2523 



HYDROGRAPHIC MANUAL 



Page 118 



locus of the vertex of the sextant angle observed at the buoy (see fig. 22). The site 

 of the buoy should be selected so that a good relation between the shore stations is 

 obtained. The strength of location also depends on the size of the angle at the buoy, 

 a strong location being obtained with an angle of approximately 90°. Angles less than 

 45° are likely to result in poor positions and should not be used. 



0wm 



Figure 22.— Buoy location by an angle at buoy and a direction from a shore station. ,4. Where one of the shore stations between 

 which the angle at the buoy is measured can be occupied. B. Where the occupied shore station is not observed from the buoy. 



Two cases involving the above principle may be encountered in practice: (1) Where one of the stations between which the angle 

 at the buoy is measured can be occupied ^see A in fig. 22) , and (2) where the occupied station is not used at the buoy (see B in fig. 22) . 

 In both cases the direction at the shore station should be observed simultaneously with the angle at the buoy. An observation of the 

 direction and velocity of the current at the buoy should also be made. 



The position of the buoy may be determined by computation or by a graphic solution. In (1) the computation is relatively simple, 

 the two observations furnishing suflBcient data for the solution of the triangle. The anchor position may be obtained by reducing the 

 buoy position as described in the last paragraph of 25U. In (2) the computations are involved and laborious and the buoy position is 

 best determined graphically as in 2515. 



If the observations at the buoy and the shore station are not simultaneous, they 

 must be reduced to the anchor position which is common to both. Having the data 

 necessary to compute the eccentric distance and direction, the reduction of the observa- 

 tions may be made on Form 382; the computation being for an eccentric station in the 

 case of the angle at the buoy, and for an eccentric object in the case of the direction 

 from the shore station. After the two observations have been reduced the geographic 

 position of the buoy anchor may be determined as described above. 



Wliere the observation station on board ship is eccentric to the buoy, the reduction 

 may be made by solving the triangle between the observation station, the buoy, and 

 the buoy anchor by computation, or graphically on polar coordinate paper, 



2523. Sun Azimuth and One Angle 



It may be desirable to obtain all observations on board the ship where there are 

 only two shore stations from which to fix the position of a buoy. It may be impossible 

 to land an observer to measure a direction from shore, as described in 2522, or inexpe- 

 dient to do so, especially if the sohre station is inaccessible. Under these conditions 

 the position of the buoy may be determined, as in figure 23, by observing the angle at 

 the buoy and observing a sun azimuth of the direction from the buoy to one of the 

 shore stations (see 4526). 



From these data the position may be computed to obtain a more accurate determi- 

 nation than is possible graphically. The azimuth between the two shore stations is 

 known, or may be computed by an inverse computation (see 2511). The azimuth 

 between the buoy and one of the shore stations was obtained from a sun-azimuth, 

 observation. This latter is corrected by Aa plus 180° to obtain the reverse azimutli 

 and the difference between this and the azimuth between the shore stations is the angle 



