Plane Table and Forest Maps 451 



the sights are often several miles in length, it is worth while to 

 make corrections for curvature and refraction which can be read 

 from a simple table. The signals will, in many cases, be used 

 as turning points, the elevation of the point being taken as that 

 of the flag, a reduction to ground level being made when this 

 value is needed. When any station is occupied, the difference 

 between the height of the instrument and height of signal must 

 be determined. The angles which are used should be as large 

 as possible. An error of 1' in the vertical angle at 2° would give 

 a resulting error in elevation twice what the error for 1' would 

 be at 6°. 



Secondary Vertical Control. — The instruments for obtaining 

 secondary vertical control are : Forest Service Standard Com- 

 pass, with slot in staff head, Abney level, or the telescopic ali- 

 dade. The compass is used either on a peg fitting the traverse 

 board tripod head or set directly on the tripod when the new 

 style traverse board which fits the regular tripod head is used. 

 The Abney level when fitted to a staff head is to be preferred to 

 the compass. 



Use of Tables. — The attached table (adapted from Wilson's 

 "Topographic Surveying") and chart will save considerably 

 calculations. For the primary work the table would give supe- 

 rior accuracy, but one multiplication being necessary. The accu- 

 racy of reading the values from the chart will equal that to 

 which the secondary angles can be read. Mounted on cardboard, 

 it can be carried in the plane table or traverse board case. 



For secondary work the following general formula may be 

 of use : 



h =: .02 X (I'd, less 4% for angles up to 10°, 

 where h is difference in altitude in feet, a' is the vertical angle 

 expressed in minutes, and d is distance in chains. For angles 

 from 10° -14° reduce the result by 2 per cent and angles over 

 14° reduce by 1 per cent. In order to do away with the percent- 

 age reductions in slide rule computations, the following values 

 may be used : 



h = .0192 X a'd, up to 5°59' 

 h = .0193 X a'd, from 6° to 8°59' 

 h — .0194 X a'd, from 9° to 11° 59' 

 h = .0196 X a'd, from 12° to 16° 



