THE USE OF THE PLANE TABLE IN MAKING FOREST 



MAPS 



By S. B. Locke 



This discussion will be limited to the application of plane 

 table methods of mapping to mountainous country with broken 

 cover, such as is typical on many of the National Forests in 

 central Idaho, where the valleys are generally narrow, the moun- 

 tains in high ranges with rocky crests, and the secondary drain- 

 age basins long and crooked. Only methods of field work which 

 experience has shown valuable and which are not generally 

 found in textbooks are here to be described. About twelve 

 months were spent in actual field work during the years 1910, 

 1912, and 1913, on the Salmon and Sawtooth National Forests 

 in Idaho. A topographic map was made of about 10 townships 

 showing cover types, and a timber estimate completed for about 

 4 townships. 



The instruments used during this time for primary triangula- 

 tion were the Johnson plane table, size 24"X31", telescopic ali- 

 dade, with a telescope having a focal length of 15 inches, giving 

 an inverted image, and having a power of 20 diameters. The sec- 

 ondary work was done with a brass sight alidade about 7 inches 

 long with 6 inch sights used on a 15"X15" traverse board. In 

 some cases the telescopic alidade was used on the traverse board. 

 A Forest Service Standard compass was used on a peg, fitted to 

 the traverse board to measure vertical angles for secondary work. 



The maps made by these methods are of two classes, the dif- 

 ference being that on one contours having a 100-foot interval 

 were sketched, while on the other no contours were drawn, the 

 elevation of the stations being calculated. The drainage sys- 

 tem and ridges are shown in detail, and a cover or type map made 

 for both classes. In rough country with broken cover, the dif- 

 ferent cover areas are in this way located more accurately than 

 would be done by paced compass lines. The scale on the field 

 sheets in both cases was 2 inches to the mile. 



Methods of Planning Control. — It is sometimes possible to lo- 

 cate a triangulation system over the area from bench marks and 

 triangular stations used by the U. S. Geological Survey. The 

 accuracy of location of these points is greater than will be 



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