RULES OF THUMB 253 



There are two steps involved in these rules of thumb for board feet: 

 First, a rule or formula is required, which gives an approximation of actual 

 board-foot contents of logs of different sizes. This can only be obtained by rules 

 on cubic instead of board-foot contents (§39). Taking a fixed per cent of 



^0.6D\2 



the contents of all logs, the last rule above quoted reduces to I 1 . 



The second step is to get the dimensions of an average log in a tree, thus averaging 

 large and small, or top, butt and middle logs together. Empirical results rather 

 than mathematical soundness has u-sually been the basis of all such rules of thumb. 



Practically all these rules of thumb for board feet are based upon the log unit, 

 as might be expected. A more scientific application of a universal rule of thumb 

 is that devised by F. R. Mason (Ref. Rules of Thumb for Volume Determination, 

 Forestry Quarterly, Vol. XIII, 1915, p. 333). This rule is as follows: 



5. The volume of a tree of each diameter and height class will correspond 

 closely with the volume as obtained by averaging the scale of the butt and top 

 logs and multiplying by the number of logs, usmg 16 feet as the standard log length. 



Mason states that this rule has been in use by Minnesota cruisers. Its superior 

 accuracy is based upon the fact that it conforms to the form quotient of the tree 

 as well as to its diameter and height, by introducing upper diameters at two points. 

 For Douglas fir this rule was 3 per cent below actual scale; for cedar, above 24 inches, 

 10 to 15 per cent high. For white pine, spruce, yellow pine, larch, lodgepole pine 

 and fir, average results were within 5 or 6 per cent of actual volume for individual 

 trees of all sizes, a result which is closer than niay be expected in the use of average 

 volume tables for single trees. The only difference between this rule and the tally 

 and computation of each log in the tree is elimination of the need for tallying logs 

 lying between butt and top. The size of the top log is constant where a fixed top 

 diameter is used. Mason states that 3^?^ is the approximate board-foot contents 

 for 16-foot logs over 24 inches in diameter. 



6. A rule given by J. W. Girard is, "add 6 inchas to the D.B.H., divided by 2 

 and use this result as the diameter for the average log in the tree. Multiply the 

 scaled volume of this log by number of logs for the tree volume." This rule holds 

 good for white pine and spruce cut to 6-inch top and for larch cut to 8-inch top. 

 For Douglas fir cut to 8-inch top, add 4 instead of 6 inches. For lodgepole cut to 

 6-inch top, add 5 inches. For yellow pine under 20 inches, add 6 inches; 20 to 25 

 inches, add 8 inches; 26 inches and over, add 10 inches. 



Any rule of thumb should be based upon the log rule and standard of utilization 

 in use. Such rules are largely worked out as a matter of personal efficiency by 

 individuals and should be tested carefully before placing too much rehance upon 

 them. 



References 



The Biltmore Stick and Its Use on National Forests, A. G. Jackson, Forestry 



Quarterly, Vol. IX, 1911, p. 406. 

 Notes on the Biltmore Stick, Donald Bruce, Proc. Soc. Am. Foresters, Vol. IX, 



1914, p. 46. 

 The Biltmore Stick and the Point of Diameter Measurements, Donald Bruce, Proc. 



Soc. Am. Foresters, Vol. XI, 1916, p. 226. 

 A Folding Biltmore Stick, W. B. Barrows, Journal of Forestry, Vol. XVI, 1918, 



p. 747. 

 Relative Accuracy of Cahpers and Steel Tape, Normal W. Sherer, Proc, Soc, Am. 



Foresters, Vol. IX, 1914, p. 102. 



