428 MEASUREMENTS FOR FOREST MANAGEMENT 



the two diameters taken ; for trees often present a broader 

 face one way than another. 



As the height is taken in feet, the basal area must there- 

 fore be reduced to feet, and the two multiplied together, e.g., 



D 2 (in inches) TT 



- x x height in feet. 

 144 4 



In practice this will be obtained from tables. 



This will give the true contents of a cylinder equal to the 

 total length of the tree, and having a diameter, all the way 

 up, equal to the diameter at breast high. 



Thus the contents as found will be far too great. The 

 correct contents are only a fraction of this amount. The pro- 

 portion which the actual contents bear to this cylinder is known 

 as the form factor. It is, therefore, necessary to multiply the 

 product of the height and basal area (at breast high) by the 

 form factor, in order to arrive at the actual contents. 



The tables of average form factors in general use are 

 all calculated on measurements taken at breast high ; there- 

 fore, in applying them, it is very necessary to be strictly 

 accurate in taking the measurements at breast high, or the 

 results will not be correct. 



These form factors are usually referable, either to the 

 total contents, including all small branches, or else to such 

 contents as have a diameter of 2\ inches 1 and upwards, over 

 bark. It is only these latter that are of any practical applica- 

 tion in this country. 



It must be clearly understood that these average form 

 factors cannot be taken in order to find the accurate contents 

 of any single particular tree. They are the average form 

 factors obtained from measurements made on innumerable 

 trees growing in close-canopied high forest. 



Therefore, if only a single tree or a small number of trees 

 is being measured, it is not correct to apply the form factor 

 from average tables. 



Under such circumstances, the mean diameter (for timber 

 only) must be estimated by the eye, just as the taper of a 



1 7 centimetres. 



