1212 HYDROGRAPHIC MANUAL Page 10 



1212. Instructions for Geodetic Control 



Copies of progress sketches of all previous triangulation within the area will 

 ordinarily be forwarded with the project instructions, as well as copies of all geodetic 

 positions and descriptions of stations which the Chief of Party is likely to need to 

 carry out his instructions. The project instructions will ordinarily state whether 

 new triangulation is necessary, and if so, the requirements as to order of accuracy. 

 If arcs of new triangulation are contemplated, the junction to be made with previous 

 triangulation will be specified. 



The instructions will often require that a search be made for, or that descriptions 

 be written for, specific stations concerning which there is a known deficiency in the 

 records at the Washington Office. Wliere geodetic control of other organizations is 

 known to exist within the area, specific instructions for a connection thereto are generally 

 included. Where there are objects of particular landmark value or important stations 

 whose geodetic positions are known to be desired, the instructions will specify that 

 these be located. 



Unless otherwise specified by the project instructions, the principal geodetic control 

 for all coastal hydrographic projects shall consist of stations located by second-order 

 triangulation at intervals of about 5 miles along the coast, supplemented by inter- 

 mediate stations located with thnd-order accuracy at intervals of about 2 miles alcng 

 the coast. If the character of the terrain makes triangulation impracticable, the prin- 

 cipal geodetic control of hydrographic projects may consist of second-order traverse with 

 stations established at intervals of 2 miles along the coast. 



All geodetic control shall consist of recoverable stations permanently marked by 

 standard station marks, and natural objects which are recoverable without special 

 marking. All geodetic control shall be established in accordance with the specifications 

 and instructions contained in Special Publication No. 145, Manual of Second- and 

 Third-Order Triangulation and Traverse, except as amended by section 22 of this 

 Manual and by the project instructions. 



1213. Instructions for Topography 



The project instructions will ordinarily specify to what extent topographic surveys 

 are to be made, whether air photographic surveys of the area are anticipated in the 

 near future, and whether the survey is to consist of a graphic control survey merely for 

 the location of the hydrographic signals, a complete planetable survey of the shoreline 

 and adjacent topographic features, or a revision survey of the shoreline and a verifica- 

 tion of the other details from former topographic surveys of the area. 



The scales of the topographic sin-veys are usually specified, even though they are 

 ordinarily dictated by the scales of the inshore hydrographic surveys of the respective 

 areas. A Chief of Party has the authority to make the topographic survey of any 

 important part of the area at a larger scale than specified if, in his opinion, the larger 

 scale is needed to provide for an adequate and fully developed hydrographic survey of 

 the area. 



The project instructions will generally specify where junctions are to be made with 

 prior surveys and what steps are to be taken, if a check is not obtamed at the junctions. 



Photographic copies of prior topographic surveys which may be needed by the 

 Chief of Party are furnished with the instructions. Blue-line prints of planimetric 

 maps, if available, will ordinarily be furnished for possible revision of the shoreline and 

 adjacent topographic detail. 



