253 HYDROGRAPHIC MANUAL PaGE 120 



For supplemental distances in an area where the principal control has been established 

 by more accurate methods, relative distances between buoys may be determined by 

 single or double log runs (see 3374 and 4453). The accuracy of log distances is 

 definitely limited, and except in emergencies they should be used only to measure 

 relative distances, which may be adjusted to other data. 



The azimuth between two intervisible buoys is determined by sun-azimuth obser- 

 vations while on range with the two buoys. The geographic position of one buoy 

 referenced to another is determined by computation on Form 27, using the measui-ed 

 distance and the observed azimuth between buoys, the latter being reduced to geodetic 

 azimuth, that is, to read from the south. The azimuth from the known position to 

 the new buoy must be used. This azimuth is frequently measured in the reverse 

 direction and in such case a correction is necessary (see 2523). 



253. Other Methods of Locating a Single Buoy 



2531. Referencing One Buoy to Another Close By 



When a sono-radio buoy (see 643) becomes insensitive during use, it is practically 

 impossible to service it in situ, when weather conditions are unfavorable, because of 

 the likelihood of dragging the anchor. In these circumstances it is preferable to anchor 

 a new buoy near the inoperative one and locate the former by a distance and an 

 azimuth from the latter, whose position is known. This method is also used in locating 

 a reference buoy near an isolated sono-radio buoy when there is a possibility of the 

 latter's anchor dragging. The relation between the two may be used later to check 

 their relative positions for any indication of a shift in position. The geographic position 

 of the new buoy is computed on Form 27. 



Before referencing one buoy to another, both buoys must have been anchored 

 for sufficient time for the current to affect both buoys similarly; otherwise appreciable 

 inaccuracies will be introduced into the measured distance and bearing. 



The azimuth between the two buoys may be determined by several different 

 methods, the choice being dictated by the distance between them, the scale of the 

 survey, the purpose of the determination, the Ivnown accuracy of the instrumental 

 equipment, and other factors which might influence the accuracy of the determmation. 

 If the two buoys are comparatively close together, a compass or gyro bearing may be 

 sufficient; but if the distance between buoys exceeds three-fourths of a mile, a sun- 

 azimuth determination is preferable. In either case the observed value is reduced to 

 geodetic azimuth. 



A short distance between buoys may be determined either by measuring depression 

 angles (see 3362) or by the use of a rangefinder (see 455). The shorter distances may 

 be measured more accurately by rangefinder than by depression angles, but only with 

 clear observing conditions, a smooth sea, and, most important of all, an experienced 

 observer. In measuring distances thus, the ship may be placed close to one of the 

 buoys and on range with the other, and angles or distances may be measured to both 

 buoys simultaneously. The distance between buoys is the difference between the two 

 resulting distances. 



If the distance between buoys is so great that the data so obtained are not sufficient- 

 ly accurate, depression angles, or rangefinder distances, to each buoy may be measured 

 from a ship observation station on line midway between the two buoys. There is some 

 difficulty in placing the ship exactly on the range between the two buoys. If there are 

 gyro repeaters on both wings of the bridge, the ship may be maneuvered so that the 



