108 



HYDROGRAPHIC MANUAL 



Figure 46. — Drafting instruments. A. 

 scale. D. Hairspring dividers. E. 

 measure. J. Beam compass. 



Meter bar. B. One-quarter meter scale. C. Latitude and longitude 

 Spacing dividers. F. Proportional dividers. G. Opisometer. H. Map 



pass fixtures which slide on the bar and can 

 be clamped in any desired positions. It is 

 used for measuring distances too long for 

 ordinary dividers and to swing distance cir- 

 cles on hydrographic sheets. It is essential 

 in making projections in the field. Bars are 

 available in lengths from 24 to 60 inches, in 

 multiples of 6 inches, and can be furnished 

 in much greater lengths for special purposes. 

 When using a beam compass to draw dis- 

 tance circles on a smooth sheet (see 5-11), 

 the Edmonston Beam Holder (Fig. 47) should 

 be used when available. 



3-140 Miscellaneous drafting instru- 

 ments. — Various other drafting instruments 

 will be found in the plotting room of a sur- 

 vey ship. Steel straight edges in lengths 

 from 6 to 72 inches are available. Before 

 using a straight edge in any operation re- 

 quiring a truly straight line, it should be 

 carefully tested. 



To test a straight edge, two fine dots 

 should be pricked on a thick sheet of paper 

 at a distance apart slightly less than the 

 length of the straight edge. With the fiducial 



edge centered on the dots, draw a fine line 

 along it with a chisel-edged pencil held firmly 

 against the edge and at a constant angle with 

 the paper. The straight edge is then turned 

 end for end and again centered on the 

 pricked points. If the fiducial edge coincides 

 with the pencil line throughout its length, 

 it is straight, unless it happens to have a 

 symmetrical reverse curve. 



Most of the lettering on smooth sheets, 

 planetable sheets, and sketches must be done 

 with a mechanical lettering set. The Leroy 

 lettering .set has been adopted as the stand- 

 ard for most work of this kind although the 

 Wrico sets are used in a few cases. 



Curves made of heavy plastic are available 

 for drawing distance arcs when it is im- 

 practical to draw them with a beam com- 

 pass (see 5-11). The Shoran curves are 

 furnished at a scale of 1 :20,000 and with 

 radii at one-mile intervals from 11 to 70 

 statute miles. EPI curves are furnished at a 

 scale of 1 : 100,000 ranging from 450 to 4,100 

 micro.seconds in multiples of 50. Curves for 

 other scales and distances can be provided if 



