LIMITS OF ACCURACY IN TIMBER ESTIMATING: 301 



possible if the latter are cut to a fixed diameter, or if made to conform 

 to average utilization, are used to get these diameters; e.g., for a tree 

 38 inches D.B.H. containing eight logs, the upper diameters are 

 respectively, from the table, 32, 30, 28, 25, 22, 18, 14, and 10 inches, 

 and are so recorded, each log under its proper log grade. (See § 207 

 for form of tally.) 



The determination of the number of board feet of each standard 

 grade in logs of each diameter and grade, and the total scale for each 

 lumber grade, is based on the contents given for these log grades from 

 mill-scale studies of log contents. The purpose is to obtain the per 

 cent of each grade, regarding the total contents of the logs tallied as 

 100 per cent, and then to apply these per cents to the volume estimated 

 for the tract. These per cents can be obtained more accurately if over- 

 run is included in logs of each separate size (§46). The mill-scale 

 study will show the amount of over-run in logs of different diameters 

 and standard lengths. The scaled volume of these logs should then be 

 increased by this per cent of over-run, before the division into lumber 

 grades is made. On the total sawed contents thus obtained, the per 

 cent of each grade is based. 1 



Even if considerably in error, the value of an estimate expressed 

 by grades of lumber is much greater than one which entirely ignores 

 the quality and consequently the relative stumpage value of the tract. 



In the absence of specific information on grades, a record of the sizes 

 of the trees, their clearness of bole, and the density of the stand may 

 furnish a basis for approximating the probable grades. 



236. Limits of Accuracy in Timber Estimating. Purely ocular 

 estimates vary in accuracy up to errors of 100 per cent, dependent 

 upon how far the method is stretched from its original limitations. 

 This does not include errors due to inexperience, inefficiency or careless- 

 ness. 



In mechanical methods of measurements, serious errors may occur 

 in computations. Such errors, of course, are inexcusable, but their 

 avoidance requires careful checking. The mechanical errors due to 

 the operation of the law of averages have been pointed out as a function 

 of the factors influencing these averages, the chief of which is the size 

 of the area unit. 



The degree of accuracy must be based upon the standard of utiliz- 

 ation. It is entirely unfair to judge the accuracy of estimates based 

 upon one standard against the results of sawing attained by the appli- 

 cation of an entirely different standard. Where the standard is the 

 same in both cases, the present demands of timber estimating require 



1 The details of this method are taken from the article by Swift Berry, Journal 

 of Forestry, Vol. XV, 1917, p. 438. 



