CALCULATION OF THE VOLUME OP MATERIAL. 



Type-trees and form-factors.— The number of trees in each 

 size-class having been ascertained, either by counting and 

 measuring each individual tree in the whole area or by means 

 of sample plots or lines, the volume of material in each class 

 and, consequently, in the whole crop, may be calculated 

 when required by means of type-trees, as follows :— 



We may assume that, on an average, in one and the same 

 crop, trees of the same diameter and height have the same 

 cubical contents. For all practical purposes, therefore, 

 we may assume, without any great error, that if we 

 select trees that are representative of each size-class, ascertain 

 their contents and multiply this figure by the number of 

 trees in the class each represents, the sum gives the volume 

 of material in the whole crop. Such trees are called type- 

 trees, and may be selected either by eye, by taking an 

 average-tree, fairly representative of the crop or class as re- 

 gards size, or, more accurately, as follows : — 



Calculate, with the aid of tables, the sectional areas at 

 the base, where measured, of all the trees in the size-class 

 and total the whole. Divide this figure by the total number 

 of trees in the size-class, and the quotient will be the basal 

 area of the type-tree required. Calculate the diameter 

 corresponding to this basal area ; fell several trees of this 

 diameter and ascertain their contents by actual measure- 

 ment. The mean of their aggregate contents multiplied by 

 the total number of trees in the size-class will give the total 

 volume of material in that size-class. Proceeding in the 

 same way with each size-class, the sum of the volumes obtain* 

 ed gives the total volume of material in the whole crop. 



By hasal area is meant the area of the section of the stem of a tree measured 

 on a plane at right-angles to the axis of the stem and at breast-height, or, as usually 

 taken, at 4^ feet from the ground. 



This area is approximately equal to one-fonrth the square of the mean of the 

 longest and shortest diameters of the tree at this height multiplied by 3'1416.* For 

 the mean diameter is {- ^t^ )> where D and d are respectively the greatest and 



least diameters of the stem. 



Hy form-factor is meant the proportion which the true contents or volume of a 

 tree bears to a cylinder having thf same basal area and heiglit. The volume obtained 

 by multiplying "the basal area by the height of a tree is of course that of a cylinder ; 

 but, as trees taper, the true contents of the stem are something less than this. If 

 a=basal area of the trunk at breast-height, ^=tlie height of the bole or length of 

 stem taken, then the volume (C) of an ideal cylinder whose basal area is a and height 



*Vaiue of T s » 



