252 THE MEASUREMENT OF STANDING TREES 



Graves gives the following cubic rule of thumb for white pine: 



Square the breast-high diameter in feet and multiply by 30. The rule gives 

 approximately correct results for trees 10 to 14 inches in diameter and 80 feet 

 high, 16 to 20 inches by 85 feet, 22 to 28 inches by 90 feet, and 30 to 36 inches by 

 95 feet. Other heights require a correction varying between 5 and 6 per cent, 

 for each 5 feet of length. It can thus be seen that both simplicity and accuracy in 

 these rules of thumb are seldom obtained in the same formula without considerable 

 cumbersome modification and it would seem that a volume table could be referred 

 to almost as easily and give as accurate results. 



The use of rules of thumb based on board feet is primarily caused by lack of 

 suitable volume tables. This is illustrated by the development of rules of thumb 

 based upon the Doyle log rule. These board-foot rules are efforts to obtain the 

 total board-foot contents of the trees from the sum of the contents of the logs which 

 they contain and were usually formulated before volume tables had come into use. 



The simplicity of the formula for obtaining the contents of a given log in the 

 Doyle rule, namely, " subtract 4 inches from the upper diameter inside bark, square 

 the remainder, and the result is the scaled contents of a log 16 feet long" (the length 

 used in estimating), was an inducement to supplement this rule so as to obtain 

 the contents of the average log in a given tree. There are two rules for this. 



1. Take the average diameter of the top and stump inside the bark for the 

 diameter of the average log. Scale this and multiply by the number of 16-foot 

 logs in the tree. 



2. Multiply the diameter at breast-height inside the bark by the same diameter 

 minus 12. Multiply by the number of logs in the tree. This gives the scale of 

 the tree (C. A. Schenck). 



Schenok also gives a rule which ignores height, as follows: For "tall" trees, 

 volume =•§ diameter squared, measured at breast-height. 



Efforts to formulate general rules of thumb, not based on the Doyle rule are 

 illustrated by the following examples: 



1. Subtract 60 from the square of the estimated diameter at the middle of the 

 merchantable length of the tree. Multiply by 0.8 and the result is the contents 

 in board feet of the average log in the tree. Multiply by the number of 16-foot 

 logs for the total scale. (Graves' Mensuration, p. 153.) 



2. Average the base diameter of the tree and the top diameter of its merchant- 

 able timber. Get the scale of a log of that diameter, 32 feet long. Multiply by 

 the number of 32-foot logs less J log. (Cary's Manual of Northern Woodsmen.) 



3. Board feet = , 



60 



when D = inches and L = feet. 



(A formula method of estimating timber, E. I. Terry, Journal of Forestry, 

 Vol. XVII, No. 4, p. 413.) This formula, according to author, requires modification 

 by substitution of a divisor of 



70 for trees from 12 to 19 inches D.B.H. 

 60 for trees from 20 to 29 inches D.B.H. 

 55 for trees from 30 to 35 inches D.B.H. 

 50 for all trees above 35 inches. 



4. To base diameter, add one-half of base diameter and divide by 2; multiply 

 by 0.8, square and divide by 12. The result is the number of feet in the stick per 

 foot of its length. Three to 5 per cent may sometimes be added for contents above 

 the point stated. 



