Page 103 control and signal building 2395 



After the selected point on the photograph has been pricked, its position may be 

 transferred to the printed boat sheet by one of the following methods: 



(1) Method of adjusted distances.— Select two'pairs of points, all four of which can be positively identified on the photograph and 

 the printed boat sheet. The lines connecting each pair should intersect approximately at right angles and fairly close to the new 

 station (see fig. 16). The points themselves should be not far from the station and should be in the same plane (not necessarily level) 

 with it. Measure AB and CD on both the photograph and the printed map in order to establish two proportional relationships. 

 Next measure AX on the photograph, correct the measured distance for the relationship in that direction and swing an arc on the 

 printed map. Then repeat the procedure for CX and the intersection of the arcs on the boat sheet will be the location of the new 

 station. Check by arcs from points B and D and adjust proportionally to distance. 



(2) Paper-strip method. — This method is especially useful for low oblique photographs or photographs whose scales differ consider- 

 ably from the hydrographic sheet. Select four known points (as A, B, C, and D, fig. 17) which can be positively identified on both 

 the photograph and the map or hydrographic sheet, and which are nearly in the same plane (not necessarily level) with the new sta- 

 tion. Weight down a paper strip at a convenient place across the photograph, and with a straightedge mark on the edge of the paper 

 strip where radiating lines from one of the known points (as A), passing through the other known points and through the new station, 

 meet the edge of the paper strip (ab, ac, ad, ax). Then draw lines from point A on the map or sheet through the other known points 

 B, C, and D, continuing them a short distance beyond each point (see fig. 17). Next place the paper strip on the map or sheet and 

 adjust it in position so that the marks on its edge coincide with the corresponding lines on the map or sheet. With the strip in this 

 position draw a line on the map passing through the mark on the paper strip corresponding to the new station (as AX). Repeat this 

 procedure using other known points as origins (as B, C, and D, and making marks 6a, be, bd, bx, etc.), thus locating the new station 

 by intersection. 



(3) Radial-line method. — If overlapping vertical photographs are available and a considerable number of points are to be located, 

 a supplemental radial plot should be made. A piece of tracing paper is placed over one of the photographs and fine lines are drawn 

 radiating from the center (principal point) of the photograph through the images of points shown on the sheet and through the new 

 stations whose locations are desired. The tracing is then oriented on the sheet by moving it about until the radial lines corresponding 

 to the known points pass through them. The radial lines to objects whose positions are desired are then transferred to the sheet. This 

 procedure is repeated using the other overlapping photographs. The intersection of the lines to each object, radiating from the centers 

 of the several photographs, is the new position desired. 



(4) Range method. — It should be kept in mind that any perspective of a straight line is a straight line. Any three objects which are 

 in line with each other in nature (not necessarily at the same height) should be in line on any photograph whether vertical or oblique, 

 and on any map projection used for surveying. Full use of this principle to locate stations or check the accuracy of the surveys should 

 be made whenever practicable. 



2395. Graphic Control Survey 



If photographic survey data are not available, or if temporary marks for the hydro- 

 graphic signals are missing throughout an extensive area, a graphic control survey by 

 planetable (see 237) may be necessary to locate signals for the hydrography. If the 

 project instructions do not specify the amount of shoreline or other topographic detail 

 to be mapped under such conditions, supplemental instructions should be requested. 



24. SHORE HYDROGRAPHIC STATIONS 



A hydrographic station is a definite point on the surface of the earth whose geo- 

 graphic position has been determined by methods ordinarily used only in hydrographic 

 surveys and with an accuracy less than third-order and often less than that of a topo- 

 graphic station. Hydrographic stations on shore are generally located by sextant 

 observations. 



241. General Statement 



The shore stations used to control a hydrographic survey are ordinarily located 

 by triangulation and topography. One of the purposes of the topographic survey 

 along the coast is to locate the control for the inshore hydrographic survey. It is only 

 occasionally to supplement this control located by triangulation and topography that 

 the hydrographer needs to determine the positions of stations by hydrographic methods. 



From the hydrographer 's offshore viewpoint a natural or artificial object unnoticed 

 by the topographer may appear particularly desirable for use in controlling the hydro- 

 graphic survey, and if the topographic survey has been completed it is only by hydro- 

 graphic methods that its position can be conveniently determined for such use. 



In general, hydrographic stations shall not be used as control from which to deter- 

 mine the positions of other hydrographic stations. 



