220 FOREST VALUATION 



almost invariably guess at the contents of the tree, either as 

 a whole or by their knowledge of the contents of logs of cer- 

 tain sizes. A better method is to use tables giving volumes in 

 board feet, or in other units, for average trees of given diameters 

 and heights, when such tables are available and are known to be 

 reliable. 



219. Strip Methods. By far the greater amount of timber 

 estimating is done by methods involving the measurement of 

 only a portion of the stand, in an effort to get an average which 

 will apply to the whole area. Even if all the trees on a tract 

 are observed, they may be merely counted, and an average 

 volume obtained by guess or by tallying the sizes of a certain 

 proportion of the stand. 



The most widely used methods consist of running parallel 

 compass courses through the area at definite intervals, and meas- 

 uring the trees standing on a strip of a definite width with the 

 course as a center. In dense brush, or with small timber, this 

 strip may be but 4 rods wide. Where tree callipers are used 

 to get the diameter of each tree and the men employed to do 

 the work have but little experience in timber cruising, this 

 width of 4 rods is seldom exceeded. More experienced woods- 

 men, who estimate the timber by eye, usually cover a strip 

 8 or 10 rods wide. The number and contents of the trees on 

 these strips form the basis of the estimate. Should the strips 

 so measured be accepted without question as giving an average 

 stand for the whole area, the total estimate is found by using 

 the multiple determined by the per cent of the area measured. 



220. Errors in These Methods. But this is seldom the case. 

 Timber is never uniformly distributed even over small areas, 

 but varies enormously from acre to acre in size and density. 

 Topographic and site factors exercise a marked influence on 

 the stand. The timber tends to run in belts along stream 

 bottoms and slopes. Chances of obtaining a better average 

 are improved by arranging the strips to cross these belts, which 

 object is most frequently attained by running up and down 

 slopes rather than along them. 



For a given per cent of the total area estimated, the chances 



