188 VOLUME TABLES FOR BOARD FEET 



159. Construction of Board-foot Volume Tables. The basis agreed 

 upon as to the top diameter to use, if merchantable heights are utilized, 

 will determine the height class into which each tree falls. The steps 

 in construction are the same as for tables of total cubic volume (§ 131) 

 with the following exceptions. 



Compute the volume of each tree by means of the log rule chosen, 

 by scaling each 16-foot log. In volume table work, this scale per log 

 should preferably be interpolated to T V-inch values, for which purpose 

 the values of the log rule can be tabulated for the given interpolations. 

 The last or top log if 8 feet long is scaled as one-half the volume of a 

 16-foot log of equal diameter. If the logs are not scaled to iVinch 

 they are rounded off to nearest inch above or below (§ 137) but where 

 but a few trees are measured in each size class, this incurs the risk of 

 unnecessary variations in volume of the tree classes. 



When merchantable heights are taken to fixed lengths, the variable 

 at this point will be the top diameter. Therefore, the average top 

 diameters should be shown for each diameter and height class. These 

 tops may later be averaged solely on the basis of diameter at breast 

 height. 



160. Data Which Should Accompany a Volume Table. Because 

 of the errors possible in misapplying tables for merchantable volumes, 

 as set forth, the use. of such volume tables presupposes knowledge of 

 their reliability and applicability. For this purpose the following data 

 should always accompany the tables: 



Species. 



Region or locality where measurements were taken. 



Age of trees to which values apply, when distinguished. 



Sites or quality to which values apply, when distinguished. 



Unit of volume used. 



Log rule if in board feet, or mill tallies specifying character and 



thickness of lumber included. 

 Specifications, if for piece products. 

 Number of trees measured as basis, by diameter classes. 

 Height of stumps. 



on the only occasion on which he had attempted it, the table gave just half of the 

 true estimate. This was unquestionably due to the cause explained above, that is, 

 trying to coordinate large top diameters with a table made to smaller tops. The 

 first impression, in using a table constructed to a small top diameter is that it 

 "secures a greater volume per tree." The error is just the reverse of this — it 

 under-estimates the timber. If, on the other hand, the top diameters in the table 

 are larger than those applied in the field and the per cent of total contents less, 

 the error in applying the table is an over-estimate equally great. These possi- 

 bilities of error in the use of volume tables based on merchantable length have 

 been commonly overlooked in practice. 



