Page 863 miscellaneous 9432 



anchors may be assumed to be the same as the azimuth between the respective buoys, 

 and the lead of the anchor cable may be assumed to be the same as the direction of the 

 current. Let the difference between the latter and the azimuth of the buoys be angle A. 

 Then the scope multiplied by cosine A is the correction for distance and the scope mul- 

 tiplied by sine A is the eccentric distance normal to the line between buoys. The first 

 value is used to correct measured distances to distances between buoy anchors; the 

 second is used in correcting observed azimuths to azimuths between buoy anchors 

 (see 9432). 



Care must be taken to apply the correction for distance with the proper sign. It 

 is advisable to make a sketch of the line of buoys, showing the relative direction of the 

 current at each buoy, for use as a guide in applying the corrections. 



9432. Correction of A zimuth for Scope 



If the scale of the survey or the desired accuracy warrants it, the azimuths ob- 

 served between buoy structures may be reduced to azimuths between their anchors 

 by applying corrections for eccentricity. The scope is found as in 943 and reduced to 

 a distance normal to the line between buoys as in 9431. The eccentric correction is 

 based on the relative positions of the two buoys and their respective anchors, bearing 

 in mind that the measured distance and azimuth are between buoy structures. The 

 eccentric correction for azimuth may be found from the follow^ing relation: 



_ eccentric distance normal to line of buoys 

 ~ distance between buoys 



in which a is the small angular correction to azimuth, and the two distances are in the 

 same units of length. 



If the two eccentric distances normal to the line of buoys are combined, by addi- 

 tion if in opposite directions and by subtraction if in the same direction, only one com- 

 putation is required to find the angle a. 



If logarithms are not used in the formula, the result will be the natural tangent of 

 a, which may be divided by 0.00029 (the natural tangent of 1 minute) to find the value 

 of a in minutes. Using natural functions, the entire computation may be made with 

 sufficient accuracy on a slide rule. 



944. Buoy Traverse 



Buoy traverses are essentially the same as random traverses on land, except that 

 the azimuths of the various portions of a buoy traverse are each measured independ- 

 ently, instead of being carried forward from an initial line through conventional hori- 

 zontal angles measured at the turning points. The horizontal distances between buoy 

 stations are measured by taut wire or log. With the azimuths and distances between 

 adjacent buoy stations known, the positions in a traverse may be computed by one 

 of the methods described in 9441 and 9442. 



Traverse computations are not difficult but attention must be paid to detail. The 

 reduction of the distances and azimuths for scope, the computation of the positions, 

 and the adjustment of a traverse, are operations in which errors are easily made unless 

 the strictest attention is paid to the proper application of the various corrections. 

 All abstracts should either be carefully checked or be prepared independently by two 

 different persons as a check. Computations of buoy positions by the field party are 

 assumed to be correct — they are not checked at the Washington Office. 



