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HYDROGRAPHIC MANUAL 



Page 142 



mic observations. This gives sufficient accuracy for charting the survey, })ut the 

 principal object is to coordinate the survey as a whole. 



Approximale Scale 



Figure 38.— Buoy control for an isolated offshore shoal. 



The buoy station to be located by astronomic observations should be anchored 

 near the center of the area or in shoal water near the critical depths, as at A in figure 

 38. Observations to determine the position of this buoy should be made as described 

 in 2535. From this buoy as an origin, buoys are anchored in lines approximately along 

 the axis of the shoal so as to furnish control in the most efficient manner. Taut-wire 

 sun-azimuth measurements are made along these lines of buoys, which are computed 

 as unadjusted traverses. (See 2525.) It is wasted effort to locate the end stations by 

 sextant astronomic observations for the purpose of adjusting the traverse between 

 them, because the traverse measurements are much more accurate than the astro- 

 nomic observations. 



25 72. Station s for R.A.R. Con trol 



Along certain types of coasts it is practicable to establish R.A.R. stations far 

 enough offshore so that surveys can be controlled by this method to the desired offshore 

 limits. Along the Pacific Coast and the coast of Alaska the positions of sono-radio 

 buoys may be determined on clear days at a considerable distance offshore by three- 

 point fixes on high mountain peaks whose positions are known. 



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X y 



Approximate Scale 



Figure 39. — R.A.R. stations located by three-point fixes and acoustic distances. 



Frequently only one line of R.A.R. stations is necessary to furnish the control 

 for the desired offshore area, but stations must be so distributed that returns from 

 at least three R.A.R. stations may be expected throughout the area to be surveyed. 



