THE CHOICE OF A SYSTEM FOR TIMBER ESTIMATING 261 



comparison. None of the trees is actually measured except on the 

 per cent estimated. Using this estimated strip as a standard, the 

 estimate upon the remainder is taken as equaling, exceeding or falling 

 short of the stand per acre upon the estimated strip, and its volume 

 is obtained by applying a correction to this estimated stand per acre. 

 210. The Choice of a System for Timber Estimating, with Relation 

 to Accuracy of Results. All systems of timber estimating involve the 

 choice, first, of one of the three methods for determining the contents 

 of the trees and second, of one of the three methods of covering the area. 

 There are many different systems of timber cruising, involving the 

 possibility of an endless combination of these six elements Each of 

 these systems represents a decision as to the per cent of area required 

 to get the average stand per acre for the total area, the method of cover- 

 ing the area to obtain this per cent, and the question as to acceptance 

 or modification of the stand per acre as applicable to the whole tract; 

 it also involves the further reduction in the work of measuring dimen- 

 sions to get the volume of trees by substituting averages for height, 

 a per cent of total tallies for total tallies and average volumes for 

 individual volumes. These two groups of factors are closely inter- 

 related. For instance, where the per cent of area covered is reduced 

 to a low figure, the area which is actually estimated must be covered 

 thoroughly by careful measurement of distances and widths of strips, 

 the diameter of every tree should be measured or tallied, and each tree 

 may be tallied for height, especially if merchantable heights are used. 

 Where, on the other hand, all of the area is covered, it may be sufficient 

 merely to count the trees, substituting the method of an average tree 

 or log for the more detailed and time-consuming method of measuring 

 each diameter. The gain in accuracy in one of these factors may be 

 offset against possible inaccuracy in another, the sum of the factors 

 being determined by the total cost of the method. These points may 

 be briefly summed up as follows: 



Area — 



Full estimate, 100 per cent. 

 As modified by averages. 



Sample plots taken as the average. 

 A given per cent accepted as the average. 



A given per cent estimated as a basis for obtaining the remainder by compari- 

 son and correction. ' 

 Trees — 



Full estimate, 100 per cent tallied for both diameters and height. 

 As modified by averages. 



Average height obtained from sample measurements. 



Volume of average tree obtained from tally of dimensions of a fixed per cent 



of the total stand. 

 Volume of average tree obtained by inspection, from sample tree, or average 

 tree on sample plots. 



