CHAPTER XVI 

 FORM CLASSES AND FORM FACTORS 



170. The Need for Form Classes in Volume Tables. Trees which 

 have the same D.B.H. and total height may vary in form, as shown, 

 according as the tree is full boled, with " good " form, or concave 

 boled, with " bad " form. These gradations of form correspond with 

 differences in cubic volume. In order to further classify the volumes 

 of trees of the same D.B.H. and height, this range of volume due 

 solely to form must be separated into arbitrary classes or divisions. 

 Such a series is based on measurable differences in form, and the 

 classes thus established are termed form classes. The adoption of 

 form classes as a third variable in constructing volume tables has 

 been retarded in this country by the necessity for expressing volumes 

 in terms of board feet, by the labor of constructing even the simpler 

 tables based on diameter and height, and by the belief that the vari- 

 ations due to form could be more simply overcome by averaging them. 



A second difficulty lay in the application of such form-class tables 

 in timber estimating, since cruisers were unaccustomed to judging 

 upper diameters by eye with the accuracy needed to distinguish between 

 the form classes. Differences in taper were readily recognized, but 

 differences in form were further obscured by the method of using 

 merchantable top diameter limits instead of total height. Practical 

 cruising did not seem to require such tables. But with the increasing 

 use of the cubic foot and the cord for pulpwood and in second-growth 

 timber, and the need for closer estimating, the desirability of distinguish- 

 ing form classes in volume tables is increasing. Such efforts as have 

 been made so far in this country follow standards prevailing in Europe, 

 where the universal use of the cubic unit, close utilization and high 

 values have made it necessary and possible to obtain more accurate 

 measurements of the standing timber. 



One great possibility in this field is the demonstration that when 

 form classes are distinguished and the true form of the tree inside the 

 bark is made the basis, all species of trees will be shown to have practi- 

 cally the same forms and total volumes for the same form classes; hence 

 a single general table so classified would suffice for all field work. Were 

 this fact established, a basic table might then be constructed for each 



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