2231 HYDROGRAPHIC MANUAL PaGE 76 



2231. Frequency of Triangulation Stations 



The frequency of second-order triangulation stations needed for the control of 

 third-order triangulation and other control surveys of a lower accuracy cannot be 

 governed by any fixed rule. Visibility, the character of the coastal area, and the 

 practicable distance inshore from the coast at which the inshore side of the scheme can 

 be established to give strong figures, are the main controllmg factors. However, a 

 scheme consisting of stations established so that the lengths of the side lines average 

 about 5 or 6 miles will usually suffice. 



For the control of planetable topography, second- or third-order triangulation 

 stations shall be established along the coastline at intervals not to exceed 2 miles, except 

 in areas of extremely unfavorable terrain where this would be exceedingly difficult and 

 costly. In such cases the spacing may be increased, but shall never exceed 5 miles. 



The control required for air photographic surveys will vary with the type of camera 

 used, the scale of the photography, and the character of the terrain. 



To control surveys made from air photographs taken with the nine-lens camera, 

 recoverable triangulation stations shall be spaced as follows: 



(a) For coastal surveys of comparatively flat areas at a scale of 1:10,000, stations shall be spaced 

 not more than 4 miles apart along the shore, supplemented by inshore stations spaced not more than 

 6 miles apart. 



(6) At intervals of about 8 miles an additional station, or stations, shall be established so as to 

 furnish at least three stations within an area about 4 miles in diameter in order that each fifth photo- 

 graph in the line of flight may be fixed by at least three control points within its limits. 



(c) Approximately two-thirds as many stations must be established along the inland portion of the 

 photographic strip as are provided along the shore. 



(d) In harbors and in areas of special importance, at least 50 percent more control shall be provided 

 than is required elsewhere. 



(e) For a photographic survey on a scale of 1: 20,000, the spacing may be increased to approxi- 

 mately twice that which is required for a 1: 10,000 scale. 



Where the triangulation scheme spans a waterway with one side of the scheme on 

 each shore, the same general spacing of stations, as outlined in (a), (b), and (c), is 

 required, with additional or supplemental stations provided at intervals of 6 to 8 miles 

 along the inland edge of the photographs for a survey on a scale of 1:10,000. The 

 supplemental stations may be intersection or three-point fix stations. 



The nine-lens photographs are 35.4 by 35.4 inches in size, and if the approximate 

 centerline of a flight is known, the limits of the area photographed can be determined on 

 a map or projection of suitable scale. Having determined these limits, the triangulation 

 stations can be more effectively located to control the photographs. The spacing between 

 centers of successive photographs along the line of flight will be 7, 12, or 14 inches, 

 depending on the ruggedncss of the terrain. The spacing between parallel flight lines 

 will be 12 or 14 inches. If there are two or more paraUel flights, the fixed photographs 

 required in (b) should preferably be staggered along adjacent flights. 



That an area is to be photographed for mapping does not exempt the observer of a 

 triangulation unit from obtaining the usual cuts to conspicuous natural and artificial 

 objects. These interesection stations will serve as valuable checks to the photographic 

 plot and may considerably expedite the office compilations. In areas where accurately 

 contoured topographic maps are not available, vertical angles shoidd be measured to 

 determine the elevations of the principal scheme and intersection stations, whenever 

 this is feasible without materially delaying the progress of tlie triangulation party. 



