5. HYDROGRAPHY 



133 



to the scale of the survey should be according 

 to Table 9. The spacing of Raydist distance 

 circles shown in the table is suitable for 

 presently assigned frequencies and may be 

 changed when different frequencies are 

 employed. 



5-13 Circle sheets for plotting sextant 

 fixes. — Since electronic positioning equip- 

 ment has been adapted to control of hydro- 

 graphic surveys, the use of visual control 

 methods is limited almost exclusively to in- 

 shore surveys. Sextant fixes can be plotted 

 with three-arm protractors quickly and easi- 

 ly in most instances. However, the hydrog- 

 rapher may at times be required to use long 

 extensions on a protractor when plotting 

 sextant fixes on large scale surveys. This 

 is a time consuming procedure; and, if the 

 projection is distorted, the plot will be in- 

 accurate. To facilitate the plotting and 

 avoid inaccuracies due to distortion, a sys- 

 tem of intersecting arcs of circles, some- 

 times called circles of equal angle, may be 

 drawn on the sheet, each circle being the 

 locus of some angle between two control sta- 

 tions. A position may then be plotted at the 

 intersection of the loci of the two observed 

 angles, each locus being found by interpola- 

 tion between the adjacent arcs drawn on 

 the sheet. A control station common to both 

 angles is ordinarily used, but this is not 

 necessary. 



Circle sheets have been used extensively 

 in the past to plot large scale offshore sur- 

 veys although few if any of the signals could 

 be plotted on the sheet. Modern survey prac- 



Figure 52.- 



Angle D=180°- -f 

 X^^angular difference 

 between azimuths 

 PA 2 and PB2 

 P 



-Drawing arcs with a metal three-arm 

 protractor. 



tices are such that this condition should 

 rarely exist, therefore only the simplest form 

 of the problem will be discussed, that is; 

 when all stations and the lines of centers fall 

 on the sheet. 



To construct the circles, scale or compute 

 the distance "a" between the two stations 

 A and B (Fig. 53) and plot the mid point 

 "D" of the line joining the two stations. 

 Erect a perpendicular at the mid point. The 

 center of any circle generated by an angle 

 ex is on this line at a distance "d" from the 

 mid point "D" and the radius of the circle 

 is apparent. Compute "d" distances for the 

 range of angles required by substituting in 



Table 9. — Distance circle intervals and units 



Scale of 

 Survey 



10,000 



20,000 



30,000 



40,000 



60,000 



80,000 



100,000 



120,000 



160,000 



200,000 



300,000 



400,000 



500,000 



800,000 



ShoTan 



Sta. 



Sta. 



Sta. 



Sta. 



Sta. 



5 Sta. 



5 Sta. 



5 Sta. 



10 Sta. 



Mi 



Mi 

 Mi 

 Mi 

 Mi 

 Mi 

 Mi 

 Mi 

 Mi 



Circle Intervals and Units 

 EPl 



50 M 

 50 M 

 50 M 

 100 M 

 100 M 

 200 M 

 200 M 

 250 M 

 500 M 



icroseconds 

 icroseconds 

 icroseconds 

 icroseconds 

 icroseconds 

 icroseconds 

 icroseconds 

 icroseconds 

 icroseconds 



Raydiat 



20 Lanes. 



50 Lanes. 

 100 Lanes. 

 100 Lanes. 

 200 Lanes. 

 200 Lanes. 

 200 Lanes. 

 200 Lanes. 

 400 Lanes. 

 400 Lanes. 



