GENERAL METHOD OF SAMPLE TREES. 47 



d. Sclertion and Number of Sample Trees. 



As the volume of the whole class is to be calculated from 

 that of the sample tree, it is necessary to select for the latter 

 a tree, which represents the average of the class: in other 

 words, the sample tree should have the mean height, as near 

 as possible a circular section, a fairly straight and not a 

 forked stem and an average extent of crown. Even with the 

 greatest care, it is not always possible to avoid errors in the 

 selection ; hence, it is generally advisable to take several sample 

 trees for each class, and to ascertain the average of their 

 volume for the calculation of the volume of the class. The 

 actual number of sample trees depends on the desired degree 

 of accuracy and the total number of trees in the class. At 

 the same time, the felling of many sample trees is undesirable ; 

 hence, their number should be kept within reasonable limits. 



A further requirement is, that the sample tree should show 

 a basal area, which corresponds exactly to the mean section of 

 the class. Such a tree is found only in exceptional cases ; 

 hence, it is necessary to take a tree as near as possible to the 

 true section, and to modify the volume in proportion of the 

 basal areas of the true and approximate, or false, sample trees. 

 Let r = volume of true sample tree, v' that of the approximate 

 sample tree, s and s the corresponding basal areas, then v is 

 found by the formula — 



V xv' X s : s' 



V = v'xt 

 s ' 



e. DelermimUion of the Volume of Sample Trees. 



The volume of the sample trees is determined, either by 

 felling and measuring them on the ground, or by means of 

 form factors or volume tables. 



If the trees are felled, the stem and all straight pieces of 

 branches, in fact all regularly shaped parts, are divided into 

 pieces of moderate length, from 8 to 10 feet, according to 



