66 MEASUREMENT OF WHOLE WOODS. 



diameter classes, are called for, are very rare. Generall}-, the 

 distinction of height classes is a matter of considerable difficulty, 

 unless the heights are measured. It is necessary only where 

 a very high degree of accuracy is aimed at. 



The example on the preceding page shows the manner of 

 booking in the case of two height classes. 



Section II. — Determination of Volume by means of 

 Sample Plots. 



1. General. 



Instead of measuring all trees in a wood, a certain part of 

 the area may be selected, the volume on it ascertained and 

 from it the volume of the whole wood calculated. Such a part 

 is called a sample, or type, plot. It may be defined as a 

 portion of a wood, which contains an average volume of material 

 per unit of area. 



Having ascertained the volume of the sample plot, that of 

 the whole wood can be calculated in two ways: either according 

 to area, or according to the number of trees on the sample 

 plot and in the \YOod. 



Let A = area of wood, 



a = area of sample plot, 



V =^ volume of wood, 



V = volume of sample plot, 



then the following proportion is assumed to exist : — 



V : V = a: A 



and 



V - ^ X A 



a 



Again, if 



A^= number of trees in the wood, 



n = number of trees on the sample plot, 

 then 



v:V=n: X, 

 and 



V = ^-^-^. 



n 



