Page 113 control and signal buii-ding 2512 



angles before Form 655 is employed. In this computation the distance and direction 

 between the observers and the buoy anchor should be used, but the magnetic, or gyro, 

 bearing must be changed to a geodetic azimuth before use. 



2512. Three-Point Fix at Buoy 



Weather conditions frequently prevent the measurement of the angles desired to 

 fix the position of a buoy at the time it is anchored, and it is generally not expedient to 

 delay the progress of the project by waiting for suitable observing conditions. In such 

 circumstances the buoy is anchored at the desired location and the observations are 

 made at a later date when observing conditions are favorable. 



When it is practicable to do so, it is desirable to maneuver the ship alongside the 

 anchored buoy so that the three-point fix observations may be made at that point. 

 This is rarely attempted, except when there is an exceptionally smooth sea and the 

 current is slack, because of the danger of damaging the buoy. To effect this maneuver, 

 the ship is brought alongside the buoy on the down-current side and the sextant angles 

 of the fix and the check angle are observed at the instant the' point of observation is 

 adjacent to the buoy. On a ship without a bowsprit, the ship may be maneuvered 

 until the bow almost touches the buoy, even in rough weather. The sextant fix is 

 observed and the ship backed away. The measured distance from the observation 

 station to the bow and the ship's heading are then used for an eccentric reduction. 



The direction and velocity of the current must also be noted. The direction is 

 determined with the pelorus and is observed when the ship is directly down current 

 from the buoy, as evidenced by the ripple marks on the surface of the water, or by the 

 direction of the buoy from the relieving drum, if the latter is used. The direction 

 toward which the current is flowing should always be recorded and this is usually 

 the reverse of the observed bearing, since the ship is down current from the buoy. 

 The observed bearing should be corrected by 180° to obtain the direction of the current. 

 At the same time the velocity of the current should be estimated; whether the current 

 is slack, weak, medium, or strong should be sufficient information. 



When the three-point fix is observed at the buoy, the anchor position may be 

 computed or plotted graphically, in the manner described in 2511 for an eccentric 

 observation, except that the eccentric distance is determined from the scope of the 

 anchor cable and the direction of the current instead of being measured with a tape. 

 The method of determining the scope correction is described in 943. 



When it is impossible to observe the three-point fix directly adjacent to the buoy 

 position as described above, it is obtained some little distance away from the buoy. 

 The ship is maneuvered close to the buoy on the down-current side so as to bring the 

 point of observation on board directly in line with the direction of the current, as 

 shown by the ripple marks on the surface of the water, or by the direction of the buoy 

 from the relieving drum, and similar observations are obtained. In this instance it will 

 be necessary to measure the additional eccentric distance from the observation station 

 to the buoy by a sextant depression angle (see 3362), or by a rangefinder. This 

 observation is made at the same time the sextant angles are marked so that all are 

 simultaneous. 



The anchor position may be computed or plotted graphically, as explained in 2511 

 for an eccentric observation, but the eccentric distance must be increased by the distance 

 from the observation station to the buoy ; the total eccentric distance is the distance from 

 the observation station to the buoy plus the correction for the scope of the anchor cable. 



465382 — 44 9 



