446 ESTIMATION OF INCREMENT 



period of only one year be taken ; but if Pressler's formula be 

 applied for a period exceeding one year, it is not possible to 

 correct the percentage by the above method. 



However, the method of ascertaining the percentage 

 increment for the coming year on standing timber, from 

 measurements taken at breast height, may be still more 

 simplified. Thus, if 



n = number of rings in last inch of radius 



D = diameter (at breast high, under bark) 



Ff = form factor (for total contents) 



p = the percentage increment on total contents 

 Then 



And this is Maw's formula. 



Now, the percentage thus indicated will be slightly too 

 high, both by this formula, and also by Schneider's formula 

 (as corrected). Furthermore, the error will be greater in the 

 case of trees with small diameters than in the case of those 

 with large diameters, though the increased error that would 

 otherwise arise in the former case, is somewhat lessened when 

 the increment due to the next year's leading shoot is taken 

 into account. 



This formula may be represented in terms of quarter girth 

 measurement. 



Thus, if 



QG = quarter girth (at breast high, under bark) 

 Yf = form factor 



n = number of years in last inch of radius 

 p = the percentage increment 

 Then 



Often, however, Schneider's formula, , may be 



dxn 



applied direct to standing trees, with a fair degree of accu- 

 racy by ascertaining the mean diameter (d) by ocular 

 estimation. 



