3S MEASUREMENT OF RTANDTXa TREES. 



factors. In the first place, the height of the tree ninst lie 

 determined, before the point of measurement can be fixed ; 

 secondly, the latter may he 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 Measiiremejits made at Hchjht 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 and 

 France now generally fixed at 1'3 meters = about 4 ft. 3 in.). 

 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. 



J). Delerminalion of Form Factors. 



At first form factors were estimated, taking into considera- 

 tion 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 recognised, that the variations due to age can be 

 omitted, except where great accuracy is required, as in 

 scientific investigations. 



