A FORMULA METHOD FOR ESTIMATIXG TIMBER 

 By E. I. Terry 



I. A FORMULA FOR WESTERN PINE 



Form factors have been but very little used in American mensura- 

 tion, because they are not directly applicable for determining the con- 

 tents of trees in board measure. About three years ago, while estimat- 

 ing western yellow pine in Colorado, it occurred to the writer that a 

 rule-of-thumb formula might be worked out by using the merchant- 

 able form factor and the ratio between the actual average cubic and 

 board-foot contents of the trees of each diameter class. It is well 

 understood that there is no one converting factor by which the cubic 

 contents of trees of all sizes can be reduced to board-foot contents with 

 any degree of accuracy. But for a given species in a given region, 

 where growing conditions are nearly uniform, it seems to be a reason- 

 able assumption that the ratio between the cubic and board- foot con- 

 tents (considering only the merchantable stem) of the trees of each 

 diameter class will be practically constant. By merchantable form fac- 

 tor, as used in this article,. I mean the relation between the volume of 

 the merchantable stem of a tree and a cylinder having the same diame- 

 ter as the d. b. h. and the same height as the merchantable length of 

 the tree. 



Another assumption which I have made is that only one form fac- 

 tor need be used for each diameter, regardless of height. In order to 

 determine whether this assumption be justifiable, I worked out for 

 each fifth inch diameter class the form factors for five-foot lengths, the 

 results of which appear in Table i. The values are somewhat irregu- 

 lar, but certainly no progressive change is evident from shorter to longer 

 lengths for any diameter class. 



But a formula, to be of practical use not only to foresters, but to 

 cruisers and woodsmen generally, must be simple in form and based 

 upon units of measurement which are both familiar and easily deter- 

 mined. Obviously, the dimensions best meeting those conditions are 

 the breast-height diameter in inches and the merchantable height of the 

 tree in feet. The latter is usually calculated in logs or half-log lengths, 

 ten-foot sections, etc., any of which can readily be reduced to linear 

 feet. Nor must a formula, if it is to be of general use, contain a large 

 number of variable factors which must be memorized or carried in a 



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