Page 125 control and signal building 2536 



the observation station on board to the anchor position. The latter will be the 

 station when the observations are completed, at which time a buoy is anchored at the 

 position of the ship's anchor and the ship moves away from the location. 



The observations may also be made from an anchorage near the station to be 

 located. In this case the distance and direction between the observation station on 

 board and the buoy should be determined and the scope of the anchor cable of the buoy 

 known. The ship's heading is also required to determine the direction of the current 

 for the reduction of the buoy position to the anchor position (see 943) . 



In deep water it is impracticable to anchor the ship in the vicinity of the station 

 and in this case observations must be made while hove-to near the buoy. For each 

 series of observations the necessary data must be obtained to relate the position of the 

 observation station to the position of the buoy anchor. 



The necessary astronomic observations are described in 338. 



2536. W ith Reference to Submarine Relief 



A buoy anchored near distinctive submarine relief in an accurately surveyed un- 

 changeable area, at an isolated position far removed from established control, may be 

 located with reference to the submarine relief without other connection to established 

 control. This is accomplished by making a second survey of the immediate vicinity 

 referenced to the buoy, but without regard to datum, and fitting it by means of the 

 bottom contours to the prior survey referred to the standard geographic datum. 



This method is of particular value when the survey to be made must be coordinated 

 with the prior survey, and the small amount of additional work required does not 

 justify a long taut-wire traverse from shore stations. It is used where additional de- 

 velopment is required on offshore shoals; to extend the original surveys farther offshore; 

 and, infrequently, to start the new season's work of a continuing project of offshore 

 surveys without the delay of extending control from shore stations. The last is fre- 

 quently of primary importance in R.A.R. surveys because better results are obtained in 

 many areas during the late spring months than during midsiunmer. With a rapid 

 method of establishing the control, R.A.R. surveys may be executed during the more 

 favorable period, leaving the time-consuming operation of establishing the connection 

 to shore stations for a period when R.A.R. does not function as efficiently. 



To fix the position of a station by this method, a well-controlled and adequate 

 prior survey of the area is required, in which a site can be selected which has a charac- 

 teristic bottom form. Where there is a choice of sites, the control and adequacy of 

 the prior survey should be examined critically and a site selected where the control is 

 strongest and the soundings are most accurate and most numerous. The prior survey 

 should be in sufficient detail so that the depth contours may be drawn with reasonable 

 certainty without plottable errors. 



Suitable sites for locations of this nature are limited to areas of characteristic bottom 

 relief in two directions approximately normal to each other so that the position may be 

 fixed in two directions. The submarine feature should be small enough in area so that 

 an extensive hydrographic survey will not be necessary to cover the desired details. 

 The selected site should have sufficient gradient between depth contours so that minor 

 uncertainties in the soundings would cause no appreciable horizontal displacement of 

 the contours. 



Not all well-defined accurately surveyed features on the ocean bottom can be 

 utilized for this purpose. Features which are otherwise satisfactory, but which are in 

 changeable areas where bottom shapes may be altered during storms or severe weather 



