SAMPLE PLOTS IN SILVI CULT URAL RESEARCH 81 



27. For virgin timber, both the International (%-inch kerf) and the local 

 rule should be used ; preference among local rules should be given to the 

 Scribner decimal C rule. 



28. The tree should be scaled in 16-foot logs (as in 12) plus a short top 

 section (to the top cutting limit) which is scaled as a fraction of the next 

 length. 



29. No deductions should be made for defect. 



30. No individual tree computations of volume in cords and other units of 

 minor importance should be made, since tables based on these are more readily 

 obtained by conversions of the final table (no. 35). 



31. In place of computations of volume, the percentage of d.i.b. at 10-percent 

 height intervals over d.b.h. may be used where taper tables are to be prepared. 



PREPARATION OF TABLES 



32. Separate tables should be prepared for second growth and for mature 

 timber, the line being drawn at the most probable rotation age. 



33. The possibility of separate tables for different sites should be carefully 

 considered. 



34. The cubic- foot table should be prepared as follows : 



(a) The trees should be grouped in the following height and diameter classes : 

 Diameter, 1-inch classes for species not exceeding 36 inches or 2 inches 



for species larger than this. 

 Height, 5-foot classes for species not exceeding 100 feet and 10-foot 

 classes for species taller than this. 

 (&) For each height-diameter class the average d.b.h., average height, aver- 

 age volume, volume of cylinder corresponding to these average dimensions, and 

 average form factor should be computed. 



(c) These form factors should be curved over height and diameter. 

 (cl) The final table should be prepared by multiplying the volumes of cylin- 

 ders computed for the various height-diameter classes by the curved form 

 factors. 



35. The board-measure table should be prepared in the following manner: 

 ( a ) The grouping into height-diameter classes should be as follows : 



Diameter, 1-inch classes for trees not exceeding 36 inches, and 2-inch 



classes for larger timber. 

 Height, 8-foot classes for trees not exceeding five logs in height and 16- 

 foot classes for taller timber. 

 (&) For each group the average d.b.h., average height, average volume, and 

 average frustum-form factor (based on the frustum corresponding to these 

 average dimensions) should be computed. This computation is facilitated by 

 the use of an alinement chart which permits ready interpolation. 



(c) The frustum-form factor should be curved over d.b.h. only. 



(d) The final table should be prepared by multiplying the frustum-form 

 factor values by values taken from the alinement chart for the various height 

 and diameter classes. 



36. Whatever subordinate tables are required, they should be derived from 

 the basic tables by conversion factors based on diameter and height. For 

 example : 



Table Conversion factor 



Cords Cubic feet per cord. 



Merchantable volume to 3 inches Percentage of total volume which 



is merchantable. 



Volume including bark Percentage of bark. 



Volume board measure based on total height- Length of top above the merchant- 

 able limit. 



37. The following checks should be made on all tables : 



(a) The aggregate difference between the actual volumes of the trees and 

 those corresponding thereto taken from the tables should be computed. Satis- 

 factory limits of error are one-half percent for more important tables and 1 

 percent for less. 



(6) The average deviation of the individual-tree volumes from the table 



should be computed. In this computation the volumes read from the tables 



should be interpolated to the nearest one-tenth inch of diameter and the nearest 



foot of height. This may be done readily by putting the table in graphic form. 



87471°— 35 6 



