14 A TREATISE ON DREDGES AND DRED(iI\; 



The Survey. The second important opeiation required in 

 hydraulic survey is the correct determination of the points where 

 soundings were made. This is obtained in three different ways, 

 which are: first, by triangulation from shore stations; second, 

 by sextant observations from the boat; third, by running the 

 boat over known ranges and spacing the soundings by time. 



First. The location of the various points where soundings were 

 made is determined very correctly by means of triangulation from 

 shore stations. A base line is selected along the shores so as to 

 command a large view of the locality to be surveyed. The line is 

 correctly measured either directly or by triangulation. At the 

 extreme ends of the base line are set up two transits, with very 

 distinct graduation, in order to make angle readings very promptly 

 when signals are given. The operators take simultaneous " shots " 

 or readings of the angles at the same instant that the soundings 

 are made. These are obtained by means of conventional signals 

 from the men in the boat, who indicate the moment that the 

 readings should be taken. 



Second. Soundings are located also by sextant observations 

 by the men in the boat. For this purpose three points are selected 

 on land, such as can be seen along the line of the improve- 

 ment to be surveyed and whose geographical position has been 

 correctly determined. Soundings are made by a surveying party 

 on a boat running with a velocity of 1} or 2 miles per hour; while 

 the location of the various points is determined by reading with 

 sextants the angles that the boat makes with the three land points. 

 This method is now extensively used by the U. S. Coast and Geodetic 

 Survey and it is based upon a simple principle of geometry. 



Let L, C, R (see Fig. 1) be the three fixed land points and P 

 the unknown position of the boat. The angles A and B arc read 

 with sextant. 



Draw two circles, one passing through L, P and (7, and the 

 second through C, P, R. Now from geometry it is known that 

 by connecting any point of the circumference with two other fixed 

 points on the same circumference, the angle will be constant. The 

 two circles intersect at the points C and P, and since C is one of 

 the fixed points, so there is but one point P where it is possible to 

 have the angle C, P, R=A and C, P, L=B. Therefore these two 

 angles determine the position of P. 



By setting off on a three-armed protractor the two annles A 



