SOUNDINGS AND HYDRAULIC SURVEYS 



15 



and B and shifting it until the plotted position of the points R, 

 C and L are each on the edge of an arm, the center of the protractor 

 will be on the point of sounding where the angles were taken. There 

 is but one exception to this rule ; that is, when the point P is on the 

 same circumference passing through R, C and L; then its position 

 is indeterminate (see Fig. 2); generally when signals are from 3 

 to o miles the angles read within 2 or 3 minutes of their correct 

 value, the point can be plotted within 5 or 6 ft. of its true position. 

 This is sufficiently accurate for hydrographic work, for on any scale 

 smaller than -nj-J-^nr 5 or 6 ft. is not a hair breadth and very few 

 charts are on a scale larger than this. 



FIG. 1. 



FIG. 2. 



Third. Another method is by running a boat over a known range 

 at a certain speed and taking soundings every 1, 2 or 5 minutes, 

 or at intervals depending upon the importance of the proposed 

 improvement. Thus, for instance, if a boat is running at a speed 

 of 2 miles per hour and soundings are made every 3 minutes, they 

 will be 80 ft. apart, and when the course of the boat is known it 

 is a very simple matter to locate the various soundings, thus obtain- 

 ing a survey of the bottom. 



No matter which method is used, in taking and locating the 

 soundings it is evident that a topographical map of the bottom 

 can be easily drawn from the field notes. Great care, however, 

 should be taken in reducing the soundings to an ideal level, which 

 is generally the mean low water mark. Consequently the correct 

 time at which the soundings are made should be accurately recorded 

 so as to compare them with the height of the tide at that time, and 

 all the soundings reduced to a common plan, which is the surface 

 of the mean low water. 



Measurement. .V topographical map of the bottom of the body 

 of water, indicating the depths of various points of the bottom from 

 the surface of the water, with mean low water as a datum line, is 



