763 



HYDROGRAPHIC MANUAL 



Page 702 



Wherever the locus of an angle is to be plotted, this can be done directly on the smooth sheet, 

 using the protractor centerpiece with a hole in it. Only the final position will then be pricked on the 

 sheet. Care must be taken, of course, to see that the underside of the protractor is clean. 



In (6), which usually occurs through a misunderstanding or error, the position is determined by 

 plotting separately the locus of each angle, the correct position being at the intersection of the two 

 arcs, assuming that they do not intersect at too acute an angle. 



LOCUS OF LEFT ANGLE 



\ 1/ 



i-< 



POSITION OF OBSERVATIONS '-°'^^^ °'' '"°"'' *'^'^'-^ 



Figure 159.— Sextain fix plotted by intersection of loci. No common center object. 



In (c), the recorded data may be complete, but the fix is weak, giving an inadequate determination 

 of the position (see 3332). There are two types of such fixes. In one type, the fix can be plotted in 

 one position but the result is erroneous because of the weakness of the fix in conjunction with other 

 factors. In the other type, known as a swinger or revolver, the fix can be plotted in any one of several 

 positions. In either type, the recorded data must be supplemented by the dead reckoning in order to 

 determine the most probable position. In the first case it is usually necessary to disregard that part of 

 the recorded data which has resulted in the erroneous position, while in the second case all of the re- 

 corded data may usually be used to plot a locus of the position. 



In (d) , it is generally not possible to obtain a three-point fix, and other data are usually recorded 

 by the hydrographer for plotting the position, such as an estimated distance from the high-water line, 

 or an estimated distance and direction from a control station. All such data should be plotted and 

 used in conjunction with the dead reckoning in order to arrive at the most probable positions, bearing 

 in mind that the distances are not measured distances, and that the general tendency is to under- 

 estimate distances over the water from a stationary position. (See also 335.) 



763. Plotting R.A.R. Positions 



Radio Acoustic Ranging (R.A.R.) is a comparatively new method of control for 

 hydrographic surveys. Notable changes have been' made in recent years in the design 

 of the equipment and in the field methods. 'These are described in detail in chapter 6. 

 Likewise, improvements have been made in the methods and procedure of plotting the 

 smooth sheet. It is expected that these will continue, but at the present time the 

 methods described herein are considered the most satisfactory and shall be used until 

 other methods are proved preferable. 



R.A.R. positions are determined by the intersection of distance arcs whose centers 

 are the R.A.R. stations from which the bomb returns were received. Like the distance 

 circles described in 7341 these arcs are plotted in units of time. Because of uncertainties 

 inherent in the R.A.R. method, due principally to the lack of complete knowledge of 



