2554 



HYDROGRAPHIC MANUAL 



Page 13G 



The positions should be computed on forms designed for the computation of 

 geodetic positions and the observations on each side of the buoy hne should be computed 

 independently to obtain the distances between successive buoys, but the mean length 

 between buoys should be used each time to compute the advance triangles, unless the 

 positions are to be corrected for scope. Where the buoys are anchored in comparatively 

 shoal water, the anchor cables will usually be so short that no appreciable error from 

 this source will enter into the positions. If it is necessary to correct the positions for 

 scope, two sets of positions should be computed from the observations on the two sides 

 of the scheme. The corrections for scope should be made after the positions have been 

 determined from each set of observations and the mean of each pair of corrected positions 

 should be taken as the accepted position. (See 2511.) 



2554. Buoy Trianguladon — Single Triangles 



A scheme of buoys arranged in triangles, may be used to furnish the control for 

 hydrographic surveys in water areas from which shore signals are* not visible. If 

 such accuracy is warranted and there are triangulation stations on two land areas 

 with a water area between, the buoy control may be located from the shore stations 

 by a chain of well-shaped single triangles, as illustrated in figure 32. This method 





A 



•^--■^ 



' '" ^- 



D 



5 Miles 



Approximate Scale 



Figure 32.— Buoys located by sextant triangulation (single triangles). 



and the method of buoy triangulation by quadrilaterals described in 2555 are especially 

 useful when the survey vessel is not equipped with a taut-wire apparatus for measuring 

 accurate traverses. 



For greatest accuracy of positions in a scheme of this sort, the buoys should be 

 arranged in a series of equilateral triangles, as in figure 32 between buoys A and B and 

 buoy J whose positions may be strongly determined by observations on shore stations. 

 The buoys must be anchored so that those in any one triangle are intervisible; hence the 

 size of the triangles depends on the visibility at the locality. The scope of the anchor 

 cables must be kept as short as possible and they should be moored, as explained in 

 2836 and 2851, if there are strong erratic currents in the locality, or if a large-scale 

 survey is planned. Mooring should not be necessary, however, where the direction and 

 velocity of the current are approximately uniform throughout the entire area, because 

 the buoys would m.aintain the same relative positions to one another. 



Three-point sextant fixes are first observed at buoys A and B at one end of the scheme. A check angle on a fourth shore station 

 should be measured at each of these fixes, if practicable. Current observations should be made in the vicinity at the time these fixes 



