76 REPORT OF THE 



HEIGHT MEASUREMENT. 



In order to calculate the quantity of timber in a forest it is necessary not only 

 to determine the number of trees of each diameter class of each species, but the 

 average height of the trees of each diameter class should also be determined. 



There are various instruments for measuring the height of a standing tree, all 

 based upon the principle of similar triangles, a principle familiar to all mathema- 

 ticians. The most convenient of these is Faustman's hypsometer, a small instru- 

 ment which can be carried in the pocket. In using this instrument, the observer 

 selects a convenient spot where he can distinctly see the top of the tree. Then 

 measuring his distance from the base of the tree, and arranging the instrument 

 accordingly, he looks at the top of the tree through an eye-piece on the instrument 

 and reads off the height of the tree as indicated by the thread of a plumb line rest- 

 ing against a scale. 



A " height party " consists of two men. One uses the hypsometer, while the 

 other takes the diameter with the calipers and measures the distance between the 

 trees and the observer. A party can measure from 200 to 400 trees per day. 



From 1,000 to 2,000 trees of each commercial species should be measured on a 

 township of, for instance, 30,000 acres of our forest. The greater the number, the 

 value, and the average diameter of the trees of a species, the greater should be the 

 number of heights taken. 



In taking heights, it has been found most convenient to measure on'^ species at a 

 time. It is not necessary to go regularly through the forest, but care should be 

 taken to measure trees growing under all conditions of soil, elevation, exposure, etc. 



METHODS OF WORKING UP THE RESULTS OF MEASUREMENT. 



Form Factors. 



A few words in regard to form factors are necessary for a clear understanding 

 of the methods of measurement described in this paper. 



The term " form factor" means the ratio between the volume of a tree and that 

 of a cylinder having the same base and height as the tree. Let a be the cross area 

 of the base of the tree, li its height, / the form factor, and v the volume; then, 



volume of cylinder ^ a x h 

 volume of tree = a X h X f 



V 



form factor == 



aX h 



