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HYDROGRAPHIC MANUAL 



shall survey station marks of other organiza- 

 tions be altered or amended by stamping. 

 Neither shall they be moved, replaced, or re- 

 set unless the project instructions so specify. 

 If need be, such marks should be reinforced 

 to prolong their existence, but special pre- 

 cautions must be taken not to disturb or 

 move them, either horizontally or vertically. 



4-7 Theodolite cuts. — A triangulation 

 station must be of third-order accuracy or 

 higher, and any stations located by geodetic 

 methods, which do not comply with triangu- 

 lation standards, are classified as topographic 

 stations. Among the latter are: (a) any 

 station located by a single triangle, one angle 

 of which is concluded ; (b) an intersection 

 station located by an insufficient number of 

 directions to provide a check on its position; 

 (c) a station located by theodolite by the 

 three-point problem, without a check; (d) 

 a temporary station which is unmarked and 

 nonrecoverable ; (e) any station located by 

 any method or scheme, which depends in 

 part on floating or moveable objects, such 

 as ship-to-shore triangulation. If stations of 

 this class are marked, a topographic station 

 disk shall always be used, never a triangula- 

 tion station disk (see 4-24). The records 

 and computations for such stations shall be 

 forwarded with the topographic or photo- 

 grammetric data and reports. 



4-8 Use of air photographs in reconnais- 

 sance.— Reconnaissance for the selection of 

 sites for control stations may be greatly 

 facilitated by the use of air photogrraphs. 

 The photographs should be matched to form 

 a temporary mosaic and studied under a 

 stereoscope. If straight lines are drawn, pre- 

 ferable with white ink, between possible 

 sites for triangulation stations on each 

 photograph, the lines will fuse to form lines 

 in space when the photographs are viewed 

 under a stereoscope. It will then be im- 

 mediately apparent whether the lines are 

 clear, blocked, or grazing. Reconnaissance 

 on the ground may then be reduced to an 

 examination of the doubtful lines. The study 

 of the photographs will also furnish infor- 

 mation for the location of hydrographic sig- 



nals, the best landing places, and routes to 

 inland triangulation stations. 



4-9 Topography. — A topographic survey 

 is the determination of the positions, on the 

 earth's surface, of the natural features and 

 the culture of a locality and the delineation 

 of them on a plane surface called a topo- 

 graphic map. Although the depths in the 

 water area are the most important data on 

 a nautical chart, the topographic features of 

 coast line are only slightly less important 

 because, in sight of land, the mariner is 

 chiefly guided by, and determines his posi- 

 tion from, the aids to navigation and the 

 shore landmarks which he can identify. A 

 chart should include the topographic details 

 of the adjacent shore and all the salient 

 features of landmark value that are visible 

 from a ship at sea. 



Virtually all topographic surveys are now 

 being made photogrammetrically. Instruc- 

 tions for photogrammetric field work are 

 contained in a series of Photogrammetric 

 Instructions which have been promulgated 

 over a period of several years. Office proce- 

 dures are described in Special Publication 

 No. 248, Topographic Manual, Part II, 1949 

 edition. Planetable methods are still used 

 when expedient, but primarily for the loca- 

 tion of signals (see 1-17). Complete instruc- 

 tions and specifications for topographic sur- 

 veying by planetable methods are contained 

 in Special Publication No. 144, Topographic 

 Manual. Part I of the new Topographic Man- 

 ual, when published, will supersede Special 

 Publication No. 144 and the series of photo- 

 grammetric instructions. Except as modified 

 in the project instructions and in this Manual, 

 the requirements stated in the above publi- 

 cations shall be strictly followed by hydro- 

 graphic parties engaged in topographic 

 surveys. 



4-10 Photogrammetry. — Except in Alas- 

 ka and similar remote areas where water 

 transportation is essential to successful op- 

 erations, the Photogrammetry Division will 

 accomplish all field work required to produce 

 advance manuscripts (shoreline, planimet- 

 ric, or topographic) prior to the beginning 



