152 UNITS OF MEASUREMENT FOR STANDING TIMBER 



volume, is then expressed in the table. The rate of taper for trees in 

 different height classes within any diameter class, as 20 inches D.B.H., 

 need not be shown in such tables. If measured, it will be found to differ 

 by arbitrary fractions of inches instead of by exact 1-inch classes per 

 standard log. 



Height classes may be based on total height, or on the length of the 

 merchantable bole. In the former case, height classes are based on 

 either 5- or 10-foot gradations, using the same system of rounding 

 off as for diameters, e.g., the 70-foot height class with 10-foot gradations 

 includes all trees 66 to 75 feet in height. With 5-foot gradations, it 

 includes trees 68 to 72 feet in height. When merchantable heights 

 are used, these lengths are commonly standardized to conform to a 

 common log length such as 16 feet and expressed as 1, 2, 3 or more log 

 trees. The log length used is always stated. Half-log lengths may be 

 differentiated. With valuable hardwoods of variable merchantable 

 length, there is some need for closer classification of merchantable 

 lengths, but volume tables are seldom constructed for intervals of less 

 than 8 feet. 



129. Diameter Alone, Versus Diameter and Height, as Basis of 

 Volume Tables. To. separate or classify the volumes of trees of each 

 given diameter class into from 4 to 10 height classes requires the measure- 

 ment of from 250 to 1000 trees, in order that the average volume in 

 each of these numerous classes may be found with some accuracy 

 (§ 137). This makes it impossible to take the time to construct such 

 tables for local or immediate use. Hence many volume tables have 

 been based on diameter alone, averaging together trees of all heights. 

 Sometimes the average heights of the trees of each diameter class are 

 shown, often they are omitted. 



For timber of uniform age and density of stand and growing on the 

 same quality of site, individual trees of the same diameter will still 

 differ considerably in height and volume; yet an average height for 

 each diameter may be found, which will indicate quite closely the 

 average volume for that particular stand or type and age class. But 

 such a volume table is quite worthless for application to any other 

 stand, age class or type, unless it can first be shown that the average 

 heights based on diameter are the same in both cases. Lacking, 

 first, the knowledge of the average heights used in the table, and second, 

 the demonstration that these coincide with those of the stand to be 

 estimated, the only possible procedure is the preparation of an entirely 

 new volume table. 



But with a table based on a classification of heights and correspond- 

 ing volumes under each diameter class, stands of any degree of density 

 or age, and growing on any site, may be estimated by use of this table, 



