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In the US, some clinometers have a scale that is set so that tree height can be read directly from a clinometer if the user is standing exactly 66 or 100 feet away (for example) from the tree (horizontal distance). In Western Canada, it was/is more popular to use the percent scale as indicated below while ensuring the person stands at least as far away from the tree as the tree is tall. That is, if the clinometer reads over 100 percent, then the user is not far enough away from the tree for an accurate measurement. Distance (HD-distance from tree) and tree height (HT) are in meters.
The following explanation is quoted verbatim from “The Forestry Handbook BC, 2013 Part 2”
Total tree height is the distance between the ground and the tip of the tree, commonly recorded in meters. Tree height is closely related to the quantity and quality of the wood in the bole of the tree and may be used to indicate site quality (discussed later in this chapter).
As well as total height, the height to the live crown (the distance between the ground and the lowest living branch) is sometimes measured to indicate tree vigor and growth. Also, merchantable height between the ground and some upper stem diameter is often measured.
Direct measurements of height using a measuring tape or telescoping pole can only be obtained easily for small trees. For larger trees, direct measurement requires falling the tree, and measuring along the bole with a measuring tape.
To avoid felling large trees to obtain measurements, indirect measurements of height calculated from angles and the horizontal distance can be obtained (Figure 2).
To obtain the measurements needed, the observer stands at a known distance from the tree, at a position where both the base (or breast height) and the top of the tree can be seen. In order to make angle measurements with reasonable precision, it is a good idea to keep the angles being measured below 45° or 100% of horizontal distance (percent scale). On flat ground, this means being at least as far away from the tree as the tree is tall. For sloped ground, more precise measurements can be obtained by standing up-slope from the tree base. Using the angles measured in percent scale, the formula to calculate height is:
where HT is the height of the tree in m, HD is the horizontal distance of the device away from the base of the tree in m, %UP is the angle in percent measured to the top of the tree, and %DOWN is the angle in percent measured to the base of the tree. Note that the angle to the tree base will be negative if the base of the tree is down slope from your eye level. If the angles are measured in degrees, the appropriate formula is:
HT = HD × [tan(up) – tan(down)]
where tan(up) is the tangent of the “up” angle and tan(down) is the tangent of the “down” angle. As above, the “down” angle is considered negative if it falls below the horizontal