Page 117 control and signal building 252 



252. Directions and Sun Azimuths in Locating a Single Buoy 

 2521. Observations at Shore Stations 



A buoy, if not too distant from the shore, may be located by theodolite or sextant 

 directions measured at shore stations. It may be necessary to start or end a buoy 

 traverse at a point along a coast where sextant angles to three shore stations cannot be 

 observed from the ship, or where the extension of the traverse to a locality where such 

 a fix could be obtained is impracticable. At such places it may be possible to locate a 

 buoy by simultaneous directions from two or more shore stations. 



The accurate location of a buoy in this manner is difficult because the directions, 

 measured at widely separated points, must be observed simultaneously; otherwise 

 current data must be obtained which will permit the reduction of the directions to a 

 common point. The most accurate method is for the observers, occupying the two or 

 more shore stations, to mark their directions to the buoy simultaneously at a signal 

 from the ship or by any other means which may be available. The ship assumes a 

 position near the buoy so that the signal to mark may be seen by all the shore observers, 

 but without interfering with the observations on the buoy. The buoy should not 

 be anchored so far offshore that the observations will be too difficult. This is especially 

 important where there are strong currents that may cant the superstructure so that the 

 normally expected visibihty is reduced. Before the ship leaves the buoy, the direction 

 and velocity of the current should be observed. 



When the observations have been made simultaneously, the position of tlie buoy 

 is best determined by the usual geodetic computations for the position of an unoccupied 

 station. The buoy position is then reduced to the anchor position from the current 

 information obtained. When shore observations have not been observed simultaneous- 

 ly, each direction should first be reduced for an eccentric observed object on Form 

 382, Reduction to Center, using the direction and scope of anchor cable data obtained 

 on the ship. The subsequent position computation, made in the usual manner, will 

 be the position of the buoy anchor. 



2522. Angle at Buoy and Direction From Shore Station 



A special condition arises when only one shore station can be occupied to observe 

 a direction to a buoy, and only one satisfactory angle can be observed at the buoy. 

 This is frequently the case where surf landings are difficult or shore stations are in- 

 accessible and difficult to occupy, and at places where there are insufficient natural 

 objects or structures for a strong sextant fix at the buoy. 



The principle by which offshore positions are determined graphically by this 

 method is that a direction from one station to a buoy and the locus of an angle taken 

 at the buoy between two stations will intersect at a point when properly related stations 

 are used in the observations. The theoretical strength of location depends on the 

 angle of intersection of the direction and the locus, and the theoretically strongest 

 location is obtained when they intersect at right angles. In order that they may 

 intersect at or near right angles the shore station, at which the direction is observed, 

 should be selected so that the direction will pass near the center of the circle which is the 



