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HYDROGRAPHIC MANUAL 



Page 126 



conditions, should be used with extreine caution. Likewise, features that are the 

 result of scouring- action of currents and features on banks and shoals, where the existing 

 currents or other conditions are such that there is a probability of change, should be 

 eliminated from consideration. 



In an area where the submarine relief is broken and moderately rugged, a station 

 may be located near a locality where the general direction of the depth contours changes 

 (station A, fig. 27). The change in direction of the contours should be at least 50° in 

 order to obtain an accurate location in both directions. In an area of similar type a 

 station may be located at a change in bottom, gradient where the distance between 

 depth contours changes (station B, fig. 27). The accurate position of a station at such 



Figure 27. — Buoy station located with reference to submarine relief. 



a location may be obtained only at a pronounced change in bottom gradient. A sta- 

 tion may be established on a shelf near the edge of a valley, to be located in one direction 

 by depth with respect to the depth contours on the shelf and in the other direction by 

 distance from the edge of the valley; provided the axis of the valley is approximately 

 normal to the depth contours on the shelf and provided the depth of water is within 

 the accurate range of the sounding equipment. A site in the vicinity of a small con- 

 fmed crest or small caldron with rather steep sides is generally suitable. If such a 

 feature is small enough, the new survey duplicates the survey of the entire feature 

 and not only fixes the position in each direction but also affords a check on the probable 

 accuracy. Satisfactory sites in a suitable depth of water may generally be found at the 

 upper edge of the continental slope. Irregular submarine features are frequently found 

 there, provided the break of the slope is abrupt. 



Before the buoy is anchored, some preliminary sounding lines may be necessary 

 to find the feature, but it will generally suffice to run an accurate dead-reckoning line 

 from a known inshore position across the previously surveyed area on a course so as to 

 traverse the intervening area approximately at right angles to the depth contours. 

 When the required depth is reached, the ship is stopped and the buoy anchored. It 

 will generally be found to be near enough to the submarine feature, but its position will 

 have to be changed if it is not. 



If the survey of the features to which the buoy is to be referenced is extensive, a line 

 of buoys may be required which are accurately referenced to one another by traverse 

 methods (see 2525). The soundings may then be located with reference to the buoys by 

 sextant or by R.A.R. methods. Such a survey is without datum, the stations being 

 merely plotted accurately with reference to one another by graphic methods w itli only 



