Tools for Measuring Tree Diameter
A girth tape measures diameter indirectly. The tape is wrapped around the tree to measure the circumference. This value is divided by PI (3.1415….) to estimate diameter. Often the tape will have normal units (mm and cm) on one side and PI units on the other side.
The tape should be held relatively firmly (but avoid stretching). The tape should also be wrapped around the bole in a perpendicular plane to the stem axis. Keeping the tape numbers right side up (as in the photograph) reduces the chances of incorrectly reading the scale – when upside down errors like x.4 being recorded as x.6 are common.
Calipers are comprised of a fixed arm, scale, and moveable arm. The fixed arm is placed along one side of the tree at the desired height. The moveable arm is then placed flush against the other side of the tree and the scale is read directly. The calipers must be located perpendicular to the stem axis.
The length of the caliper arms must be at least half the diameter of the tree. Biased estimates (underestimates) of the diameter will result if the arms are less than half the tree diameter. This limitation may restrict the use of calipers to smaller trees – the large diameter trees often found in natural eucalypt forests would require calipers arms in excess of 1 m which would be inconvenient.
There are a wide variety of calipers available. The most widely used type is the light, hardwearing, metal alloy caliper.
Spiegel Relaskop (Relascope):
Commonly referred to as a Relaskop. A sophisticated, compact and robust device for measuring range, tree height and diameter, and stand parameters. It is relatively expensive.
A research quality instrument that is no longer in production. The Telereskop is similar in principle to the [Relaskop] except that it includes 5 x optical magnification.
A heavy height, diameter and range measuring instrument. The Criterion uses laser light to determine the distance from a tree. The user enters numbers read from a rifle-scope and a simple computer chip calculates diameter.
A medium weight and cost instrument for measuring tree diameter. The Pentaprism uses a moveable prism to superimpose an image of the tree bole over an original image. When the sides of the superimposed image line up with the original image, the diameter can be read directly from a scale which measures the displacement of the moveable prism.
Many users experience difficulties with the Pentaprism. They have difficulties lining up the sides of the tree for the two images.
Ranking tools for ease, effectiveness, and cost
Parameters for Measurement of Tree Bole
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.
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 the radius are possible.
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 (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, the 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.
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 the 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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