Page 133 control and signal building 2552 



evergreen trees buoyed and weighted to float upright; the tree branches and foHage 

 make a perfect signal against the horizon. 



This same method will sometimes serve to locate buoys much farther offshore, 

 such as are treated in 2552, especially when the atmosphere is exceptionally clear. 

 Since the buoys are much farther offshore in this case, the sextant angles may have to 

 be observed from a higher elevation on the ship to be able to observe on shore stations 

 from such great distances. The three-point fixes at the buoy stations can sometimes 

 be observed from the crow's nest at the time the buoy anchors are dropped, or at a 

 later date when the ship is maneuvered to a position alongside each buoy. 



2552. Sextant Locations Beyond the Visibility of Shore Signals 



In an occasional survey project the offshore zone of the survey may be best con- 

 trolled from a single line of buoy stations approximately parallel with the shore. This 

 usually occurs where the offshore depths are moderate and the hydrography must be 

 controlled by sextant fixes, but tall shore signals cannot be seen at the required distance. 

 For use as control in such an area a line of buoys may be anchored at a suitable distance 

 beyond the limits of visibility of the shore stations and cut in from successive positions 

 of the ship, drifting or at anchor, between the line of buoys and the shore. The 

 scheme requires no special instrumental equipment and it may be used by any vessel 

 equipped to handle buoys. 



The buoys should not be anchored so far offshore that there will be difficulty obr- 

 serving them from ship positions within sight of shore signals. The buoys should 

 be anchored with the shortest practicable anchor cables that will rnaintain them in 

 their positions, in order to reduce to a minimum any errors of observations due to 

 scope. The buoys should be approximately alined, the interval between adjacent 

 buoys being slightly less than half the distance a buoy can normally be seen in the area 

 at that season (see 2541). The buoys ^ to 6^ in figure 29 are not in alinement in order 

 to show the small angles observed at the buoys. 



The ship positions should be strong three-point fixes, as indicated in 1 to 10, 

 figure 29, a check angle being observed at each if sufficient shore signals are visible. 



\1/ 



Approximate Scale 



Figure 29.— Offshore buoys located by cuts from three-point fix positions nearer shore. 



