METHOD OF FORM FACTORS. 61 



It will be seen that for the methods as described above the 

 greatest difference amounts to 1/7 per cent. Even if, in the 

 case of ordinary inch classes, all sample trees are thrown 

 together, the difference amounts only to 2*7 per cent. Under 

 these circumstances it appears that any of the methods meets 

 the requirements of measurements for the preparation of 

 working plans, and that much more depends on the care 

 bestowed upon the operation than on the particular method 

 followed. If the actual fellings show greater differences than 

 the calculations justify, they are frequently due to extraneous 

 causes, such as the felling of the trees at some distance above 

 the ground, careless working up of the material, inaccurate 

 measurement of the fall, theft of material, etc. 



In all cases where special accuracy is required, as for 

 instance for scientific investigation or for the determination of 

 the sale value of woods, the classes or groups should be small 

 and the number of sample trees large. In this way greater 

 accuracy is likely to be obtained than by making a distinction 

 between the different methods which have been described. 

 This conclusion appears justified by the facts that in regularly- 

 grown woods the basal area is a most powerful factor, and 

 that the height is to a sufficient extent a function of the 

 diameter or girth. 



4. Determination of Volume by means of Form Factors and 

 Volume Tables. 



Instead of felling and measuring sample trees, their volume 

 can be ascertained by means of form factors or taken from 

 volume tables. This applies to all the methods given above. 

 In all these cases the volume is ascertained according to the 

 general formula : 



V=SxHxF. 



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

 obtained has already been explained. The mean height of a 



