943 HYDROGRAPHIC MANUAL PaGE 862 



The difference between a forward jind back azimuth is designated Aa. Its value 

 varies with Latitude, azimuth, and the distance between buoys; its value is zero for 

 north-south azimuths and a maximum for east-west azimuths. Aa in buoy traverses 

 is identical with the same value in geodetic computations (see Form 27). Its approxi- 

 mate value may be found with sufficient accuracy for use in computing buoy traverses 

 by using the dead-reckoning positions of the buoys, the taut-wire distances between 

 them, and the sun azimuth. The computation may be made on Form 27, using five- 

 place logarithms. 



Where the back azimuth is needed, the obsers^ed azinmth should be corrected 

 by Aa, as found above, and the resulting azimuth used in the position computations 

 of the buoys, after which, w^ith these more accurate data, a new value of Aa is deter- 

 mined. If an appreciable difference is found, the geographic positions should be 

 recomputed, using the azimuth corrected for the second value of Aa. 



943. Scope of Buoy Anchor Cable 



Survey buoys anchored with only one cable, as is usual in the Coast and Geodetic 

 Survey (see 283), are free to swing around their anchors to various positions, depending 

 on the direction and velocity of the current. The horizontal distance between the 

 buoy and its anchor is called the scope. 



The scope varies with the length and weight of the anchor cable, the type of buoy 

 structure, the depth of water, and the method of anchoring (see 2836), as weU as with 

 the velocity of current. Scope is difficult to determine accurately because of the many 

 variable factors; and accurate experiments have not been made, on which to base 

 empirical formulas. Where scope is appreciable, an allowance should be made for 

 it in buoy computations and even where their positions are determined graphically. 



Scope should be determined and allowed for in a standard manner. "VVTiere 

 buoys are anchored with the cable recommended in 2832, the following formulas have 

 been found to give approximate scope with sufficient accuracy for various estimated 

 current velocities, and for galvanized wire rope and chain. 



In a weak current ^=1.8288 (L — D). 



In a moderate current 5=1. 3716(L-D) + 0^4072Vi'^-.D2. 

 In a strong current 5=0.9144(L-D + V^^-£>2 ). 



in which S is the scope {in meters), D is the depth of water (in fathoms), and L is the 

 total length of anchor cable {in fathoms). 



Where a relieving buoy is used, the horizontal distance {in meters) between the 

 relieving buoy and the buoy structure must be added to the resulting S values. The 

 length L must be reduced where the anchor cable is found to be fouled on the anchor, 

 or where its effective length is reduced from any cause. 



Provision may be made to measure the approximate scope, by anchoring a small 

 marker buoy to the anchor of the survey buoy by a light line. The length of this 

 line should only slightly exceed the depth of water plus the range of tide. The scope 

 can then be derived from simultaneous depression angles measured to the two buoys 

 when in range. 



9431. Correction of Distance for Scope 



If the velocity and direction of the current at each buoy station are estimated 

 at the time the distances are measured by taut wire, the measured distances may be 

 corrected for scope to obtain distances between the buoy anchors. The scope can be 

 computed as explained in 943. For the computation, the azimuth between two buoy 



