62 MEASUREMENT OF WHOLE WOODS. 



How the basal areas of the trees of a class or a wood are 

 obtamed, has ah-eady been explained. The mean height of a 

 number of trees, or of a whole wood, is ascertained in the 

 following way : — 



V=S xHx F = SiX hiX /i + .So X ho X f.2 + S3 X //3 X ^3 + . . . 



hence ^^ .siX/^iX ./i + s.2 X //.2 X./2 + • • • 



SXF 



If it is assumed, that the form factors are the same through- 

 out, the above formula reduces to the following : — 



^^ _ 8 1 X //I + g2 X //2 + • • • . 



in words: "the mean height is equal to the total volume of 

 cylinders erected over the trees divided by the total basal 

 area." This formula holds good in the case of the several 

 trees of one class, as well as for calculating the mean height 

 of several classes, or of a whole wood. 



If a somewhat smaller accuracy suffices, one of the following 

 methods may be adopted : — A number of trees are selected, 

 which show about an average diameter and height, their 

 heights are accurately measured and the mean taken, which 

 represents the mean height of the class, or wood. Or the 

 height of the tree with the average diameter (or basal area) 

 is taken as the mean height of the wood. It remains to be 

 noted, that the heights obtained by means of these simplified 

 methods are generally a foot or two smaller than those obtained 

 according to the formula : — 



H =z ^i2^lA^ -\- SoX h.2-\- . . . 



S 



How the form factors of single trees are ascertained, has 

 been described above. Similarly, form factors for whole woods 

 can be determined according to the formula :— 



r = sxiixF. 1-=^, 



Example. — See pages 60 and 61. 



