HYDROGRAPHIC SURVEYING 



123 



The outline of a body of water is determined by means of a 



traverse and offsets from the line of survey, as shown in Fig. 1, 



or by triangulation, an 



example of which 



method is shown in Fig. 



2. A carefully meas- 

 ured base line, as 1-8, is 



selected, and the angles 



of all the triangles are 



measured. From the 



triangle 1-2-8, in which 



the side 1-3 and the 



angles are known, the* 



sides 1-2 and 2-3 are 



computed by trigonometry. Then, in the triangle 2-3-4, the 



side 2-3 and the angles are known and the other sides are calcu- 

 lated; and so on with the other triangles. In the 

 last triangle, a side, as 5-7, may be measured as a 

 check on the work. 



FIG. 2 



SOUNDINGS 



The configuration of the bottom of a body of 

 water is determined by means of soundings. For 

 depths of 18 ft. or less, sounding poles are used. 

 The lower portion of one form of sounding pole is 

 shown in Fig. 3. It is made of white pine 3 to 

 3^ in. in diameter at the bottom and 2 to 2$ in. at 

 the upper end. It is fitted with a disk-shaped iron 

 r^J| shoe, which prevents the rod from sinking into soft 

 V i mud. The bottom of the shoe is sometimes hol- 



\ lowed out for the purpose of bringing up samples 



of materials. 



For depths greater than 18 ft., a lead line is 

 used in making soundings. It consists of twisted 

 hemp or closely plaited linen, about f in. in diam- 

 eter, to the end of which is secured a weight called the lead. 

 One form of lead L having the cross-section 5 is shown in 

 Fig. 4. It is molded around an iron rod R to which small 



FIG. 3 



