258 HYDROGRAPHIC MANUAL PaGE 146 



be practicable to connect the offshore end of the scheme with another similar scheme 

 approximately parallel to it extending offshore from the adjacent land area. 



At times when, and in areas where, the R.A.R. method of control operates efficiently, 

 the size of the quadrilaterals may be large, but they must be correspondingly reduced 

 in an area in which the method is not so efficient. Banks and irregular depths in the 

 inshore areas frequently interfere with the measurement of subaqueous distances and 

 it is necessary to start the scheme from a closed taut-wire traverse. The traverse need 

 extend offshore only far enough to establish reliable positions beyond the area of uTegu- 

 lar depths, the remaining stations in the scheme being located from the positions of 

 sono-radio buoys in the traverse (see 2573) . 



In a scheme of this character it is likely that the lengths of certain lines cannot be 

 measured by subaqueous methods because of shoals or intervening irregular depths 

 within the area. Such a line may be measured by taut wire or by one of the two meth- 

 ods illustrated in figure 41. The diagonal between buoys E and G could not be meas- 

 ured, but the position of buoy G, determmed by measurements of the distances from 

 F to G and H to G, may be verified by a distance to buoy G obtained from position 6 

 which in turn has been located by acoustic distances from C, F, and E. A position for 

 this purpose must be carefully selected so that it may be accurately determined and the 

 measurements must be made with extra care. 



A position on the shoal, as shown at position 7, shoidd be used with extreme cau- 

 tion. A return will probably be received from buoy G but, because of the intervening 

 shoal, the path of the sound wave is uncertain and consequently there will be uncer- 

 tainty in the computed distance. 



In the adjoining quadrilateral neither of the diagonals could be measured directly 

 because of an intervening shoal, but from the supplemental buoy / it was possible to 

 measure distances to the fom- buoys at the corners of the quadrilateral. With these 

 and the sides of the quadrilateral measured, an adjusted graphic plot will determine the 

 coordinated positions. 



Positions in a scheme of this type are determined by a graphic plot on one or more 

 aluminum-mounted sheets as explained in 2573. 



Offshore areas are likely to contain shoals which require thorough development 

 and lines of soundings more closely spaced than is practicable by R.A.R. On or near 

 such an area a line of buoys may be anchored to furnish control for three-point fixes. 

 The buoys should be established in a line parallel to the axis of the shoal. The end 

 buoys, L and M in figure 41, are located by subaqueous distances from the adjacent 

 sono-radio buoys. The intermediate buoys in the line are located by sun azimuths and 

 taut-wire distances and their positions are computed as an adjusted traverse between 

 the end buoys (see 2561). The positions of the survey buoys, when located in this 

 manner, are referenced with sufficient accuracy to the positions of the sono-radio buoys 

 so that the positions in the entire scheme are adequately coordinated. 



258. Statistics of Buoy-Controlled Surveys 



Hydrographic surveys in areas where buoy stations are required for the control, 

 either for sextant fixes or R.A.R. methods, are different from hydrographic surveys 

 controlled from shore stations. They require a different planning and a different 

 technique of execution. The tabulation is intended as a guide to the requirements and 

 possible results to be expected under different conditions in different areas. The statis- 

 tics are from actual typical projects controlled by the different types of buoy schemes 

 described in 255, 256, and 257. General information regarding each project is given. 



