Caliper

Forest mensuration lecture notes

[+] Definitions in forest inventory

[+] Functions and models in forest inventory

[+] Measurement devices

[+] Single tree variables

[+] Stand variables

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Figure 1 Principle of dbh measurement with a caliper (Kleinn 2007).

The caliper is one of the most efficient tools to measure stem diameters directly wherever one has direct access to the stem. An ordinary caliper, constructed of metal or wood, consists of a graduated beam with two arms perpendicular to it. One arm is used at the origin of the scale and the other arm slides (Figure 1). When the beam is pressed against the tree and the arms are closed, the tree diameter can be read on the scale.

There are some calipers with a set of fixed arms also. The graduations are calibrated so that, when the fork is placed on the tree, the points of tangency indicate the tree diameter. Nowadays, aluminum calipers have replaced the wooden and steel calipers. As wooden calipers have tendency to wearing and tearing whereas steel ones are too heavy to carry in forest.

Typical example of a caliper (60 cm).

Electronic calipers had been developed that measure diameter electronically; the diameter is then either read from a display or directly stored in a memory chip or the measurement is via wireless directly transferred to a mobile computer that the field crew carries along. There are various successful examples of using electronic calipers; but in most inventories the benefit in terms of rationalization of procedures is relatively modest compared to the high cost of electronic calipers.

For an accurate reading, the beam of the caliper must be pressed firmly against the tree with the beam perpendicular to the axis of the tree stem and the arms parallel and perpendicular to the beam, so that it minimizes the effects of maladjustment of the moveable arm. It is important that all types of calipers be held perpendicular to the axis of the tree stem at the point of measurement (breast height, usually). If the stem cross sections are irregular, some times two caliper readings are recommended at right angles; then, the mean value of the two readings is taken as diameter.