FORM FACTORS 429 



tree is estimated in the British method of measuring stand- 

 ing timber ; and the height up to the timber limit must also 

 be ascertained, and the contents of timber will be : 



Timber height (in feet) x (mean diameter)* ^ 



144 4 



Hence it will be seen that the continental method has no 

 advantage over the British method, where a single tree, or 

 only a small number of trees, is being individually measured, 

 except that the height is measured more accurately if an 

 hypsometer be used. 



The form factor is really the proportion which the 

 basal area of a cylinder, having the contents of and being as 

 long as a given tree, bears to the basal area of that tree at 

 breast high. If trees have the same height, the form factor 

 will nearly always be different in the case of trees having 

 different diameters ; though all trees, whatever their height 

 or diameter, would have the same form factor if the degree of 

 taper from the tip of the tree to the ground were uniformly even. 



In well-grown, close-canopied high forest, it is found that 

 the girth of trees of the same species will usually vary accord- 

 ing to the height, during the period in which active height 

 growth is taking place ; and the form factor, as found in 

 average tables, is usually made referable to the height of a tree. 



Now, if a crop of timber were not uniform in any respect, 

 it would be necessary to measure every tree individually, and 

 no use could be made of average form factors or any other 

 average data. 



But, where crops are fairly uniform, the contents of any 

 area, whether large or small, may be found through the use 

 of average data by the following methods : 



(1) Measurement of Sample Trees. 



(2) Measurement of Sample Plots. 



(3) By estimation from Average Yield Tables. 



1. Measurement of Sample Trees. 



This excellent method involves, in its simplest form, the 

 finding of the true average stem and multiplying the contents 



