CHAPTER XXI 



METHODS OF IMPROVING THE ACCURACY OF TIMBER 



ESTIMATES 



225. The Use of Forest Types in Estimating. When only a part 

 of the area of a tract is covered in estimating, the accuracy of the 

 resultant estimate depends upon the success with which the actual 

 average stand per acre has been obtained. Although the per cent of 

 area taken has been properly chosen to fit the topographic conditions 

 and character of the timber and although the measurement of the timber 

 upon this area and the width of the strips has been accurately carried 

 out, so that no avoidable error remains in the work done, yet the esti- 

 mate may still be in error by the failure to secure the same proportion 

 of the different types and variations of stand on the strips as exist on 

 the area as a whole. On account of the prohibitive expense of running 

 a sufficient per cent of the .area to get this average mechanically, a 

 margin of error in timber estimating is permitted, and is gaged by the 

 value of the timber and the purpose of the estimate. Any modification 

 which will secure the required degree of accuracy and at the same time 

 avoid incurring an unreasonable expense will necessarily become a 

 part of the system employed. 



The more uniform the stand as to sizes and density of stocking, the 

 better the averages. This applies to the use of all six of the classes of 

 averages cited in § 209. 



For the purpose of securing a greater degree of uniformity in the 

 stand on those subdivisions of total area to which the estimates obtained 

 on strips or plots are applied, the distinction of forest cover types is 

 indispensable. A forest type includes all stands of similar character 

 as regards composition and development due to given physical and 

 biological factors, by which they may be differentiated from other 

 groups of stands. A cover type is the forest type now occupying the 

 ground, whether this be temporary or permanent. Timber estimating 

 concerns itself only with the existing forest cover. 



The factors which are reduced to greater uniformity by the sepa- 

 ration of forest types in estimating are composition of stand as to species, 

 and consequent relative per cent of total volume of stand represented 

 by the different species, a vital consideration in timber estimating. This 



288 



