RULES OF THUMB 251 



204. Rules of Thumb for Estimating the Contents of Standing Trees. 



A rule of thumb represents an attempt to formulate a simple rule which 

 can be memorized and by the use of which the contents of trees of any 

 diameter and height may be found. Such a rule would enable the 

 cruiser mentally to compute the volume of average trees without looking 

 them up in a table. It is also desired as a substitute for a universal 

 volume table because of the difficulty of finding volume tables for the 

 different species. 



The factors of variation in tree form are exaggerated by application 

 of units of product and the variation in board-foot log rules, and the 

 further differences in the per cent of total contents utilized in trees of 

 different sizes make it impossible to devise rules of thumb which are 

 as accurate as good volume tables; but since their use in ocular timber 

 estimating frequently accompanies methods of cruising by which a 

 close degree of accuracy is not attained, a slight possibility of error 

 in application is not considered a sufficient drawback to offset the 

 advantage of simplicity. They are especially desired in judging by 

 eye the contents of single trees. 



Rules of thumb must be based upon either the cubic or board-foot unit. The 

 simplest forms ignore the influence of height and are therefore inaccurate except 

 when applied to trees within a given range of heights. 



The effort is always made to devise rules which may be applied to the dimensions 

 measured by the eye; that is, to diameter and height. Rules which require the 

 use of basal area call for tables. 



For cubic contents, the following rules of thumb will serve as illustrations: 



1. To obtain cubic feet multiply the basal area in square feet by the height 

 and divide by 2. This is based on the theory that the cubic form factor of trees 

 will average 0.5 which is the form factor for a paraboloid. 



2. For trees averaging 80 to 100 feet in height, with a form factor of 0.49, the 

 contents in cubic feet equals the radius in inches squared (B. E. Fernow). For 

 "average" trees, volume in cubic feet equals one-fifth of the diameter squared 

 (C. A. Schenck). 



Both of these rules of thumb are good only for trees of a given height and form 

 factor. They are similar to the European rule of thumb — volume in cubic meters 

 equals the diameter squared divided by 1000. In this rule, D is measured breast- 

 high in centimeters. This rule applies to pine 30 meters high, beech, oak and 

 spruce, 26 meters high, and correction factors are, indicated as follows: for 

 each additional meter of length above or below these heights, for pine, a 3 per cent 

 correction; for beech, 5 per cent; for spruce and fir, Z\ per cent. Hersche's rule 



(h \ 

 of thumb reads, cubic meters = Z> 2 1 r+1 ), using meters. This applies to trees 



50 to 115 feet in height. \ 3 / 



possible to estimate the whole stand in the same form class, the smaller dimensions 

 a little higher and the larger dimensions somewhat lower than the average, e.g., 

 0.70 for overtopped trees, 0.675 for intermediate and co-dominant trees, and 0.65 

 for dominant trees (§ 171). The highest and lowest form classes will never occur 

 as an average, but only for single trees. 



