2513 HYDROGRAPHIC MANUAL PaGE 114 



2513. Three-Point Fixes on Range With Shore Stations 



The position of a buoy may be determined by observing three-point fixes at three 

 or more ship stations, each of which is on range with the buoy and a shore station (fig. 

 19). This method is particularly useful on larger vessels which are difficult to maneuver 

 alongside a buoy to obtain a fix. The position of a buoy so located is easily deter- 

 mined by graphic plotting; it may be computed, but such computations are rather 

 involved. 



The shore stations, with which the buoy is to be brought successively on range, 

 must be selected to give the best possible intersections at the buoy. A fix is observed 

 when the ship observation station is exactly on range with the buoy and shore station, 

 at any convenient distance (usually a fourth to a half mile) from the buoy. The fix 

 on each range may be repeated several times, if desired, while steaming slowly directly 

 away from, or toward, the buoy but keeping the observation station exactly on the 

 range. The observers should stand together but, by shifting their position on board, 

 they can keep the point of observation on range as the ship veers across it. In order 

 that the angles will be observed w^hen exactly on range, the observer who is viewing 



SHIP STATIONS 



Figure 19.— Buoy location from ship stations in range with shore stations. 



the buoy direct — not reflected — should give the mark. This is not possible when the 

 buoy is on range with the right object, but in this case the ship is generally far enough 

 from the buoy so that it is possible to view the buoy reflected and thus mark the angles 

 at the proper instant. One observation for the direction and velocity of the current 

 is usually sufficient, but this should be taken immediately before or after the other 

 observations. If the direction of the current is changing rapidly, two observations to 

 determine it should be obtained, one before and one after the angle observations, and. 

 the mean of the two should be recorded for the determination of the scope during the 

 period of observation. 



The ship positions may be plotted graphically as explained in 2511. After each 

 fix is plotted, a line is drawn from it toward the shore station which was in range with 

 the buoy. If more than one*fix was obtained on a single range, a mean of the resulting 

 lines should be used if they do not coincide exactly, rejecting any questionable fix. 

 If the observations have been accurate, the range lines will intersect at one point 

 which is the position of the buoy. If desired or necessary, the correction for scope of 

 anchor cable may be applied graphically before scaling the position from the sheet. 



The computation of such a position is very time consuming. The position of 

 each fix on each range must be computed in the manner described for the compu- 

 tation of the position of the buoy anchor (see 2511) using Forms 655, 25, and 27. 

 From these computations are derived the azimuths of the ranges, or the azimuths 

 from the buoy to the respective shore stations, and the differences between adjacent 



