Constructing Volume Tables. 219 



over, exceptionally susceptible to distortion, since the extreme 

 values are not only unsteadied by other values except from one 

 side, but are almost certainly the average of fewer measurements. 

 Thirdly, the error in shape of the ends of one set of curves will 

 often result in error of location of the second set drawn. 



It is this danger of improper shape or direction of the curves 

 that may be minimized by the use of the frustrum form factor. 

 What it can do for us is to set a standard curve form with which 

 we can compare our average tree measurements. It is a simple 

 matter, for example, to draw a set of curves showing the volume 

 in board feet of frustrums of cones having the same upper and 

 lower bases as the average of the trees on which our table is to be 

 based. Each of these curves and the corresponding curve of the 

 volume table will be similar in shape and position. From our 

 conclusion above that variation of frustrum form factors is least 

 within the individual diameter classes, it is obvious that this 

 similarity will be most marked in the corresponding set of curves, 

 amounting, indeed, to an essential parallelism. 



How best we may use the frustrum form factor constructively 

 depends chiefly upon the number of tree measurements which we 

 have at our disposal. In the case under consideration at the be- 

 ginning of this article it is obvious that the field data required 

 must be kept to a minimum. In such a case the following pro- 

 cedure is suggested. 



a. Determine the top diameter (average) to which trees of 

 different D. B. H. classes will be cut, if possible by actual measure- 

 ments after a logging crew. Such measurements are very quickly 

 taken, and sufficient to construct a good curve may be obtained in 

 a short time. 



b. As many full volume measurements of trees as is practicable 

 should be taken. 



c. For each tree thus measured the frustrum form factor should 

 be calculated. 



d. These figures should be grouped together by diameter 

 classes, and the average frustrum form factor for each diameter 

 class obtained. 



e. These values should then be evened off by means of a curve. 



f. A table should then be constructed showing the volumes in 

 board feet of frustrums of cones having basal diameters and 



15 



