2515 HYDROGRAPHIC MANUAL PaGE 116 



warrants (see 2513). Unusual care must be exercised in plotting the positions of the 

 three-point fixes and the respective cuts to the buoy. Tlu-ee or more cuts will seldom 

 intersect at a point, and considerable judgment is necessary in analyzing them and 

 accepting the most probable position. 



2515. One Angle at Buoy and One Cut From Three-Point Fix 



A buoy station may be established during hazy weather, with the expectation of 

 determining its position by a sextant fix at the buoy when observing conditions become 

 favorable. Wlien this is attempted, only two of the three necessary shore stations 

 are found to be visible from the buoy station and it is impracticable to move the buoy 

 inshore where all may be seen. In such a case the angle between the two may be meas- 

 ured, and at a point farther inshore, selected to give a good intersection with the locus 

 of the angle obtained at the buoy, a three-point fix is obtained simultaneously with a 

 cut to the buoy (fig. 21 ). The position of the buoy may be determined from these data. 



Figure 21.— Buoy location by one angle at buoy and one cut from a ship station. 



Wlien the observations have been obtained in this manner, the position is best 

 determined graphically (see 2511). The three-point fix position should be plotted first and 

 the cut drawn at the approximate location of the buoy. The angle at the buoy is next 

 plotted on the direction line with a metal protractor, the center of the protractor being 

 moved along the direction line until the protractor arms intersect the respective shore 

 stations. The center of the protractor will be the position of the buoy. To obtain 

 the position of the buoy anchor, the buoy position must be corrected for the scope 

 determined from the current observation. This may also be done graphically. This 

 correction cannot be made accurately for both observations, since the current at the time 

 of the three-point fix will be unknown, but if the latter is observed immediately after the 

 angle at the buoy, the current may be assumed to be the same for both observations. 

 With these observations only, there will be no check on the location, so care is essential 

 in observing, recording, and plotting the data. 



