A METHOD OF USING THE ANEROID BAROMETER IN 

 CONTOUR MAPPING. 



By C. F. Korstian. 



One of the principal features of the reconnaissance work 

 done on the Snoqualmie National Forest in 1910, was the collec- 

 tion of data for a topographic description of the Forest. This 

 data was intended to check existing maps and show increased 

 topographic detail and minor features not previously located. 

 The best maps up to this time were too general for the require- 

 ments of the Forest. 



The topography of the area included within the boundaries 

 of the Forest is, in general, quite rugged. The mountain slopes 

 vary from moderate to precipitous ; cliffs one hundred to two 

 hundred feet high are numerous and the mountains often rise 

 2,500 to 4,000 feet above the valleys. In that part of the Forest 

 in which the reconnaissance was made, the absolute elevation of 

 the highest peaks ranged from 5,000 to 9,000 feet above sea 

 level. A rise of 400 to 500 feet for each strip acre, ten chains 

 in length, was not uncommon. For obtaining elevations in this 

 work aneroid barometers were used almost exclusively. Most 

 of them were small enough to be carried in the pocket and read 

 only to units of 100 feet. However, ten-foot readings could 

 easily be interpolated. A few of the instruments were gradu- 

 ated to ten-foot units and could be interpolated to one-foot read- 

 ings, but they do not register as high elevations as those with 

 graduations of 100 feet. In taking elevations for most contour 

 maps, aneroids with units of 100 feet give sufficiently accurate 

 results to warrant their use. 



Before reading the aneroid it is always advisable to tap 

 gently on the glass cover to make sure that the system of levers 

 is in proper working order. It should then be held in a horizontal 

 position directly in front of the eyes, and by glancing along the 

 needle the reading can be made directly over the point, and the 

 readings will all be uniform. 



On account of the variation in atmospheric pressure from 

 time to time, it was considered necessary on this reconnaissance 

 work to keep from tw r o to four aneroids in camp, and to read 

 them approximately every hour. There is an advantage in using 

 two or more aneroids in camp, for the average of their readings 

 gives greater accuracy. The man who stayed in camp to take 

 these readings employed the time in transferring the field 

 sketches to a four inch scale standard map sheet. The fluctuation 



