Measuring the height of a single tree ©

Height is a variable of fundamental importance when measuring or describing a tree or forest. Plants may not be defined as trees unless they have the potential to reach a minimum defined height (e.g. Specht 1970, defines a tree as a woody plant more than 5 m tall, usually with a single stem), while forests may be distinguished from woodlands by the number and potential or actual height of the trees (e.g. Specht defines woodlands and forests as both having trees, but with the projective crown cover of forests greater than 30%).

In forest mensuration, height is used to describe both the quantity and quality of tree and stand growth:

Quantity: height is a variable in tree and stand volume tables or equations used to predict volume.
Quanlity: height at a given age is the basis of site index classification, i.e. it reflects the fertility of the site.

There are various expressions of tree height:

total height
merchantable height and
bole height.

Techniques for measuring height may also vary depending on whether the tree is:

straight or
leaning and if
on flat or sloping ground.

Total height

Picture of leaning tree with vertical and slope distance

Total height is defined as:

the vertical distance from the base of the tree to the uppermost point (tip).

In forests where leaning trees are common, measuring the linear distance (or slope distance) from base of the tree to tip may be more appropriate. The linear distance will be greater than the vertical distance. However unless the lean is more than about 8 degrees, the difference between the two is less than 1% . Measurement crews and data interpeters must be aware which of these two height definitions is used.

Measurement of total height may be difficult in dense stands of trees with rounded crowns, e.g. tropical rain forest, and in young plantation tree crops before thinning opens up the canopy.

Diagram of excurrent habit Trees with an excurrent habit have a well defined tip. One can measure total height more accurately in excurrent species. Most conifers are in this group.

Diagram of deliquescent habit Trees with a deliquescent habit have a poorly defined tip. The heights of trees of deliquescent habit (i.e. umbrageous species) are much more difficult to measure accurately. The convention is to sight to the point in the crown which is vertically above the top of the bole and judged to be at the crown surface.

Many hardwood tree species (e.g. most eucalypts) exhibit an excurrent habit when young, but change to deliquescent habit when they mature.

Merchantable height

Merchantable height concerns conventionally merchantable material only and is defined as:

the distance from the base of the tree to the highest point of the main stem where the diameter is not less than some specified value or where utilisation of the bole is limited by branching or defect.

Merchantable height is therefore a variable quantity which depends on the specifications for the merchantable product.

Height measurement

Techniques for measuring tree height may be direct or indirect and essentially depend on the position or fate of the tree:

Felled trees - when the tree is on the ground, measurement of the linear distance from base to tip or to the merchantable limit is done directly with linear tape or graduated pole.
Standing trees - height can be measured by either direct or indirect methods (see below). Indirect methods are most common because the tip or merchantable limit is often inaccessible.

Direct methods for measuring the height of a standing tree

Climbing with tape and graduated pole. An accurate approach, but practicable only with some species under certain conditions. This technique is costly and dangerous and is normally restricted to experimental projects only.
Height sticks or rods. A reliable method, with an instrument error less than l%. However this method can also be expensive for trees in excess of about 20 m.

Indirect methods for measuring the height of a standing tree

Indirect methods involve the use an instrument called a hypsometer.

A hypsometer is an instrument used to calculate the heights of trees by triangulation.

Methods of indirectly measuring the height of standing trees fall into two basic groups:

Methods based on geometric principles, e.g. the JAL hypsometer is the standard instrument in some European countries (e.g. Denmark) and is considered quite reliable. Geometric instruments have traditionally not been used in Australia. The Dendrometer II, recently introduced into Australia, is a geometric-based instrument used for quick but approximate measurement of tree height.
Methods based on trigonometric principles. These are the most common methods used in Australian forestry.

Note: As a general rule, height measurement using trigonometric instruments should be made from a point on the ground such that the angle of observation to the tip of the tree lies between 30 and 45 degrees, i.e. the observer should stand between 1 and about 1.5 times the tree height away from the tree.

Leaning trees

Only a small minority of trees are truly vertical, i.e. the tip of the tree is vertically above the geometric centre of its base, and thus we are frequently concerned with measuring the height of a leaning tree.

It should be noted that most of the 'noise' in data bases of tree height occurs due to failure by measure crews to account for tree lean.

There are two general methods of measuring the height of leaning trees, direct and indirect. The direct methods are the same as for a vertical tree (i.e. climbing or using height sticks ) while the indirect methods must be modified. It is imperative that field crews involved in measuring tree heights inspect each tree closely for lean and, if a lean is detected, take appropriate action. In many cases, the lean and associated error with not taking the appropriate action will be very small and can be ignored, but in other cases serious error will occur (Table of errors ).

Precautions are necessary with the indirect methods to minimize error. Extreme error is most likely to occur when the operator stands in the plane of lean (i.e. the tree is leaning directly towards or away from the operator) and does not use the appropriate method. Depending on where the observer stands in the plane of lean, the error will vary from maximum positive to maximum negative.

Ocular estimation of height

Except for quick approximations and some advanced sampling approaches (e.g. point-Poisson or point-3P sampling), one should never need to substitute ocular estimates for measurement.

However, ocular estimates are used in South Australia on stripline assessment of site quality at age 9-10 years where the greatest height assessed is approximately 18 m. The South Australian experience is that trees up to this height can be estimated reliably by eye when done by experienced workers.

It is not uncommon practice in forest inventory to estimate log length in standing trees to the nearest 1-2 metres. Ocular estimates of length of product and height of defects are used in conjunction with measurements and models in some inventories, e.g. MARVL (Deadman, M.W. and Goulding, C.J. 1978. N.Z. Journal of Forestry Science 8(2): 225-39). These estimates are usually satisfactory to 25 m height, but beyond this, significant error may be introduced and the results are unreliable.

When estimating the height of a tree by eye, always place vertically at the base of the tree a "calibration" pole of known height (e.g. 2 or 3 metres). Stand well back from the tree when making the height estimate.

Sources of error

Measuring the height of trees is time consuming and prone to errors. Experience has shown that when indirect methods are used to measure height, measurement from two independent positions is essential.

The readings from the two positions should agree within the limits of instrumental error - this is an absolute check on instrument and operator error (sighted to correct tip, etc.). Thus, differences of up to 1 m in readings for a 40 m tree are acceptable - precision of instruments under forest conditions is no better than this.

The most common errors include:

Measuring the leaning tree as if it is vertical, i.e. distance measured to tree base instead of the to the point directly beneath the tip. The plumb line is an essential adjunct to the height measuring instrument (to establish the point beneath the tip).
Instrument error. All instruments should be checked periodically against some standard or known height and adjust as necessary.
Operator and recording error. - personal error is always likely, e.g. :
1. incorrect setting of distance or booking of angles and distances, incorrect reading;
2. forgetting to add on the section of tree below eye level or forgetting to sight to the tree base;
3. measuring to wrong tip - shaking the tree may help!
4. difference of opinion amongst observers in nominating the tip of an umbrageous crown.
Effects of wind sway. Accurate readings are impossible in high winds. Preferably sight to the tip in a plane at right angles to wind direction.

http://online.anu.edu.au/Forestry/mensuration/HEIGHT.HTM
Cris.Brack@anu.edu.au
Mon, 6 Feb, 1997