88 MEASUREMENT OF STANDING TREES. 



same proportion to the total height. There are, however, 

 various drawbacks to the employment of these form factors. 

 In the first place the height of the tree must be determined 

 before the point of measurement can be fixed ; secondly, the 

 latter may be very inconvenient in the case of very tall, as 

 well as very short trees ; thirdly, it has been found from actual 

 measurements, that the factors thus obtained are by no means 

 so regular as had been supposed, that is to say, trees of different 

 heights show by no means the same form factor if measured 

 at a constant proportion of the height. 



(3) Form factors based on Measurements made at Height of 

 Chest, called Artificial Form factors. The diameter, or girth, 

 is measured at the most convenient height from the ground, 

 namely at chest-height of an ordinary man. (In Germany 

 now generally fixed at 1/8 metres = to about 4' 3".) The 

 height of the ideal cylinder is equal to the height of the 

 tree. Owing to the measurements being taken at an absolutely 

 constant height, the form factors of two trees, which show the 

 same shape but differ in height, cannot be the same. It follows 

 that, in using such form factors for calculating the volume of 

 trees, the height of the latter must be taken into consideration. 

 Nevertheless, in practice, these are the only form factors now 

 used. 



h. Determination of Form factors. 



Formerly form factors were estimated, taking into con- 

 sideration all points which affect them, such as species of tree, 

 height, age, free or crowded position, etc. Such an operation 

 requires much skill and practice, and in fact it comes pretty 

 much to the same thing as estimating the volume direct. 

 To eliminate such uncertainty, tables have been prepared which 

 give the form factors for different species, heights and ages, 

 such tables being based upon the results obtained by the 

 measurement of numerous felled trees. Of late years it has 

 been recognized that the variations due to age can be omitted, 

 except in scientific investigations. 



