24 



HYDROGRAPHIC MANUAL 



However, there is a sequence of events which 

 is fundamental to successful operation. 



First, the adequacy of the established tri- 

 angulation should be evaluated and a pre- 

 liminary reconnaissance should be made if 

 additional control is required. If photogram- 

 metric methods are to be employed for 

 locating hydrographic stations, the character 

 of the photogrammetric coverage should be 

 studied and plans laid for this part of the 

 work which will assure the availability of 

 advance or final manuscripts when they are 

 needed for hydrography. Whether signals 

 are to be located by planetable or photogram- 

 metrically, they should be built and located 

 sufficiently in advance of the hydrographic 

 party to avoid delays. It is not advisable to 

 build signals in excess of those needed dur- 

 ing the season. 



Nearly all shoreline and topographic map- 

 ping is accomplished by photogrammetric 

 methods. Preliminary manuscripts can be 

 compiled from photographs without prior 

 identification of control or shoreline inspec- 

 tion (see 4-13). This procedure is inefficient 

 and should be avoided since preliminary 

 manuscripts must be recompiled and signals 

 relocated after control has been identified. 

 When photographs are available, field inspec- 

 tion and control identification should be com- 

 pleted one year in advance of hydrographic 

 operations. If there is no established con- 

 trol in the area, or if supplemental control 

 is required, the triangulation surveys and 

 photogrammetric work should be carried on 

 simultaneously. 



2-16 Aerial photographs for operational 

 planning. — Aerial photography and topo- 

 graphic, or planimetric, mapping will usually 

 precede hydrography. Copies of the maps and 

 photographs will be furnished as part of the 

 project data. At times the photographs may 

 be available ahead of the planimetric or 

 topographic maps. In the absence of detailed 

 maps, the photographs will provide a wealth 

 of information for planning operations, and, 

 even when the maps are immediately avail- 

 able, the photographs will provide additional 

 information useful in operational planning. 



The aerial photographs for a project may 



be either nine-lens or single-lens vertical 

 photographs. Nine-lens photographs will 

 have been taken at the anticipated scale of 

 the inshore hydrography and, once taken, 

 the scale of these photographs is fixed; it is 

 not practicable to furnish prints at larger 

 or smaller scales. Single-lens photographs 

 will usually be taken at some scale smaller 

 than the anticipated scale of the inshore 

 hydrography. Ratio prints at the latter scale 

 will be used for map compilation and will be 

 furnished the hydrographic party. When 

 single-lens photographs are used, ratio prints 

 can be furnished in the range of one-half to 

 four times the contact or taking scale. 



The Bureau is making increasing use of 

 infra-red and color photography to supple- 

 ment the panchromatic mapping photog- 

 raphy. This special purpose photography is 

 used in compilation of the topographic or 

 planimetric maps that usually precede hy- 

 drography, and the special photography can 

 be made available to the hydrographic party, 

 if needed. Infra-red photography increases 

 the contrast between land and water and 

 emphasizes the shoreline. When infra-red 

 photography is taken at the proper stage of 

 the tide, that is, for example, at low tide or 

 at high tide, it defines the low water line or 

 the high water line very sharply. These 

 photographs show clearly which features, as, 

 for example, rocks, are bare at the stage of 

 the tide at which they were taken. Infra-red 

 photography also reduces sun spots that oc- 

 casionally obscure some of the shoreline on 

 panchromatic photography. Color photog- 

 raphy has greater depth penetration than 

 panchromatic photography, and is useful in 

 outlining shoals and channel lines that must 

 be developed by the hydrographer. 



Some information can be obtained from a 

 casual examination of the aerial photo- 

 graphs, but a stereoscopic examination is 

 essential if optimum information is to be 

 obtained from them. Nine-lens photographs, 

 or larger ratio prints of single-lens photo- 

 graphs, are somewhat of a problem when 

 using the smaller stereoscopes available in 

 the field. For this reason the photographs 

 should be cut and folded for convenient view- 



