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EQUIPMENT AND INSTRUMENTS 



4821 



A model for a special-purpose scale to be reproduced at the Washington Office 

 should be prepared with great accuracy by the field party on bleached aluminum, metal- 

 mounted paper, or smooth-sheet paper. As the scale will be reproduced photographi- 

 cally, it is preferable that the model be prepared at an enlarged scale and that the 

 exact size to which it is to be reduced be accurately indicated. 



Special-purpose scales may often be used to speed the operations of plotting or 

 scaling and in such cases their use is recommended. 



4821. Meter Bar 



The meter bar (A in fig. 91), so called because its graduated scale is 1 meter in 

 length, is the standard scale for all measurements used in constructing projections, 

 plotting distance circles, or other operations involving comparatively long distances. It 

 is graduated on one side at a scale of 1:10,000, and on the reverse side at a scale of 

 1 :20,000. The bar may be used for other scales by applying a simple ratio to the distance 



1 2 3 4 5 6 7 8 9 



0^! 1 2 3 4 5 6 ^ 8 9 



Figure 92.— Metric scale, a =100.0 meters. 6 = 110.0 meters. c= 111.0 meters. d = lll. 5 meters. 



to be scaled. For example, a 10,000-meter distance on a 1:40,000 scale map will equal 

 5,000 meters on the 1:20,000 scale of the meter bar; or similarly, 2,500 meters on 

 the 1:10,000 scale. 



All meter bars now acquired are tested for accuracy and calibrated by the National 

 Bureau of Standards, the allowable error being 0.1 mm for any distance. A copy of the 

 calibration test for a bar will usually be found inside the cover of the box in which the 

 meter bar is furnished. Older bars, for which there are no data as to accuracy, should 

 be carefully verified before use by comparison with a standard of known accuracy. 



In addition to the meter bar, quarter-meter scales (B in fig. 91) and eighth-meter 

 scales are available. These are not tested for accuracy, and should be compared with a 

 meter bar of known accuracy before use. The quarter-meter scale can also be furnished 

 at scales of 1:8,000 and 1:12,000. 



The quarter-meter scale is especially convenient to use for plotting stations or for 

 scaling other comparatively short distances. The eighth-meter scale is furnished as 

 part of the equipment of a planetable alidade and is usually attached to the straightedge 

 of the alidade; separate scales, however, are also available. 



Method of use. — The length of a meter bar is divided into spaces representing 100 

 meters each at the scale of the bar (see fig. 92). The width of the bar is divided into 

 10 equal spaces by numbered parallel lines. At the left end of the bar one 100-meter 

 space is subdivided by 10 parallel diagonal lines in such a way that distances to tenths 

 of a meter may be scaled as illustrated. 



In setting a distance on the beam compass or dividers, the point of the leg contain- 

 ing the slow-motion screw should be placed on the appropriate 100-meter vertical division 



