255 HYDROGRAPHIC MANUAL PagE 132 



cost, it is advisable to replace a sono-radio buoy with an ordinary buoy structure during 

 extensive periods in port, especially during stormy seasons of the year. 



255. Buoy-Control Schemes for Location by Sextant 



2551. Buoys for Control of Inshore Hydrography 



Along precipitous shores where it is impossible, for any reason, to build or locate 

 shore stations adjacent to the water, a line of buoys may be anchored offshore and used 

 to control the inshore hydrography between them and the coastline. Control estab- 

 lished in this manner is especially useful in localities where rugged interior topography 

 prevents the establishment of triangulation stations on the shoreline, and along beachless 

 coastlines where planetable traverse is impossible or where boat landings to build signals 

 can be made only with great difficulty. To locate these buoy stations, established 

 control stations at interior points are necessary to which the three-point fixes at the 

 buoys can be observed. The principal triangulation scheme along the coast usually 

 provides stations on the tops of hills and the prominent interior features which can be 

 used for this purpose. 



^ % '^. X 



^. 



Approximate Scale 

 Figure 28. — Alongshore buoys located by three-point fixes at buoys. 



The buoys are anchored in a line from ^4 to E (fig. 28) paralleling the shore, at a 

 sufficient distance offshore so that at least three shore stations are visible from each 

 buoy station. Each should be located by a three-point sextant fix taken at the buoy, 

 with a third angle to be used as a check if sufficient shore stations are visible. Each 

 buoy station should be located by observations on shore stations exclusively; none of 

 the observations should be on other buoy stations. 



The positions of the buoy stations may be computed but such precision is rarely 

 warranted. Sufficiently accurate positions may be obtained by plotting the three-point 

 fixes with a metal protractor on an accurate projection on an aluminum-mounted sheet. 

 When an aluminum sheet is not available, the fixes may be plotted on the smooth 

 sheet, but this must be done immediately after the projection is made to avoid the 

 possibility of distortion in the sheet. For immediate use in the field the three-point 

 fixes may be plotted directly on the boat sheet. In such a scheme, corrections for the 

 scope of the anchor cable are an overrefinement unless the stations are in deep water 

 and long anchor cables are used. If scope corrections are applied to the plotted 

 positions of the buoy stations they should be made as explained in 2511 and 943. 



It is interesting to note that the Hydrographic Service of Canada makes rather ex- 

 tensive use of this method for controlling inshore surveys along heavily wooded coast- 

 lines. Instead of using specially constructed buoys, however, they anchor small 



