# Parameters for Measurement of Tree Bole

## Parameters for Measurement of Tree Bole

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**Radius**

*Radius (r)*: length from the centre to the outside of the bole. It is rarely measured in forestry. Radius cannot be measured on standing trees because the centre of the tree needs to be accurately located. Because a bole is not circular, different measurements of radius are possible.

**Diameter**

*Diameter (d)*: length from the outside of the bole, through the centre, to the opposite side. Diameter is commonly measured in forestry. Again, because tree boles are not circular, different measurements of diameter are possible.

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 diameter of the maximum and minimum axis of the bole on trees that are clearly elliptical;

- The diameter of the maximum axis and the axis at 90 degrees;

- The diameter of any two axes at 90 degrees to each other.

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**

*Circumference (c)* – also known as *girth*: the length around the outside of the bole. Circumference is commonly measured in forestry, but usually it is then used to estimate bole diameter. If the bole were circular, diameter can be estimated as circumference divided by PI. However, if the bole deviates from this ideal shape, then this calculation will overestimate the diameter. This bias is not constant and will vary with the degree and type of deviation. However, this bias is rarely considered significant.

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.

**Cross Section Area**

*Sectional area (a):* the area of the cross-section of the bole. This parameter is very important in forestry. The sectional area at breast height is used in many relationships and is called *basal area (g)*.

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.

*For correction and improvements please use the comments section below.*

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