Page 141 



CONTROL AND SIGNAL BUILDING 



257 



in one continuous operation to eliminate any error that might result if it were measured 

 in sections.- The positions of the end buoys at A and C (fig. 37) should be determined 

 from theodolite angles measured simultaneously at three or more shore stations. 



Figure 37.— Taut-wire sun-azimuth traverse to extend geodetic datum. 



The traverse, starting at buoy A, running to buoy B, and continuing to a shore 

 connection at buoy C is computed as an ordinary traverse but with extra care and 

 refinement to obtain greater accuracy. A point connection is made at buoy B by 

 simultaneous observations from three shore stations on the isolated island which, 

 combined with a base and azimuth measurement, will furnish the required data from 

 which the geographic positions of the stations on the island may be computed. The 

 accuracy of such a scheme, although not as high as where a single straight traverse is 

 used, will be indicated by the traverse closure. 



A more accurate value of the distance will be obtained if the buoy traverse can be 

 arranged to extend in one straight line from the initial station to the tie-in station, 

 passing the isolated island at a satisfactory distance so that a point connection may 

 be made. This, however, will seldom be possible or economic in practice. 



A traverse for this purpose should never consist solely of measurements in two 

 directions along the same line of buoys (from A to B and from B to A) and computed 

 and adjusted as a closed traverse, for an apparently excellent agreement in distance 

 might be obtained, even though the taut-wire calibration were considerably in error. 



257. Buoy-Control Schemes for Location by Other Methods 



2571. Astronomic Location in an Isolated System 



To survey adequately an isolated offshore shoal area of critical depths, buoy 

 stations are required for control of a system of closely spaced sounding lines. If the 

 area is so far from shore stations that a taut-wire traverse connection is impracticable, 

 the geographic position of the survey is generally based on independent sextant astrono- 



