Forest Measurement and Modelling.
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Product oriented volume Forest Measurement and Modelling. |
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The volume of wood contained in a tree bole is of primary interest to saw-millers. However these millers are interested in the product they can extract from the bole or logs cut from the bole. Product oriented volume definitions attempt to estimate the volume of direct interest rather than the true volume of the three dimensional shape. Log rules are an attempt to estimate volume that is of direct interest to wood processing industries. A log rule is a table or formula showing estimated volume, in standard units, for various log diameters and lengths. During the 1900's, at least 100 log rules were devised. Australia has not adopted the log rule approach, however several of the rules are widely adopted throughout the US. The major log rules in the US predict the production of board foot. A board foot is equivalent to a plank 1 inch thick and 12 inches (1 foot) square; it contains 144 cubic inches of wood. However, none of these rules can accurately predict the mill output of boards, except when near-cylindrical logs are sawed according to rigid assumptions on which the rules are based.  Different board-foot log rules make different assumptions about cutting patterns, saw kerf, wastage, etc. Avery and Burkhart (1983 p 44) provide a comparison of the board foot estimates for a 16-foot long log under five major log rules. For small diameter logs, the different rules can result in 300% differences between estimates. |
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Calculating log volume True volume |
A variety of equations and measurement techniques can be used for determining the (true) cubic volume of logs on the ground - the choice of technique will depend on the accessability of the log for measurement and the log shape. Logs cut from the butt (or base) of the tree tend to have a greater rate of taper and butt-flair, while logs from the main bole and above tend to have a more regular shape and less rate of taper. Volume can be estimated over- or under-bark by measuring sectional area over- or under-bark. |
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| Stacked logs |  |  | |
| Separated logs (Mid-point of log accessable) |  |  |
| Stack measurements |
Historically, logs may have been measured as stacks. This approach was normally restricted to small diameter logs of standard length and relatively low value. Logs also needed to be regular without large sweeps, bends or forks as these irregularities could significantly alter the packing of the logs and thus reduce the true volume within the stack. Stack units of measurement included:
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| Reduction in volume due to cull and defect |
Defects in the log can reduce the amount of product that can be extracted by a sawmill. The percentage of this reduction may be important for the person attempting to estimate product oriented or even true volume - converting gross to net volume. There are no standard rules for determining the amount of material lost due to defects in logs. However, Grosenbaugh (1952) suggested a set of rules that may be appropriate. Slightly modified from the original, these rules include:
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[product.htm] Revision: 6/1999 |
