You need your staff, notched in feet and inches (or meters and centimeters); or a straight stick of known length (measured by the spread of your fingers). Proceed thus (see above illustration): Stand the staff (b c) upright in the sun and measure the length of its shadow (a b). Measure the length of the shadow of the tree. Multiply by the length of the staff. Divide by the length of the staff shadow. The result is the height of the tree.
For example, say the length of the tree's shadow is 40 feet or meters, the staff's shadow ten feet or meters and the staff is the normal length of 1.73 feet or meters, the formula will be as follows:
40 x 1.73 = 69.2 divided by 10 = 6.92