2511 HYDROGRAPHIC MANUAL PaGE 112 



is closed and adjusted, all corrections will be sub tractive. Theoretically the correc- 

 tions to the angles, from the horizon closure, should be proportional to the sizes of the 

 angles, but such refinement is rarely warranted. 



2511. Three-Point Fix at Buoy Anchor 



To locate the anchor position of a buoy, it is usually preferable to observe the three- 

 point fix at the time the buoy is anchored. Wlien this is done, both angles of the three- 

 point fix and a check angle to a fourth station should be observed at the rail of the ship 

 over the point where the anchor is dropped, and the angles should be marked when the 

 anchor is released or when it touches the bottom. 



Depending on the character of the shore stations and their visibility and distance, 

 the added height of the ship's bridge or crow's nest may be needed from which to ob- 

 serve the shore stations. When angles are measured thus, from a point an appreciable 

 horizontal distance away from the anchor position, it is necessary to measure the eccen- 

 tric distance and direction between the observers and the point where the anchor is 

 released. With this in view, the observers will, when practicable, take a position near 

 the ship's pelorus or gyro repeater on the wing of the bridge or on line between the 

 pelorus and the point from which the anchor is released. The bearing between these 

 two points is measured simultaneously wdth the observation of the three-point fix. The 

 distance between them is measured to the nearest meter with a steel tape. If there is an 

 appreciable difference in elevation between the two points it may be necessary to resduce 

 the measured distance to the horizontal. If practicable, the place from w^hich the angles 

 are observed should be selected so as to facilitate the measurement of this distance. 



The position of a buoy located by this method may be plotted most easily by the 

 hydrographer. If the angles have been taken from a position vertically above the buoy 

 anchor and the position is to be located graphically, it is only necessary to plot the 

 position with a three-arm protractor on an appropriate sheet. An aluminum or an 

 aluminum-mounted sheet with accurate projection should be used for this purpose 

 (see 713). When the observations have been made eccentrically, the position may be 

 determined by graphic methods or by computations. If the scale of the sheet is suffi- 

 ciently large and care is used in plotting, the accuracy of the position determined graphi- 

 cally will approach, if not equal, that of a computed position. 



The position may be computed on forms designed for the computation of geodetic 

 positions. Natural objects, and structures such as tanks, lighthouses, and building 

 cupolas, are often more useful than marked triangulation stations when such three- 

 point fixes are required. The geographic position data seldom include all the required 

 information for such objects and it is often necessary to make inverse computations 

 for azimuths and distances between the stations used. These computations should be 

 made on Form 662, Inverse Position Computation, or on Form 27, Position Computa- 

 tion, Third-Order Triangulation. After the azimuths and distances between the respec- 

 tive stations have been computed, the three-point fix is computed on Form 655, 

 Computation of Tlu-ee-Point Problem. The resulting data are then entered on Form 

 25, Computation of Triangles, and the triangles are solved. The actual geographic 

 position is computed on Form 27, using the data derived on Forms 662 and 25. Addi- 

 tional information relative to buoy-control computations is given in 94. 



To obtain the position of the buoy anchor, when the observations are eccentric, it 

 is best to compute the position for the three-point fix and then reduce the latter to the 

 anchor position on Form 27, rather than make an eccentric reduction to the observed 



