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Exactly measuring tree height

Written and maintained by KoutaR, Jeroen Philippona and Michael Spraggon. Add a comment.

Contents

Introduction

Measuring the height of trees is more difficult than measuring diameter or girth. Many people try it with simple estimation techniques like the stick method, the lumberjack method or another easy method to get a rough estimate of the height of a tree.

Professional foresters however have, for decades, used methods based on measuring the distance from the observer to the trunk and the angle to the top. The latter can be measured with a clinometer. Instruments called hypsometers often perform these two measurements and calculate from them the height of the tree. However these methods can still result in large errors.

Here various methods of height measuring are described from the most accurate to the least accurate.

At Monumental Trees we recommend direct tape drop or the sine method as the most accurate and reliable.

Exactly measuring tree height

However, if a reliable tree height measurement is required, one of the following methods needs to be used (each one having its own caveats). All height measurements on the tree record list are made using a reliable and documented method.

Climbing the tree with direct tape drop

The most accurate way to measure the height of a tree involves climbing to the top of the tree and doing a direct tape drop. This method is used mostly by professional tree researchers and arborists, especially to measure the height of record breaking trees.

The climber needs to be able to climb to the top or within a few meters from the top. He (or she) has to take with him a high quality measuring tape at least as long as the distance he will be from the ground.

 
Climbing and measuring the one of the tallest known beech trees of Europe in the Sonian Forest, Belgium. March 2011.

This method is the most accurate possible, provided that the tape is straight, not deflected by branches, but not overstretched. This can be done by attaching a small weight at the lower end of the tape. The climber is often not able to reach the very top with the tape measure and can instead use an extensible pole to reach it. We recommend a light weight folding or telescopic pole, which can be extended to at least 3 meters to measure from the top down to a point where he can sit comfortably and safely attached to the tree before lowering the tape to measure from this point to a mark on the trunk near the ground, to which a second person can align the tape. Radio communication is helpful here!

Thus there are 3 measurements which are added together to give the total height:

  1. Distance from the top to the tape drop point (about 3 metres or more from the top).
  2. Distance from the tape drop point to the lower trunk marker near the base.
  3. Height of the lower trunk marker from the average point where the ground meets the trunk.

Please note: this method should be used only by experienced tree climbers with the relevant training and equipment, and with permission from the landowner. Certain professional arborist equipment and techniques are not suitable for measuring to the delicate tops of tall trees. These include the use of gaffs (foot spikes for climbing directly on trunks) and other heavy footwear and equipment which will damage the very thin branches and shoots when hauled through the uppermost part of the crown. For more information please visit www.treeclimbing.com.

The sine method, using a laser rangefinder

If used properly, this method is also very reliable. An important advantage over many other methods is that over-measurement is not to be expected.

The principle of the sine method is that you calculate the height of a tree by measuring the distance to the top and the angle to that top.

Height Tree = A + B

where:

A = [sin (a) * D1]
B = [sin (b) * D2]
D1 = distance to the top
D2 = distance to the base
a = angle to the top
b = angle to the base


The measurer uses three devices (or the combination of them in one device):

  1. a laser rangefinder: with this you can measure the distance from the observer to the top as well as to the base of the tree
  2. a clinometer: with this you can measure the angle from the observer’s viewpoint to the top of the tree as well as to the base
  3. a calculator: this is needed to do some basic trigonometry to calculate the height.

There are instruments (like Nikon Forestry [550 and Pro] laser rangefinders) that have these three instruments all built inside one device. The distance and the angle to the top are measured at the same time and the internal calculator does the mathematics to calculate the height. Even the adding of the height to the top and to the base can be done automatically.

If the tree has a broad crown or more than one top, it is important to scan all of the tops with the laser rangefinder to find the tallest. The top should be measured several times to be sure that the result can be repeated.

Instructions when using a simple laser rangefinder + clinometer + calculator

  1. measure the distance to the top with the laser = D1
  2. measure the angle to this top with the clinometer = a
  3. calculate with the calculator: A = D1 * sin a (A is the height from eye level to the measured top)
  4. measure the distance to the base of the tree with the laser = D2
  5. measure the angle to the base of the tree with the clinometer = b
  6. calculate with the calculator: B = D2 * sin b (B is the height from the eye level to the base of the tree)
  7. Height of the tree is A + B.

Instructions when using a combined laser rangefinder (for example Nikon Forestry 550 or Pro)

A. aim at the top or in case of a broad crowned tree one of the tops The display screen shows:
  1. the distance to the top
  2. the angle to the top
  3. the calculated vertical distance between the top and the eye level (A)
B. from the same position, aim at the base of the tree. The display screen shows:
  1. the distance to the base
  2. the angle to the base
  3. the calculated vertical distance between the base and the eye level (B)
C. the height of the tree is A + B. The laser rangefinder has an function for automatically adding these two.
Height Tree = A + B

where:

A = [sin (a) * D1]
B = [sin (b) * D2]
D1 = distance to the top
D2 = distance to the base
a = angle to the top
b = angle to the base


Note that the sine method gives the correct height even if the top is not directly above the base.

Possible sources of error:

  • In deciduous forests, especially in summer, it can be impossible to hit the tallest top with the laser when it is blocked by leaves and branches.
  • With a multi-top tree it can be hard to find the tallest top.
  • Be sure to hold the laser rangefinder in upright position: if holding the laser tilted, you can get an over-measurement.
  • Try to hold the clinometer or the laser rangefinder with a built-in clinometer as still as possible; moving the device may result in the clinometer giving a false angle.

Note that not all the lasers operate with sine method (e.g. Impulse Forest Pro, Nikon Forestry Pro with 3-point measurement). These lasers measure the distance to only one point at the trunk in combination with measuring the angles to the top and the base: this is a tangent method.

Measuring tree height approximately

Tangent method

The tangent method has been used for thee height measuring a lot by the professional forestry and forest research. It works well with single stemmed, vertical conifers but has the risk of over-measurement if used for leaning trees as well as trees with a broad top.

The principle of the tangent method is that you calculate the height of a tree by measuring the horizontal distance to the trunk (the baseline) and the angle to the top.

The measurer uses three devices (or the combination of them in one device):

  1. a clinometer: with this you can measure the angle from the observer’s viewpoint to the top of the tree as well as to the base.
  2. a measuring tape or other instrument to measure the baseline – the horizontal distance from the observer to the trunk of the tree.
  3. a calculator: this is needed to do some basic trigonometry to calculate the height.

Some devices have these three functions in one, like the Forestor Vertex Hypsometer but also some laser instruments. C 1. Instruction for tangent measurement using a clinometer + tape + calculator a. measure the angle to the top of the tree as seen with the clinometer = a b. measure the baseline = the distance from the observer to the trunk c. calculate with the calculator: A = Tan Angle a x D (A is the height from eye level to the measured top) d. measure the angle to the base of the tree with the clinometer = b f. calculate with the calculator: B = Tan Angle b x D (B is the height from the eye level to the base of the tree) g. the height of the tree is A + B.

Height Tree = A + B

where:

D = distance from eye to trunk
a = angle to the top
b = angle to the base


Why do we not consider the tangent method as reliable?

In this figure the measurer calculates the same height for the conifer and the leaning broadleaved tree if he does not realize the top of the leaning tree is not perpendicular over the base.

To measure the correct height the measurer has to cross-triangulate the actual top projection to the ground and has to make a second baseline for the top measurement.

In this figure, the top is leaning towards the observer, and it is necessary to crosstriangulate the actual top projection to the ground (the point directly under the highest top). From this point a separate baseline for the top measurement is taken, otherwise the height will be exaggerated (or underestimated if the tree is leaning away from the observer).

Other approximate methods

Other, more simple estimation techniques are for example the stick method, the lumberjack method or the photograph method.

Measuring instruments

Examples

Three instruments used for height measuring of trees:
A. Suunto clinometer
B. Haglof Forestor Vertex digital hypsometer
C. Nikon Forestry 550 laser rangefinder

The Nikon Forestry 550 Laser Rangefinder.

At the screen the measurements can be given in metres or in feet.

After measuring a tree top the screen shows:

  1. measured distance to the measured top
  2. measured angle to the measured top
  3. calculated horizontal distance to the same top
  4. calculated vertical height above the observers eye level.

Purchasing and further information on measuring equipment

   

More on tree measuring:

 

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