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Parameters for measurement of tree bole Forest Measurement and Modelling. |
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Measurement of a tree bole at a nominated height would be easy if the bole corresponded to a simple geometric shape. For example, if we could assume that the bole cross section was like a circle, then we could measure the radius (r), diameter (d), circumference (c) or the area (a). We can calculate all the other variables once we measure any one of them.![]() However, the tree bole is rarely circular (or any other simple geometric shape) and the use of the above equations will only provide approximate estimates. The selection of which parameter to measure will depend on: the use of the measurement; the resources and tools available; tradition; and the acceptable error. |
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Radius |
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Diameter |
![]() Diameter at breast height (dbh) is probably the most common measurement made on a standing tree. Direct measurement of diameter commonly measures two different axes:
The two diameter measurements are averaged using an arithmetic mean (most common) or a geometric mean (for highly elliptical boles). ![]() The measurement of diameter on one axis is often acceptable when the data is only being used to group trees into a stand table. |
Circumference |
![]() An advantage of measuring the bole girth is that there is no sampling error involved. Unlike diameter measurements, the result does not depend on which axis was selected to measure. This leads to an increase in measurement precision. In addition, if a tree bole changes by 1 cm in diameter, the girth measurement changes by 3.1415... cm (PI). Thus, finer readings of the change can be read. |
Sectional area |
![]() Sectional area could be directly measured using a planimeter, but this is rarely done. Instead, sectional area is calculated from diameter after assuming that the bole has a circular shape. If the diameter is estimated from a measurement of circumference, then the basal area estimate will be an overestimate (positively biased). If the diameter is estimated from the mean of measurements on one or two axes, then an over- or under- estimate of the sectional area is possible. The geometric mean of the maximum and minimum axes is less biased than other approaches (Matern 1956, Chacko 1961). Biging and Wensel (1988) studied ways of measuring basal area increment. They concluded that increment estimates were unbiased if measurements along the minor axis were used. |
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[doborcirc.htm] Revision: 6/1999 |