Knowledge reference for national forest assessments - observations and measurements
Annex:Brief description of measuring instruments
Instruments for Measuring Tree Diameters
The most commonly used instruments for measuring diameters at breast height are: calipers, diameter tape, biltmore stick, and sector fork. Though some of these instruments may be used to measure diameters at other points of the tree, there are other instruments that are more efficient for this purpose e.g., Finn caliper, optical calipers, optical forks, Spiegel relascope, etc.
Usually one single diameter is recorded for dbh per tree. This means that there is the general assumption that the cross section at breast height is circular. This is a model assumption which is, in a strict sense, never true, and which may lead to systematic errors. But so far, forest inventory and planning has lived well with this approximation and there is no other approach.
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. When the beam is pressed against the tree and the arms are closed, the tree diameter can be read on the scale. Calipers are often used to measure tree diameters when the diameters are not large (not more than 80 cm, say). Calipers of sufficient size to measure large trees or those with high buttresses (as in tropical regions), are awkwardly to carry and handle, particularly in dense undergrowth.
It is important that the caliper be held perpendicular to the axis of the tree stem at the point of measurement. Rapid measurement of trees is possible with calipers.
For asymmetric cross sections, we may carry out multiple dbh measurements (like two perpendicular measurements, or measurement of the maximum and minimum diameter) to record some aspects of asymmetry. Then the inventory protocol must define how to calculate the basal area from these measurements; common options are averaging the measurements to calculate the dbh (for example, when two perpendicular measurements were made), or assuming an elliptical cross section (when maximum and minimum diameter were measured).
The diameter of a tree cross section may be obtained with a flexible tape by measuring the circumference at breast height. Diameter tapes are graduated such that the diameter can directly be read. As with any dbh measurement, we assume that the tree has a circular cross section. Any deviation from this shape leads to a systematic positive error when calculating the basal area: a larger basal area is assumed than is actually there.
The diameter tape is convenient to carry and to apply. There is no upper dbh limit: however for larger trees, where the perimeter exceeds the length of the tape, it must be measured by multiple setting of the tape. Care must be taken that the tape is correctly positioned at the point of measurement, that it is kept in plane perpendicular to the axis of the stem, and that it is set firmly around the tree trunk. Metal tapes do deform less (by hauling or by temperature) than those of synthetic materials and are to be preferred.
This is a simple instrument for measuring dbh. It consists of a straight ruler that is held perpendicular to the axis of the tree stem. By holding the stick so that the 0-point of the graduation of the stick lies on the line that is tangent to the tree cross section at one side, the diameter of the tree is read at the intersection at the other end of the stick on the line that is tangent to the tree cross section at another side. One has to take care of holding the stick in consistence distance from eye and holding it properly. Also this instrument assumes circular shape of the stem at dbh.
Instruments for Measuring Tree Height
Techniques and instruments for general height measurement may be applied to tree-height measurement. However, instruments must be economical, light, portable, and usable in closed stands. There are various methods and techniques to measure tree height from simple ocular estimation to using graduated poles for short trees and using similar triangle principles for height measurement. Following are the most commonly used instruments used for tree height measurement in forestry; all use simple trigonometry: From a known horizontal distance to the tree, the slope angle to the top and to the bottom of the stem is measured. This allows to calculate the tree height.
Height measurements with these instruments require therefore three steps: (1) measuring the distance to the tree (one must find a position in the stand from which top and bottom of the tree can be viewed), (2) measuring the slope angle to the top, and (3) measuring the slope angle to the bottom of the tree.
While tree height can be calculated from any distance to the tree, most instruments provide scales from which heights can be directly read for some fixed distances (usually 15m and 20m or multiples). An optimal distance for height measurement is at about one tree height´s distance.
It consists of a gravity-controlled, damped, pivoted pointer and a series of scales on a rotatable, hexagonal bar in a metal, pistol-shaped case. It includes regular metric scales where heights can be directly read for distances to the tree of 15, 20, 25, 30m, and a slope percentage scale.
Sights are taken through a gun-type peep sight; the indicator needle is locked by squeezing a trigger, and the observed reading is taken on the scale. A rangefinder is available with this instrument.
It is similar in construction and operation to the Haga altimeter, although its appearance is somewhat different. The regular metric scales are for distances to the tree of 15, 20, 30, and 40m. A degree scale is also provided. All scales can be seen at the same time. A rangefinder is incorporated into the instrument.
It is a hand-held device housed in a corrosion-resistant lightweight alloy body. The scale is supported by a jewel bearing assembly, and all moving parts are immersed in a damping liquid inside a hermetically sealed plastic capsule. The liquid dampens undue scale vibrations. It is held to one eye and raised or lowered until the hairline is seen at the point of measurement. At the same time, the position of the hairline on the scale gives the reading.
The instrument has provision of two scales. Suunto clinometer can give the direct reading of height if the measurement is take from the certain distance, e.g. from 15 or 20 m distance from the tree.
The Spiegel Relascope, also known as Bitterlich Relascope, is a sophisticated mechanical instrument that can be used to measure diameter, tree height and stand basal area (by relascope sampling).
It has a peep-hole at the rear and a clear window at the front. Three additional windows in the lower half of the instrument allow light to enter and illuminate the scale. A brake button at the front of the instrument, allows a weighted wheel within the Relascope to rotate. When looking through the peephole, a circular field of view is seen. The scales are seen in the bottom half of this field of view and scale readings are taken where the scale touches the line halfway up the field of view. The standard metric Relascope has three scales for directly measuring (vertical) height, for distances of 20 m, 25 m and 30 m from the tree.