a smooth curve to have a correct plan of the contours. It will simplify the sketching at a station to draw the highest, lowest, and middle contours first, as they will then serve as guides for estimating the position of the others. It should be remembered that a contour never splits, as shown in PI. 14, fig. 20; nor do two contours run into one, as shown at fig. 21 ; nor cross each other, except in the rare instance of an overhanging cliff, as shown in fig. 22. When an auxiliary contour is introduced, no more of it is drawn than is sufficient to delineate the special feature which makes it necessary. A principal contour, on the other hand, can not have an end within the map ; if it commences at one edge it must terminate at another. Typical contour groups. A closed contour encircled by one or more closed contours is either a hill, as shown in fig. 23, PI. 14, or a depression, as shown at fig. 24; the arrows showing the direction in which water would run. The summits of all the hills of importance should have their elevations determined and marked on the map. All depressions without an outlet and which do not contain a pond or lake should be marked with a D at their lowest point. A series of contours, as shown in fig. 25, is either a croupe (the end of a ridge or promontory) or a valley. If a croupe, the contours will have their concave sides toward the higher ground ; if a valley, the contours will have their concave sides toward the lower ground. A combination of four sets, like fig. 26, with convex sides turned toward each other, represents a dip in a ridge, or the junction of two ridges, and is called a saddle. A pass in a mountain range generally takes the form shown in fig. 27. Order of development of contours. As the progress of topographical work is usually from the shore line inward, this affords the most favorable direction for drawing the curves of equal elevation, and as it is desirable that all work at a station shall be com- pleted when it is first occupied, so as to avoid the necessity of returning to it, the curves should be drawn by the eye from the shore-line to the points sighted and determined for position and height, to be checked by reverse drawing from those points when in turn occupied; and so from station to station, drawing to and from in reverse will check and verify, between stations not too far apart, such comparatively small errors of position as an accurate eye will soon learn to estimate. The heights of sufficiently close points must be determined to guard against any wide range of estimate of height by the eye. In abrupt slopes of considerable extent the use of a pocket clinometer is of much value to determine the degree of slope, and to draw accordingly the curves by the widths of their zones (the cosines of angles of slope) from'a paper scale prepared for the pur- pose. (See PI. 30.) Filling in. Having completed the work at a given station, the topographer proceeds with his party and instruments to an adjoining locality, where he selects a new station from which he can gather the details of an area bordering upon the one last surveyed. In this manner, by successively occupying stations over the whole expanse of the sheet, the skeleton map is filled in. Traverse lines. In a wooded country, where it is impossible to find open space with range sufficient to see enough points for determination of position by resection, it is 454 COAST AND GEODETIC SURVEY REPORT, 1897-96. necessary to run traverses along the roads, with offsets to such lateral features as it may be practicable to reach without the expenditure of excessive labor and time in opening lines of sight. The levels, when necessary, are carried along with the line by observing the vertical angles with the alidade upon some mark on the rod, taking back and fore sights at alternate stations. Main traverse. The standard table is used on main roads and whenever the details are important and numerous. The traverse line is started by occupying some point previously determined and sending the telemeter rod ahead to a place selected for its advantageous position, in reference either to the surrounding features, or facility in obtaining a new section of the traverse. Having sighted to this point, read and plotted the distance, short guide lines should be drawn along the edge of the ruler at both ends and numbered or lettered, so they may be identified from others of like character. The table is then moved to the forward station, approximately oriented with the eye, and the plotted point carefully plumbed over the one on the ground. The alidade is now placed on the table, and the table oriented by bringing the edge of the ruler close up o the guide lines; then revolving the table until the vertical wire bisects the rod or signal left for that purpose at the last station. The same processes which were employed at the initial station are now repeated; the detail mapped and the new station in advance occupied in turn; the line progressing in this manner by successive steps. In running traverses, great care should be taken to sight as low as possible upon the fore and back signals, so as to avoid any error of deflection which might arise from the inclination of the signal poles. Subordinate traverse. When the line is unimportant and few features present them- selves to be noted, an auxiliary plane-table oriented by a declinatoire or a transit, fitted with stadia wires, may be employed. When this method is pursued with a second table the forward rod station is not occupied, but another is chosen in advance of it, from which it can be .seen where the instrument is set up and oriented with the declinatoire. Sighting the alidade to what is now the back station, the distance is read and plotted along the edge of the ruler, and the point so determined represents the one occupied by the table. The pivot on which the declinatoire needle rests should be examined frequently, the least roughness as to its point will cause the needle to drag and introduces serious de- flections in azimuth. All traverse lines should start and end at well-determined points. This will serve to check the accuracy of the work. . If the closing error is not too large, the line should be adjusted by distributing it throughout its length. The line is run on a spare sheet when an auxiliary table is used; then traced, "swung in," and adjusted between the two fixed points. Determinations for hydrography. Where the topography surveyed includes the shore- line of a body of water, the hydrographic survey of which is intended to follow the topo- graphical work, as in the Coast Survey, it is the duty of the topographer to locate and determine the shore signals, and it is only necessary to state that they should be so placed APPENDIX NO. 8. PLANE TABLE MANUAL. 455 as to furnish the hydrographic party \vith as many points as is desirable for the deter- mination of positions on the water. Natural or artificial objects along the shore, or in plain sight from the water, such as fence ends, rocks, prominent houses, etc. , should be determined and marked upon the sheet. Lines to buoys and other permanent floating objects should be, as far as practicable, taken at the same stage of the tide, or direction of current. The delineation of the ordinary mean low- water mark should be aimed at, and when it is beyond the reach of the plane-table, and presents no marked points for determination, or is of a character that will not admit of putting up and working the instrument as along the swampy shores of the south, where the muddy shoals extend far seaward, and among the shifting quicksands of our great estuaries and bays it may be left to be traced by the soundings and tidal reductions of the hydrographic parties. The channels through mud flats of this character should be indicated, however, if only approximately, by cuts and tangents, or the determination of stakes at the turning points. Where the fall of the tide exposes rocks and ledges, shingle beaches, etc., their character and extent should be delineated and distinguished from the sandy beaches, as these are features most diffi- cult and laborious for the hydrographic survey to represent. High-water and storm-water line. In tracing the shore line on an exposed sandy coast care should be taken to discriminate between the average high-water line and the storm- water line. Determination of inaccessible points . On a precipitous coast, \vhere the shore line is inaccessible and can not be determined by ordinary methods, the salient features are located, when occupying commanding stations, by observing the vertical angles upon them, and drawing direction lines to them. Then using the elevation of each station as a base the distance to each feature is computed and plotted. The same method applies to outlying rocks, and is often employed where there is any doubt of their being identified from different places. Large scale surveys. As has been previously stated, i-io ooo and 1-20 ooo are the scales customarily used in the execution of the topographical work of the United States Coast and Geodetic Survey, as they are the ones best suited for the charting of the coast line and harbors of the United States. Other surveys for special purposes have been made from time to time on scales both larger and smaller, and the field practice has been modified according to the require- ments of the scale used. A topographical survey of the District of Columbia outside the thickly populated limits of the City of Washington was made between the years of 1880 and 1891 on a scale of 1-4 800. The methods pursued are here described, as they are typical of other surveys on a large scale. Based on a sufficiently minute triangulation, the plane-table and stadia, wye level, and rod were used for all determinations of details. The relief was elaborately indicated by contour intervals of 5 feet. The datum plane is the same as used by the engineer department of the District, on which is based all the levels used for grades of streets and sewers in the city of Washington, the survey being made for the purpose of extending streets and avenues beyond the city limits. 456 COAST AND GEODETIC SURVEY REPORT, 1897-98. From this datum, along all roads, avenues, and railroads, and where roads were infrequent, across country, lines of level were run, and after careful checking in the usual manner bench marks were placed in position convenient to all parts of the field. The plane-table stations were established so as to easily overlook every part of the field and so close together that each was surrounded by the others within the range of a single reading of the stadia rod. The mode of procedure was as follows: The plane-table was placed in position by a graphic solution of the three-point problem. At the same time the height of the level was determined above some near bench mark and the target of the level rod fixed, so that when it was in the line of sight of the level the bottom of the rod would rest on the ground where the elevation corre- sponded to that of some contour. The level rodsman then began his journey along this imaginary horizontal line, holding the rod for the observation of the levelmau at each noticeable change in the configuration of the ground. The levelman directed the rods- man by signals at each point until the rod was in position on the contour line, when the stadia rod was substituted and its distance read and plotted on the plane-table sheet. Both rodsmen followed the contour line in both directions from the table as far as the stadia rod could be conveniently read. Generally two and sometimes three contours were run from one level station, and on their completion a turning point was fixed and the level. shifted to a higher or lower ground, as the circumstances required. A survey of Craney Island, Virginia, was made in the same manner on a scale of i-i 200. RAPID SURVEYS. Military reconnaissance. In almost every field of operations, from the commence- ment of the civil war to its. close, the plane-table was used. Until this time very little was known, save in theory, of the value of the plane table as a reconnoitering instrument, and it is the testimony of all the officers of these parties, as the result of their labors, that for rapidity and accuracy in the execution of military reconnoissance it is more effective than any other instrument. The usual system adopted, in default of triangulation, was the measurement of a base with an ordinary chain and triangulating with the plane table. In detailed surveys for the Army, where a topographer averages from i to 3 square miles a day, on large scales a chained base of from one-half to three-quarters of a mile for the survey of an area of 25 square miles is found sufficient. At Chattanooga, from two different bases of about half a mile each, platted on separate sheets, and measured once carefully with the common 2o-meter chain, the same chain being used -for both measurements, after considerable intermediate plane- table triangulation carried on by two officers, two objects were determined 2}^ miles apart, common to both sheets, which were on a scale of 75500. an d the discrepancy was but about 15 meters. Many other points of junction indicated this to be the maximum error. In this case the leaves were mostly off the trees and the hills afforded good points. The sheets covered about 20 square miles each. At Nashville there was a discrepancy of about 10 meters in 2 miles. At other times, when the character of the country or the pressure of time did not admit of the measurement of a preliminary base and topographical triangulation, the APPENDIX NO. 8. PLANE TABLE MANUAL,. 457 work was commenced by starting from a single point and prosecuted by linear measure- ment with the chain or stadia, intersections from the ends of the chained lines being taken to determine objects, which, as the work progressed, could also be used as checks upon tlie chaining. Where circumstances permitted, an occasional return with the chain to a back point, either to close a series of lines upon it or to start afresh, was resorted to. This work was generally carried on over roads and the interior filled in by sketching and intersection as far as practicable. Some of the tests in this latter work, where the operations of two officers joined, were remarkably close. A very efficient topographical officer estimates that with the usual number of hands and a good sketcher for aid, in a country of average variety of detail, in which all the houses, prominent barns and outbuildings, streams, roads, general outline of woods, and approximate curves are to be shown, on a scale of TT^TTO. an area of between 2 and 3 square miles can be filled in daily, with not only sufficient accuracy for military pur- poses, but so that an accustomed eye with ' ' the map in hand would not discover any marked discrepancy." This rapidity of work, however, could not be expected in or near towns or populous districts. It is doubtful if the average work would reach more than one-half this amount. In some thickly wooded sections and where time is limited, it has been found advisable to run the main roads with the plane table and fill up with the compass, which is more rapid but less accurate than where the entire work is done with the plane table alone. The usual method employed where these methods were combined, was as follows: Where the army was stationary, or moving leisurally, one main road was run with the plane table, the operator being accompanied by assistants well practiced in the use of the compass. Upon arriving at any important road or water course an assistant 'was sent to the right and left, starting from a plane-table point, determined by the chaining, and running as far as was requisite and then returning to the main road again to repeat the operation, the compass notes, of course, being kept in a book prepared for the pur- pose. Prominent points determined by the plane table were used as checks in the compass work. The intervening topography, where no compass or plane-table work had been done, was sketched in by the chief of the party, in which accurate pacing became of great value. With compass and notebook. Plane-table methods can be utilized to advantage when compass, pencil, notebook, and ruler are the substitutes for an instrumental outfit. The book serves as the sheet and board combined, and the ruler, as it was in the early days of the art, becomes the alidade.* Photogrammetry '.f In the topographical reconnoissance made for the Alaska Boundary Survey by the Coast and Geodetic Survey, the camera with constant focal length has been used as an adjunct to the small mountain plane table. The latter was used to plat the shore line and adjacent topography, also to determine as many peaks of the interior country as possible by the intersection of lines of direction. All camera stations were determined geographically and hypsometrically, and platted upon the * See "Sketching without instruments," in Topography, Drawing, and Sketching, by Lieut. Henry A. Reed, U. S. A., 1886. t See United States Coast and Geodetic Survey Report, 1893, App., 3, and Report for 1897. Photo-Topographic Methods and Instruments, Flemer. 458 COAST AND GEODETIC SURVEY REPORT, 1897-98. plane-table sheet. The topographical details beyond the reach of the plane table were added to the map in the Office by the photogrammetric methods. The rugged mountains of southeast Alaska appear particularly well adapted for this mode of procedure, as identical points can be readily picked out from different pan- orama views, owing to the characteristic shapes of the mountain peaks, snow fields, glaciers, etc. Periods of fair weather are also very short and of rare occurrence in that locality, and a great deal of topographical material can be gathered photographically in a short time, which when platted will cover a large territory jf a sufficient number of reference points on the views have been located instrumentally. The platting proper can be carried out to any degree of minuteness and detail; the only requirement is that a sufficient number of camera stations shall have been occupied to fully cover the territory in question, so that every topographical feature of prominence has been seen or photographed from at least two stations better more. By this application of photogrammetry the plane-table methods of determining topographical details are extended to the Office, inasmuch as the same features are selected from the panorama views and platted geographically which would have been located by the plane table. But the actual time .spent in the field is reduced at the expense of the time needed for office work. Survey in advance of triangulation. Where it is necessary to make a topographical survey in advance of the determination of points by triangulation, a reconnoissance is first made for the location of a base line and selection of points to be determined with the plane table. The base is measured with sufficient accuracy and conveniently, with a steel tape which has been compared with a standard at a fixed tension, and to one end of which is attached a spring balance to secure the same tension during measurement. The suc- cessive lengths are marked by lines cut on copper tacks driven in wooden stubs firmly set in the ground. The temperature is noted at frequent intervals as the work progresses, and the corrections are applied to the length of the base when completed. The base .is then properly located on the sheet in reference to the area to be embraced and its length carefully set off. It is well at the same" time to mark in three or four different parts of the sheet lengths of i ooo meters for the purpose of determin- ing at any time the true scale of the sheet variable by the different hygrometric condi- tions of the atmosphere. Signals having been erected at the selected points, the extremes of the base are occupied with the table and the points, as far as maybe reached with good intersections, determined from them and lines of direction drawn to all the points visible, to serve as checks upon their determination from other points furnishing directions for good inter- sections. The survey then proceeds as usual. It is well at the beginning of work to set off with the declinatoire at some deter- mined point near the middle of the sheet the magnetic meridian, for the purpose of put- ting the table in approximate position at any station with the declinatoire. The manner of doing this is elsewhere described. Before finishing the field work it is important, when the sheet has no projection, to provide data for drawing a true north and south line. This is done by drawing from a point upon the sheet, when the table is in position, a line in the vertical plane through APPENDIX NO. 8. PLANK TATU.K MANUAL. 459 Polaris and the point occupied and recording the time of observation. The a/.hmith of the star at that time being known, a true north and south line can accordingly be set off. If a. small transit instrument is at hand and carefully adjusted for movement in vertical plane, an assistant with a lantern can be located where the vertical plane through Polaris and the point occupied intersects the ground, at as great a distance from the point as the ground will admit of within the limit of communication by light signals. When the assistant is in position, a stake is there driven, the direction to which from the point occupied may be determined by daylight. If, in the absence of a transit, the alidade has not vertical range sufficient to reach Polaris, ail illuminated plumb line may be used for the alignment. OFFICK WORK. All the drawing of the topographical features of a siirve)^ upon the chart should be penciled in the field, while they are still under the eye. Sketching and plotting in the office from notes, unless the country be near at hand for ready reference in case of doubt or a defective sketch, is objectionable. Where this is unavoidable, the sketch should be transferred to the sheet as soon as possible after having been taken, while it is fresh in the mind of the person by whom it was made, and by whom, also, if possible, it should be plotted. Days which, from inclemency of the weather, are unfavorable for out-of- door work should be allotted to this purpose, and advantage should be taken of them, also, for retouching any details of the sheet which may have become indistinct, as it is very important that they should not be left indefinite or become obliterated; for when the inking is done, as it generally is, at a distance from the field of operations, the necessity for this care is obvious. Nos. 4 and 5 pencils are good for this purpose, for which very hard or very soft and black pencils are equally unsuited. In the inking of a topographical sheet three requisites to its proper appearance when finished should be borne in mind: clearness, neatness, and uniformity. The lines and objects should be clean and sharply defined, nothing being left obscure or doubtful; the paper should be kept unsoiled, and erasures avoided as far as possible, and the style and strength of the drawing should be the same throughout. It is an important matter that an easy and natural appearance should be given to the map, for, as before remarked , a mere rigid adherence to conventional signs is not all that is neces- sary; while there should be no deviation in this respect, at the same time the drafts- man should strive to represent the country. There is a great difference with regard to this among topographers. Two perfectly correct charts of the same section of ground, executed by different persons, may be inked, and while one will have a stiff and ungraceful look, the other will appear artistic and natural, giving at once the impression of a sincere representation of the country surveyed. Office work should not be commenced until the topography is entirely completed, as no inked or partially inked chart should ever be used in the field. Sometimes, for the special examination of old work, or for the insertion of some recent artificial or natural changes, this becomes necessary. There is always a risk of injuring an iuked map by exposure to the weather or by using it upon a plane table. The inking should commence with the shore lines, high and low water. The high water, or shore line proper, should, in all cases, l>e full and black, the heaviest lines on 460 COAST AND GEODETIC SURVEY REPORT, 1897-98. the sheet, and in this, as in all the rest of the ink work, the lines of the survey should be strictly adhered to. The topography as drawn in the field is supposed to be correct when the chart is finished, and no office amendments or changes are admissible. The low-water line is next drawn, not so full as the former, but clear, black, and uniform, consisting of a dotted line for sand and mud and the conventional sign where it is formed by shells, rocks, or coral reefs. Grass upon flats, or shoals covered at high tide, have no distinct continuous line to mark their limits, each being represented in its proper form and within its area by its conventional sign only, but the shape should be well and correctly defined. All objects between high and low water, covered at full tide, should be represented less boldly than the rest of the map, but not faintly or indefinitely. The roads should next be inked plainly and evenly, and their sides parallel, except where the survey shows a deviation from the general width. Main thoroughfares when fenced are drawn with a full line, subordinate roads where fenced should be shown by the usual sign, and where there is no inclosure a line of dashes should indicate the road- side, and then should follow the fences and houses. In drawing the latter care must be taken that the corners and angles exhibit a sharp, clear outline, which adds much to the appearance of the map. The general skeleton of the survey being now completed, the contours are drawn with a bold, uniform, plain red line, without break, over all the other work, following accurately the full range of level of each of the contours on the sheet. After this comes the general filling in, by conventional signs, of sand, marsh, grass, cultivation, orchards, rocks, hachures, etc. Some practice is needed to execute the sand work regularly and neatly. It should never be hurriedly done, though of course rapidity in this respect follows practice. The lines representing marsh, and the delinea- tion of grass on the fast land, should always run in the same direction over the whole sheet and be parallel to the top of the sheet and the title. The appended drawings, Pis. 15, 1 6, 17, and 18, give the conventional signs as adopted by and now used in the Coast Survey. The most difficult part of the inking for a beginner is the lettering, which now follows, and for which samples are given (PI. 19). It is expected that every topographer shall have learned to draw sufficiently well to ink his sheet in a clear and distinct manner and letter it with some regard to neatness and graphic effect, as the appearance of an otherwise well-inked sheet is sometimes marred by careless or indifferent lettering. The location of the names upon the sheet should be such as not to cover or obliterate any detail or feature of the survey, and the letters should be put in neatly and grace- fully, and in point of size and form according to the specimens furnished. The title should finally follow, with such notes as may be necessary to explain any peculiarity of the sheet or survey. This title and lettering should, as far as practicable, be so placed, that when the sheet is held with the top (usually the north or east end of the map) from you it can be easily read; in other words, as nearly parallel to the top or upper end of the sheet as the nature of the work will admit. All names well established and recog- nized in a neighborhood, both general and local, should be collected during the survey, and their correct orthography ascertained, and in case of any doubtful or disputed orthography a report should be given of any traditions or any authorities which may APPENDIX NO. 8. PLANE TABLE MANUAL. 461 bear upon the subject. No illuminated or German text, old English, or what is known as "fancy printing," should be indulged in, a strict adherence to simplicity being the greatest ornament. The minutes of the parallels of latitude and meridians of longitude should be marked in figures at the upper and right-hand ends, respectively, the degrees on the center parallel and center meridian only. Where the .buoys are determined by the topographer, and their names, colors, numbers, or kind are known, they should be lettered upon the map. The triangulation points should also be lettered, first being surrounded by a small red triangle. Barns, houses, prominent trees, and other objects determined by the plane-table that may be used as points of reference in making additions to the sheet subsequent to the survey should be indicated by a small blue circle. NOTE. Plates No. 20 to No. 29 were selected from a collection prepared by the late Assist- ant E. Hergesheimer to illustrate the topographical features in various portions of the United States. The full collection is to be found in Appendix 14, United States Coast and Geodetic Survey Report, 1883. 462 Cs>a.tt aiui Geodetic Survey Report 1897-98 Appendix 8. Shoreline Low Water Oak Rocky Ledgea Deciduous and, Undergwwth Pine SrvdedlBarik. JPdbnetto Sand and Shingle Sand-Dunea Cacti, Topograpliical Symbols Cuaxt caul. Geodetic Survey Report 1897-38 Appendix 8. No. 16. Salt Marsh Cypress Swamp Salt Pond Grass t'r&sh Marsh and fresh .Pond, Orchard Oyster -Bed Ricelhkes M ^ H ^^^. t 2 2 ^^ ^^ ^ L ^ U * o> to " * * O (^ J o ft. o 5 5 t> ^ ENGRAV DIVISION CAPITOL HARBOR ISLANDS RIVERS SOUNDINGS DIRECTIONS *oints POINTS 'reeks CREEKS bannels CHANNELS loals SHOALS PQRSTU ^PQRSTU B >"S CO ^) 55 O P H tf HOC J fc^j U M W \ " &H ^H H Q U ffi HI h- 1 2 cc ^^ M Q h e 8 3 H ^ S*^ 2 o H (O in in o in in 3 in o CO m O irt c*i o f\j -< rH -4 Q ^ g9 O CD ffl ? O s^ 1 a f UNIVERSITY V. a: iC P Coa.it and. Geodi*Lc Survey Report 1897 98 AppetvdLx 8. . 20 'f^':*v ? 't*V/^r* 8 *~*.v Sparsely settled, Tort,SaltMarsh^Pcn Woods ^Dttchjes.Fenoes, and. TTrideftned. Roajds( Brrcnswvck, Coast and Geodetic Survey Re-port 1837-98 Appendix 8. No. 21 Rtairoeulfi,GinaJs,fnmi liridf/rs, Rocky C&ffs,jfutrirer drift, Waters-worn rncks Mixed woods over hULcm-vcs. f'mint Hint <;,;,'"'< HW7-3H Appendix 8. No. drift, ba,n,k,s. with, bou,lder-Si set free; a*ui scrub (lecidu-oiis woods. (Guy HeaudL) No. 25 -Ui' Slit v,'Y Ht'fMfl HW7 '.' ,1/y>mch with], vw Dnru?s, FrcstiWater Pond* Meadow Gi-(iss, Sr.ir/e Brush and. A (S. Coast. Coast and Geodetic Survey Report. 1837-V8 Appetuli.v 8 Blocking of' Cities. -Lfi7- BiiUdings, SvJncrbcm Vittas an,d Grounds. Fresh Marsh- <-?fei-vporL,ft.I.> unit dcinli-itr Stirvry ll,-fH,rt Ift'.i? ,98 AfipfiuUx. 8. No. 2 7 V-o.s ;. r 1 to 57 57. 29 1145 . 8 349.1 2* 1 to 29 28.64 572.8 174. 5 3* 1 to 19 19.06 381.6 116. 3 4* 1 to 14 14. 30 286.0 87. 1 5 1 toll 11.43 228.6 69.7 10T 1 to 6 5.67 113.4 34.6 1ST 1 to 4 3.73 74.6 22. 7 20' 1 to 3 2 .75 55.0 16.7 25* 1 to 2 2 . 14 42.8 13.1 30* 1 to 1.7 1.73 34.6 10. 5 35 1 to 1.4 1.43 28.6 8 .7 40* 1 to 1.2 1 .19 23.8 7. 2 45' 1 to 1 1 .00 20.0 6.1 50* 1 to 0.8 0.84 16.8 5.1 55' 1 to 0.7 0.70 14. 4.3 60* 1 to 0.6 0. 58 11.6 3.6 UJ :r c/) LJ DC ID (f) 6 Q CO V) a < o Coast and Geodetic Siu-vey Report lftV7-<18 Appendix 8. THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO 5O CENTS ON THE FOURTH DAY AND TO $1.OO ON THE SEVENTH DAY OVERDUE. 27 FEB 17 1943 LD 21-50w-l/33 <:>". .''I