2563 



HYDROGRAPHIC MANUAL 



Page 140 



2563. Traverse With Added Shore Connections 



Where buoy stations are established for inshore hydrographic control it is frequently 

 possible and advisable to obtain additional shore connections where the configuration 

 of the coastline permits. Occasional shore connections increase the accuracy of posi- 

 tions in a buoy traverse and should be made when they require little additional time. 



In the scheme illustrated in figure 36, established for the control of hydrography beyond the visibility of shore signals, it is possible, 

 or necessary, to establish additional buoys in two lines extending inshore from buoy C and at each inshore buoy it is possible to obtain 

 a strong three-point fix to shore stations. The same taut-wire sun-azimuth measurements are required in this type of scheme, but 

 he computations differ in that a mean position of buoy C is determined to which all the lines are adjusted. 



Approximate Scale 



Figure 36.— Buoys located by traverse with additional shore connections. 



Each buoy line, from A to C, F to C, G to C, and E to C, is computed from the fixed position of the inshore buoy in the respective 

 line, resulting in four independent positions of buoy C. A mean position of buoy C is found by weighting the four positions inversely 

 according to the lengths of the respective lines. The positions in each traverse line are then adjusted proportionally to the fixed jwsi- 

 tions of the inshore buoys and the mean position of buoy C. 



Also illustrated in figure 36 is a short line of buoys extending from buoy B to buoy H. This is called a spur line and all of the 

 positions in such a line are located with the same accurate observations, but it is adjusted by applying the correction to buoy B to 

 all buoy positions in the spur line, without additional adjustment. 



2564. Traverse to Establish Datum 



A carefully planned buoy traverse may be used to extend a geodetic datum to an 

 isolated island, where it is impossible or difficult to do so by triangulation. If the 

 distances are measured with an accurately calibrated taut-wire apparatus and the sun 

 azimuths are observed carefully, th^ datum provided by the traverse will be suf- 

 ficiently accurate for charting purposes. 



The lines of buoys should be planned to extend from the adjacent land areas to 

 the isolated island by the shortest practicable distances, and from points where strong 

 determinations may be obtained from established triangulation stations. (See fig. 37.) 

 Each buoy traverse should be practically straight and the two should be approximately 

 at right angles to each other. The interval between buoys should be a maximum in 

 order to reduce azimuth errors to a minimum. The sun azimuths must be observed 

 with unusual care and the necessary data must be obtained for correcting the azimuths 

 to the anchor positions of the buoys (see 9432). Each buoy traverse must be measured 



