Geoscience Reference
In-Depth Information
DID YOU KNOW?
By keeping careful lumber tallies of boards cut from various sized logs, any sawmill may con-
struct its own empirical log rule. Such rules may provide excellent indicators of log volume at
the particular sawmills where they are compiled.
Because no industrial lumber organization or government agency has control over the measure-
ment of logs, districts and individual buyers have devised their own rules to fit a particular set of
operating conditions. Thus, in the United States and Canada there are over 95 recognized rules
bearing about 185 names. In addition, there are numerous location variations in the application of
any given rule.
Three methods are employed to develop a new log rule. The most obvious is to record the volume
of lumber produced from straight, defect-free logs of given diameters and lengths and accumulate
such data until all sizes of logs have been covered. These “mill scale” or “mill tally” rules have the
virtue of requiring no assumptions and of being perfectly adapted to all the conditions prevailing
when the data were obtained. Their disadvantage, aside from the amount of recordkeeping required,
is that they may have been produced in such a restricted set of conditions that the values are not
applicable anywhere else.
The second method is to prescribe all of the pertinent conditions (e.g., allowance for saw kerf and
shrinkage, thickness and minimum width and length of boards, taper assumptions) and then to draw
diagrams in circles of various sizes, representing the sawing pattern on the small end of a log. These
“diagram rules,” of which the Scribner is an example, will be good or bad, depending on how well
the sawmilling situation fits the assumptions used in producing the diagrams.
The third basic procedure is to start with the formula for some assumed geometric solid and then
make adjustments to allow for losses to saw kerf, edgings, and so forth. These are referred to as
“formula rules” and as is the case for any type of rule their applicability will depend on how well
the facts fit all of the assumptions
The development of a rule may involve more than one of these procedures; thus, the step-like
progression of values in a mill tally or diagram rule may be smoothed out by fitting a regression
equation. Or, the allowance to be used for slabs and edgings in a formula rule may be estimated
from mill tally data. Finally, there are the “combination” rules such as the Doyle-Scribner, which
uses values from the Doyle rule for small logs and from the Scribner rule for large logs. The aim, of
course, is to take advantage of either the best or the worst features of the different rules.
13.4.2.5 Basal Area Measurements
The cross-sectional area at tree stem breast height is called
basal area
. This is important because
tree-stem measurements are often converted to cross-sectional areas. To compute tree basal area,
one commonly assumes that the tree stem is circular in cross-section at breast height. Thus, the
formula for calculating basal area in square feet (where the DBH is measured in inches) is
2
π
(
DBH)
2
)
2
Basalarea (ft
)
=
=
0 005454
.
×
(
DBH
(13.10)
4
×
144
If metric units are used, basal area should be expressed in square meters (m
2
), and DBH is measured
in centimeters:
2
π
(
DBH)
2
(DBH)
2
Basalarea(m
)
=
=
0 00007854
.
×
(13.11)
4
×
10,000
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