132 



HYDROGRAPHIC MANUAL 



Table 8. — Statute miles to meters 

 (Conversion factor: 1 statute mile = 1,609.347219 meters.) 



pass through the three points at equal 

 distance. Points on the other circles to be 

 drawn can be located by sub-dividing the 

 radial lines. All computations should be re- 

 tained for use in constructing the smooth 

 sheet. 



The distance circles can be drawn by one 

 of three methods. Plastic arcs of varying 

 radii are available for drawing EPI and 

 Shoran circles, or can be constructed if there 

 is sufficient need to justify their cost (see 

 3-140). Other types of standard curves may 

 be used. When the arcs are not long enough 

 to permit drawing a continuous curve the 

 full length of a sheet, four points should be 

 computed and plotted so that the curve can 

 be placed to coincide with three points along 

 the arc. 



If the station is only a few feet from the 

 edge of the sheet, the station center may be 

 located by drawing intersecting arcs from 

 the computed points on the nearest circle. 

 The boat sheet should be laid flat and secured 

 in position at one end of the drafting table 

 and a section of bristol board or other suit- 

 able material fastened to the table at the 

 approximate location of the station. The arcs 

 from the three nearest points should inter- 

 sect at one point and should be checked from 



other computed points before the circles are 

 inked. 



A third method of drawing the circles is 

 shown in Figure 52. Two pins are set firmly 

 at positions A and B. The angle D equals 

 180° — 1/2X and this holds true for any 

 point along the arc AB. The angle D is set 

 on a metal protractor using the movable arm 

 that can be closed to a zero reading. A pen- 

 cil is centered in the protractor and the arc 

 is drawn by moving it along with the arms 

 sliding against the pins. The arc EC can 

 be drawn in a similar manner using the 

 angular difference in azimuths of the lines 

 PB and PC as X. The portion of the arc 

 that falls beyond A or C is plotted with the 

 protractor set at '^X. This method is ap- 

 proved for boat sheet plotting only. 



5-12 Distance circle intervals. — Odessey 

 protractors (see 3-22) are used to plot posi- 

 tions determined by electronic ranging sys- 

 tems. Although protractors can be con- 

 structed to fit any desired interval of 

 distance circles, the general practice is to 

 draw the circles at intervals which are ap- 

 proximately equal to latitude projection in- 

 tervals so that the protractors will be of 

 moderate size and easy to manipulate. The 

 spacing of the distance circles with relation 



