Page 861 miscellaneous 941 



Where there are buoy positions computed from observations to shore stations, it 

 is advisable to use a fourth binder for these computations. 



Where the total of the buoy positions is small, all of the above forms and computa- 

 tions can be conveniently kept in one binder. 



Neither the observations nor the computations of a buoy traverse are self-checking, 

 as are triangulation observations and computations. The accuracy of a buoy traverse 

 is only known from its closure error, and frequently much surveying is done using buoy 

 stations of a traverse before it is finally closed and this error is known. In order to 

 ensure that errors are not made in the computations, every step must be checked, and 

 duplicate position computations should be made for the geographic positions (see 9441). 

 Particular care should be given to the verification of traverse adjustments (see 944). 



941. Reduction of Sun-Azimuth Observations 



The azimuths of a buoy traverse are usually derived from inclined angles measured 

 between the sun and two buoys in range. The sun's altitude above the visible sea hori- 

 zon must be measured simultaneously with each inclined angle. The inclined angle 

 must be measured to the horizon in line with the buoys and not to some point on the 

 superstructure of a buoy which is above or below the visible sea horizon. 



The fundamental formula for the reduction of an inclined angle to the horizontal is: 



cos i 



cos A = 1 



cos h 



in which A is the approximate horizontal angle, i is the observed inclined angle, and h 

 is the observed altitude of the sun. This is the first formula given in 3338. 



For precise results such as are needed in buoy traverses a small correction known 

 as the dip correction must be applied to each horizontal angle computed from the above 

 formula. Buoy-traverse azimuths are computed on Form 720, Azimuth by Inclined 

 Angle. The dip correction to be applied is explained on the back of this form and a 

 graph is included to assist in the derivation of the correction. 



Occasionally, it is necessary to determine the azimuth between a buoy and an ele- 

 vated shore station beyond the visible sea horizon. An additional correction is requu'ed 

 in such cases. Where the observed point on the shore station is above the visible 

 sea horizon the correction, which is always positive, may be found from the following 

 formula : 



,. /. ■ ^ \ h' tan h 



correction {m minutes) 



sin A (approximate) 



in which h' is the measured angular height, in minutes, of the observed point above 

 the visible horizon, and h and A are as in the preceding formula. 



In the case of an elevated shore station, as described above, the corrected hori- 

 zontal angle may also be obtained from the second formula given in 3338. 



942. Back Azimuth Correction 



The sun azimuths in a buoy traverse are observed in the direction which is most 

 convenient, depending on the relation between the directions of the line of buoys and 

 the sun. The traverse, however, may not be computed in this same direction and 

 when it is not, it is necessary to determine back azimuths just as in geodetic com- 

 putations. 



