Page 61 control and signal building 2115 



2115. Hydrographic Station 



A hydrographic station is one whose geographic position has been determined by 

 methods ordinarily used only in hydrographic surveys and with an accuracy usually 

 less than that of a topographic station. 



The positions are generally determined by sextant observations — three-point 

 fixes, intersection of cuts, or astronomic observations. 



Also classed as hydrographic stations are those located by taut-wire traverse, 

 log-distance runs, R.A.R. distances, and other less accurate methods. 



Hydrographic stations are ordinarily not permanently marked; but if they are 

 marked, a standard topographic station mark is used and a description is written 

 (see 245). 



2116. Hydrographic Signal 



A hydrographic signal is any signal used as a target in measuring sextant angles 

 for the control of a hydrographic survey, irrespective of the method by which the station 

 was located. 



212. Frequency and Spacing of Control 



The spacing of control depends on the nature and character of the coastline. 

 The number and distribution of stations shall be such that all topographic and hydro- 

 graphic features and soundings within the area of the survey can be located with 

 sufficient accuracy for charting at the largest scale likely to be necessary. 



Triangulation or traverse stations must be frequent enough along the coast to 

 control the planetable or air photographic surveys. The required frequency depends 

 on the scales of the surveys and the configuration of the coastal area, or the area covered 

 by air photographs. 



Bases must be measured at intervals along the arc or scheme of triangulation in 

 order that the strength of the triangulation may be in accordance with the specifications 

 for the class of triangulation being established, as specified in the requirements for 

 horizontal control given in table 3 in 223. 



Where practicable, main-scheme (second-order) triangulation stations shall be 

 spaced about 5 miles apart along the coast, with supplemental stations of third- 

 order accuracy about 2 miles apart for topographic control (see 2231). 



In addition to the triangulation control, a considerable number of intermediate 

 stations are necessary for hydrographic control. Experience is the best guide in the 

 establishment of adequate control for this purpose. Stations should be located at 

 prominent points and at the heads of small bights, and in general so spaced that strong 

 fixes will be available throughout the area of the survey. But an excess of stations 

 should be avoided, for it leads to confusion and increases the difficulty of identification 

 when taking sextant angles. 



For launch hydrography, stations spaced about 400 meters apart along the shore 

 will usually suffice. Stations should be located well inshore from the beach, if practic- 

 able, so that strong sextant fixes may be obtained when the launch is near the shoreline. 



For offshore visual ship hydrography, control stations may be several miles apart 

 if structures are buUt over them, or if they consist of peaks, mountain tops, or prominent 

 natural objects. 



Hydrographic surveys of moderate depths, out of sight of land, are usually con- 

 trolled by sextant fixes on buoys, spaced from 2% to 5 miles apart, depending on the 

 visibility, the scale of the survey, and the importance of the area being surveyed. 



