256 



HYDROGRAPHIC MANUAL 



Page 138 



256. Buoy-Control Schemes for Location by Taut-Wire Traverse 



2561. Line of Traverse Buoys Parallel to the Coast 



Buoys anchored in a line approximately parallel with the general direction of the 

 coastline can often be used advantageously to supplement the shore control in areas 

 where the latter alone does not provide strong three-point fixes. This type of scheme 

 provides economic offshore control between two coastal localities at which the end 

 stations of a line of buoys may be located by sextant fixes to prominent shore objects, 

 the hydrography in the intermediate area being controlled by buoy stations. The 

 scheme may also be used to advantage along a broken coastline where it is necessary to 

 establish control for inshore hydrography. 



10 Miles 

 J 



ApproximatP Scale 



Figure 34.— Line of buoys parallel to the coast. 



The scheme consists of a number of buoys anchored in a line between A and H (fig. 34) where strong position determinations 

 may be made by sextant fixes. A uniform accuracy of the positions of all buoys in a scheme of this character is obtained by observing 

 sun azimuths between adjacent buoys, measuring the distances between buoys with taut wire, and computing and adjusting the scheme 

 as a traverse. The position of buoy A is computed from the sextant angles by the three-point problem and reduced to the position of 

 the anchor. (See 2512.) This position is used to compute the succeeding positions through the traverse, using the observed sun 

 azimuths and measured taut-wire distances, to arrive at a traverse position of buoy II. The three-point sextant fix at buoy // is also 

 computed by the three-point problem and reduced to the position of the anchor, which is held fixed. The difference between the 

 latter and the traverse position of buoy // is the traverse error. The position of each intermediate station in the traverse is corrected 

 by a proportional amount.of this error according to its distance from the origin of the traverse at buoy A. (See 2554.) 



In some such schemes it may be possible to measure at each buoy one large sextant 

 angle whose locus is approximately parallel to the line of buoys. Where this is possible 

 the azimuth of the line can be controlled much better by the angles than by sun azimuths. 

 In such cases the positions should be determined from a graphic plot on an alummum- 

 mounted sheet. The locus of the angle at each buoy station is plotted on the sheet (see 

 7625) and the distances between successive stations are plotted on these loci, from the 

 fixed position at one end of the line to the fixed position at the other end, the adjust- 

 ments for closure being made graphically. The three-point fixes and the loci are 

 plotted with a metal protractor. 



In such a scheme, weak three-point fixes of doubtful accuracy should not be used for 

 locating intermediate buoys nor used to adjust the traverse in sections, because larger 

 errors are probable than should result from an adjustment between the two end stations 

 where reliable positions are obtained. 



