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EQUIPMENT AND INSTRUMENTS 



4826 



The spaces between the vertical lines representing speeds should be large enough so that tenths of knots may be interpolated by 

 estimation and so that none of the diagonal lines intersects them at too acute an angle. 



A second type of speed scale (fig. 94), used principally by verifiers in the Washington OflSce, is better adapted for use on surveys of 

 larger scales. Its principle is the same, but it is constructed so that the diagonal lines represent the various rates of speed and the 

 horizontal scale may be graduated for use at a number of different scales. 



Speed in knots 



Figure 93.— Graphic speed scale for field use. 



The scale is a grid on which the spacings along the horizontal axis represent distances in meters, the spacings along the vertical 

 axis represent minutes of time, and the diagonal lines represent speed in knots. Such a scale is constructed for a selected survey scale, 

 e. g., 1:10,000. To construct the grid, lay off along a horizontal line 100-meter distances at the desired scale. At the zero point erect a 

 perpendicular and arbitrarily divide it into equally spaced intervals representing minutes of time. Complete the grid by drawing 

 vertical and horizontal lines through the points of graduation. The minute intervals may be further subdivided for closer determina- 

 tions. 



( DISTANCE IN METERS 



Scale 1:10,000 



30.9 S T 

 Time in minutes 

 Speed in knots 

 Distance in meters 



Figure 94.— Graphic speed scale for oflSce use. 



The diagonal lines may be drawn as follows: Select any speed, e. g., 10 knots. In 3 minutes at 10 knots a vessel travels ^ nautical 

 mile or H minute of latitude. In the case illustrated, this is 926.5 meters. From this point on the horizontal line representing 3 min- 

 utes, for a scale of 1:10,000, draw a diagonaUine to the origin. This diagonal line represents a speed of 10 knots. The positions of 

 other diagonal lines representing other speeds may be similarly determined, or the distance along any horizontal line from the zero 

 vertical line to the diagonal line representing 10 knots may be subdivided into 10 equal parts and these points may be connected with 

 the origin of the scale by diagonal lines, each of which will represent its respective speed. 



