550 



JOURNAL OF FORESTRY 



In comparing these two measurements from this point of view it is 

 interesting to measure them against d.i.b. at 16 feet from the stump. 

 This, while impracticable in the field on account of the difficulty of 

 measuring diameters at that point, is of great theoretical interest be- 

 cause it is probably the optimum single diameter measurement as an 

 index of board foot volume. 



For this comparison thirty tree measurements of a single species and 

 region were selected at random from a single merchantable height 

 class, by the following plan : The first ten were chosen all in the same 

 one-inch diameter class, and the average d.i.b. stump for them was 

 found. The next ten trees were selected in the d.i.b. stump class 

 thus indicated. In a similar way the last ten were chosen in the 

 d.i.b. 16-foot class, which was found to be the average for the first 

 twenty trees. The final result is a group which is essentially uniform 

 in height and about equally uniform in diameter at each of the three 

 points of measurement. 



Since it has been found that frustum form factors vary but little 

 with small changes of diameter and probably not at all with height, 

 such factors for this group should be uniform in so far as they are 

 determined by these two influences. Variations of frustum form 

 factors among these thirty trees may then be taken as due chiefly to 

 "form," or in other words, to the failure of the single diameter 

 "measurement to express volume. 



Four species were investigated. The resulting pfobable errors, 

 which appear in Table 1, should well express the relative accuracy as 

 indices of the three points of diameter measurement. 



Table i. 



Computed by least square approximation formulae E 



.8435 $v 

 Vn (n-1) 



