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HYDROGRAPHIC SURVEYING 



error is on the arc, that is, if the index is to the left of the zero 

 mark, the error is subtractive, or minus, and should be sub- 

 tracted from all readings. 



EXAMPLE. The angular distance between two objects, as 

 measured with a sextant, reads on the vernier 35 36' 30*. 

 What is the true angular distance if the index error of the sex- 

 tant is: (a) + l' 20"; (&)-!' 40"? 



SOLUTION. (a) Since the vernier reading is 35 36' 30" and 

 the index error is + 1' 20", the true angular distance is equal to 

 35 36' 30"+ 1' 20" = 35 37' 50". 



(b) Since in this case the index error is 1' 40", the true 

 angular distance is equal to 35 36' 30"-!' 40" = 35 34' 50". 



VOLUME OF A RESERVOIR 



By means of the platted soundings a contour map is prepared 

 in the manner explained under the heading Topographic 



Surveying; the outline of the reservoir being the surface con- 

 tour. The contour interval is fixed according to the slopes of 

 the valley and the degree of accuracy required. The volume 

 of water included between two plane surfaces passing through 

 two adjacent contours is that of a prismoid whose bases are 

 those surfaces included by the contour lines and whose height 

 is the contour interval. The sum of the volumes of the sev- 

 eral prismoids will be the volume in the reservoir. When the 

 number of prismoids is even, the following expression which is 

 based on the prismoidal formula, will give the total volume V. 



