2283 HYDROGRAPHIC MANUAL PaGE 86 



standard methods, it is doubtful if the difference in accuracy obtainable would warrant 



it, since results approaching third-order accuracy may be obtained by this method. 



This type of control does not meet the requirements for third-order triangulation 



because the ship stations are not stationary and there is one concluded angle in every 



triangle. Stations located by this method shall be termed topographic stations, and 



shall be marked by standard topographic-station marks and be described on Form 524. 



(See 211.) 



2283. Buoy-Intersection Method 



A method similar to the ship-intersection method is by the use of a row of buoys, 

 or floating signals, anchored at strategic positions along the coast to form one side of 

 a scheme for a short distance. This method should be resorted to only when the type 

 of coast is such that the positions of the desired stations cannot be determined economi- 

 cally by standard methods and the water is not too deep to prevent anchoring buoys far 

 enough offshore to provide good strength of figures. The buoys shall be placed a 

 sufficient distance offshore so that the smallest distance angle in any triangle shall not 

 be less than 30°. 



^:^i^l^^^^;V:^ -'^^^ C^ig^^ galgj E j)^^^ 



Figure 10. — Establishing shore control by the buoy-intersection method. 



In figure 10, A, B, and F are established triangulation stations and control is desired between B and F. Intervisible stations are 

 located at C, D, and E and floating signals are anchored at W, X, Y, 'and Z. The method of observation is apparent from the figure. 

 Enough consecutive stations should be occupied at one time to permit simultaneous observations on any one buoy. Six to eight posi- 

 tions with a second-order direction theodolite should give results approaching third-order accuracy. No length checks are obtained 

 by this method, unless twice as many buoys are used for control (see 2282). A check on the work is obtained, however, when the 

 scheme is closed on F. Any discrepancy between the two determinations of F should be adjusted and proportioned among stations C, 

 D, and E. If a topographic survey by planetable traverse is contemplated, a field check of the positions will also be obtained as the 

 traverse proceeds from one station to another, the principal check being when the traverse reaches station F. 



Stations located by this method shall be marked and described as in 2281. 



2284. Offset Traverse 



Traverse is not to be considered a satisfactory substitute for triangulation when 

 the location of important control is involved. It may be resorted to, however, in 

 localities where triangulation is impracticable, or along stretches of shore where the 

 principal scheme of triangulation veers inland away from the coast. Complete 

 instructions for third-order traverse may be found in Chapter 4, Special Publication 

 No. 145, 



Where the terrain makes both triangulation and traverse by taped distances 

 impossible, a careful offset traverse may be substituted, which will approximate third- 

 order accuracy. A typical area where offset traverse may be advantageously used is 

 illustrated in figure 11. 



A scheme of triangulation is impracticable along the shore of the southern end of 

 Onward Island but it can be carried overland, spanning a depression between two 

 divides, to make a connection between the triangulation on the east and west coasts. 

 The coast between A and M rises abruptly to an elevation of about a thousand feet, 



