A simplified procedure for measuring tree heights
West. J. Appl. For.
The most commonly used technique for measuring the height of a tall tree involves "shooting" the top and bottom of the tree with an Abney level or clinometer. Typically, these angles are measured as percent slope. The percents are multiplied by the horizontal distance from the tree to the observation point. The two vertical distances that result are added or subtracted depending on their algebraic signs, to arrive at an estimate of total tree height.
Long, J.N. and P. Mohai. 1986. A simplified procedure for measuring tree heights. West. J. Appl. For. 1:21-22.