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the charts present a general idea of the area, the representation of specific details may 

 be considerably distorted. 



1332. Air Photographs for Study of Project 



Good air photographs will furnish extraordinarily detailed information, thus supple- 

 menting prior surveys and reports, particularly when those using them are experi- 

 enced in the interpretation of photographs and acquainted with similar types of terrain. 

 The photographs available may vary from a few nonstereoscopic obliques, taken casually 

 during flights over the area for other purposes, to complete stereoscopic coverage by 

 vertical photographs taken for plotting on the same scale as the intended hydrographic 

 survey. The information obtainable for planning the project will vary correspondingly 

 from general data on the character of the region to detail almost sufficient for planning 

 day-to-day operations. The project instructions will indicate the available air photo- 

 graphs, mosaics, maps, or hydrographic sheets prepared from air photographic surveys. 

 Usually an index chart or mosaic will be furnished from which the approximate loca- 

 tion and area of each photograph may be determined. 



In the absence of sufficiently detailed prior surveys, the following data for advance 

 study and planning of the project may be obtained from an examination of suitable 

 photographs or preliminary maps and mosaics compiled from them : 



(a) The approaches to the coast and routes along it both for the ship and the small boats; whether 

 there are off -lying islands, rocks, reefs, or shoals, and the probable clear channels between them; 

 protected areas possibly suitable for anchorage, and the best landing places for small boats. 



(b) The locations of sites suitable for camping; the boats and equipment which will be needed for 

 landing and for inshore hydrography; objects available for hydrographic signals; the proportion of 

 control stations that may be expected to be furnished by the photographic survey and the amount of 

 supplemental planetable surveying likely to be needed; the kinds and amounts of the various signal- 

 building materials required; and the probable rate of progress of the inshore operations. This infor- 

 mation may be determined from the appearance of the shore with more or less certainty depending on 

 previous experience. A detailed discussion of control from air photographs for use in hydrographic 

 surveys is included in 239, and the use of air photographs to detect dangers to navigation is discusssed 

 in 3624. 



(c) The photographs of the terrain adjacent to the shore will show the roads, trails, rivers, settle- 

 ments, etc.; special means of transport and supply other than by water, which may be desirable; 

 whether the terrain is bare or wooded; the relief, landmarks, and sites suitable for triangulation or 

 signals to control offshore hydrographic surveys; the distances and best routes to such sites, and the 

 character of signals, materials, and outfitr needed for the operations ashore. The use of photographs 

 in connection with triangulation reconnaissance is discussed in 2221. 



A stereoscope, if available, should be used in studying the photographs wherever 

 they have suitable overlap. Landmarks, the tentative locations of old and new sta- 

 tions, landing places, campsites, and any questions regarding the interpretation of the 

 photographs should be noted in pencil on them, or on the mosaics or preliminary 

 compilations, during the advance study. 



The field reconnaissance required to complete the information needed to conduct 

 the season's operations (such as the depth of the channels indicated by the photographs, 

 depth and holding bottom of protected anchorages, doubtful lines for triangulation 

 observations) will become apparent during the advance study. After arrival on the 

 working ground, the notes made during the advance study will facilitate the detailed 

 study of the photographs for the planning of day-to-day operations. As the party 

 becomes familiar with the locality, additional precision in interpretation and benefit 

 from the use of the photographs will be obtained. 



