4. CONTROL AND SIGNAL BUILDING 



4-1 Basic control. — All hydrographic 

 surveys must be controlled by a system of 

 geodetic triangulation or traverse established 

 with an accuracy not less than that pre- 

 scribed for third-order control (see 1-15). 

 Although triangulation stations have been 

 established in most coastal areas of the 

 United States and the territories, a hydro- 

 graphic party frequently finds it necessary 

 to establish supplemental control or to re- 

 establish lost stations. Second-order methods 

 shall be used to locate all main-scheme sta- 

 tions, whether new or re-established. Second- 

 order, Class II, accuracy is desired on the 

 main-scheme triangulation, but third-order 

 accuracy is acceptable where reoccupation of 

 stations to obtain second-order triangle clos- 

 ures and side checks will be unduly expensive. 

 Supplemental triangulation stations estab- 

 lished solely for control of photogrammetry 

 and hydrography, permanent fixed aids to 

 navigation, prominent landmarks, and elec- 

 tronic control stations shall be located with 

 at least third-order accuracy. 



A complete discussion of and specifications 

 for classification of horizontal control sur- 

 veys is contained in the 1959 edition of Spe- 

 cial Publication 247, Manual of Geodetic 

 Triangulation. 



All first and second order triangulation 

 surveys, base measurements, and azimuth ob- 

 servations shall be accomplished in accord- 

 ance with instructions contained in Special 

 Publication 247. Third order triangulation 

 and traverse shall comply with specifications 

 in Special Publication 145. 



4-2 Control by traverse. — Under certain 

 conditions it is more expedient to establish 

 control points by traverse methods. This is 

 particularly true where the beach is bordered 

 by precipitous mountain areas and in fiords 



surrounded by heavily wooded hills. In such 

 cases, control can be established by Tellurom- 

 eter traverses in accordance with procedures 

 described in Publication 62-1, Tellurome- 

 ter Manual, or by taped traverses as de- 

 scribed in Special Publication 145, Manual 

 of Second and Third Order Triangulation 

 and Traverse. The latter method is time- 

 consuming and expensive, and shall be used 

 only as a last resort. 



4-3 Spacing of control. — Triangulation 

 or traverse stations must be frequent enough 

 to provide basic control for the project at 

 hand. The use of photogrammetric and elec- 

 tronic methods of controlling hydrographic 

 surveys has lessened the need for a dense 

 coverage of triangulation stations, but a cer- 

 tain minimum should be established and 

 maintained for future use. The required fre- 

 quency depends on the scales of the surveys, 

 the configuration of the coastal area, and 

 requirements to control the photogrammetric 

 plot. Triangulation stations are used in sur- 

 vey work by other engineers, public and pri- 

 vate, and the density of control should be 

 adequate to meet anticipated needs for con- 

 trol in the area. In general, main scheme 

 triangulation stations should be established 

 at intervals of 5 to 8 miles along the coast, 

 with supplemental third-order stations as re- 

 quired to provide adequate control for the 

 project. When planetable methods are to be 

 used to locate supplemental control points 

 for hydrography, triangulation stations 

 should be established at intervals of 2 to 5 

 miles. 



4-4 Recovery of old stations. — The sites 

 of all previously established control stations 

 in the project area shall be visited, if prac- 

 ticable, and a thorough search made for each 

 station and its reference marks. A report 



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