You can obtain a rough guess of a tree's height by:
- comparing with a measureable object nearby the trunk, eg. a pole or a house of which you know or can measure the height and by looking from a distance how many times that objects fits the tree. You could also do this on a photograph that was preferably taken from a distance as large as possible (to have the smallest perspective distortion of the tree possible) with the largest zoomfactor your camera has (to have the least lens distortion possible).
- a more correct method is based on goniometry:
you start to walk away from the base of the trunk until you see the tree's top from an angle of 45°
(which you can check using your arm).
The height of the tree is then the distance from the tree to where you're standing (eg. 80 ft) + your eye height (the distance from the ground to your eyes, eg. 5½ ft).
The idea is that if there's an angle of 45° (the angle between your line of sight and the ground) in a right-angled triangle (a triangle with an angle of 90°, the corner tree-ground), then both small sides have an equal length. This means that the height of the tree then equals the distance tree-observer. But since your eyes are not on the ground, you need to add your own eye height as an extra (see image).
Provided you train yourself a couple of times in making steps of eg. 3 ft, you can relatively quickly determine the height of something. If you make your steps correctly (or use a tape to measure your distance from the tree) and if you're sufficiently able to determine an angle of 45° (or make you use of an inclinometer or tilt meter), then such a height guess can be quite good theoretically. But keep in mind that height measurements should be looked at very critically.
If you are not sure your guess of the 45° angle is any good, you can do it like this too: take a stick of about 1 to 2 ft and keep it vertically with a straight, horizontal arm. Walk away from the tree until the top of the tree corresponds with the tip of the stick, from your point of view. Then turn the stick horizontally and remember with which spot (as far away from you as the tree is) the tip of stick corresponds. The height of the tree equals the distance from the tree to that spot.
These methods assume the tree is not growing on a slope and the top of the tree can be determined (by surrounding trees or by the round shape of the crown the actual top of the tree can remain hidden for the observer). Also, 'distance to the tree' is not entirely correct: it should be 'distance to the ortogonal projection of the tree's top on the ground'. The difference between the 'distance to the tree' and that projection is half the diameter of the tree near the ground.
- a third and the most accurate way consists of climbing to the top of the tree and doing a direct tape drop.
Of course this is something that should only be doing taking extreme precautions.
This method is rarely used. On the photo on the right you can see Ronny Schreurs climbing a giant sequoia in
military area Massy in Houthalen-Helchteren (Belgium) to determine its height.
Video material of the tape drop of Hyperion, the tallest tree in the world.
Professional arborists measure trees' height by using a inclinometer like the Suunto clinometer or the Forester Vertex, by which a number of distances
and angles are measured. In more recent years handy laser equipment is used and is rapidly replacing the traditional inclinometers.
Using analoguous formulas from goniometry (like mentioned above) the height can be calculated.
A tape is used much more infrequently, except for specific trees like record trees or felled trees.
Above (left) you see the Nikon Forestry 550, a professional laser based range finder that can be used to measure the height of trees accurately and quickly. On the right see a climber working his way to the top of the tallest known beech tree in the Sonian forest to measure its height using a tape.
More on tree measuring: