Page 115 control and signal building 2514 



azimuths are the angles at the buoy. With the angles at the buoy known, its position 

 is computed, as previously described, by using Forms 655, 25, and 27. The compu- 

 tation of the anchor position from the buoy position is the same as for an eccentric 

 observation (see 2511). 



It is apparent that the above computations are laborious and they are an un- 

 necessary refinement unless the large scale of the survey requires the additional ac- 

 curacy. A graphic determination, when the observations have been made in this man- 

 ner, will result in a buoy position sufficiently accurate for all but the largest-scale surveys. 



2514. Observations Betiveen the Buoy and Shore 



A buoy may be located by sextant at a maximum distance froni shore stations 

 by observing throe-point fixes at successive ship stations between the shore and the 

 buoy simultaneously with cuts to the buoy (fig. 20). This method of location is par- 

 ticularly adapted to areas where only one line of buoys, parallel to the shore, is required 

 for control (see 2552). 



SHIP STATIONS 



°\ ;CUT ANGLE / ' CUT ', ' 

 V r ANGLES X 



Figure 20.— Buoy location by cuts from ship stations. 



The method requires unusual care in execution to avoid an uncertain position of 

 the buoy. The successive ship stations should be selected with care, so that the 

 position determinations will be strong (see 3332) and the cuts to the buoy will intersect 

 at good angles. For best results the angles should be taken so as to close the horizon, 

 wherever possible, and the horizon closure should be adjusted; all angles from each ship 

 station should be measured simultaneously; and the angles at each ship station should 

 be repeated at least once, as a check. 



Five observers are required to observe all the angles simultaneously to close the 

 horizon; two for the shore station fix, one for the check angle, one for the cut to the 

 buoy, and one for the angle closing the horizon. One additional observer is required 

 for each additional buoy. If there are not sufficient observers or the angles are too 

 difficult to measure simultaneously, the ship should be anchored for each set of obser- 

 vations. 



With the ship drifting, the difierence in distance and direction between sets of 

 repeat observations will give valuable information on the velocity and direction of 

 the current for use in reducing a buoy position to its anchor position. 



To compute the position of a buoy that has been located in this manner is quite 

 laborious and it is not recommended for general use. The location may be determined 

 graphically with ease and in most cases with an accuracy as great as the method 



