124 



HYDROGRAPHIC SURVEYING 



cross-bars are attached to prevent the lead from slipping. 



At the bottom is a cup C covered with a washer W, which pre- 

 vents samples of material from being 

 washed out while the lead is being drawn 

 to the surface. 



Methods of Locating Soundings. 

 Soundings are usually made on well-defined 

 lines called ranges. The position of each 

 sounding is located by various methods, 

 depending on local conditions, the degree of 

 accuracy required, etc. The most import- 

 ant methods are as follows: 



1. By Time Intervals. The soundings 

 are made at stated intervals of time from 



a boat moving at uniform speed along a range. The distance 



between the end soundings being known, the position of each 



sounding can be deter- 



mined by proportion. 

 2. By One Angle 



Measured on the Shore. 



The ranges are fixed 



with regard to ashore 



base line AB, Fig. 5, 



and the position of a 



sounding as C is found 



by the intersection of 



FIG. 4 



the range line with the 

 line AC, the angle of 

 which with AB is meas- 

 ured with a transit loca- 

 ted at A. 



3. By Two Angles 

 Measured Simultane- 

 ously on Shore. A tran- 

 sit is also placed at B, 

 Fig. 5, and the angle 

 CBA is measured simul- 

 taneously with the angle CAB, the position of C being deter- 

 mined by the intersection of the lines AC and BC. The ranges 



FIG. 



