Tools for measuring tree diameter

Tools for measuring tree diameter
Forest Measurement and Modelling.

Fiberglass tape: A girth tape measures diameter indirectly. The tape is wrapped around the tree to measure 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: 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.

Telerelaskop: 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.

Criterion Laser: A heavy height, diameter and range measuring instrument. The Criterion uses laser light to determine distance from a tree. The user enters numbers read from a rifle-scope and a simple computer chip calculates diameter.

Pentaprism: 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 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

I have ranked the above tools used for measuring diameter at heights not within easy reach. My interpretation of 5 main factors:
  • price
  • precision
  • robustness
  • ease of sighting
  • speed of use
  • compactness

[toolsd.htm] Revision: 6/1999