Place a stake in the ground next to the tree so that the tree's shadow does not obscure the stake's shadow. Ensure the stake is far enough into the ground that it does not move easily.
Measure the exposed length of the stake with a tape measure. Assume for this example that the exposed stake measures 4 feet in height.
Measure the length of the stake's shadow from the base of the stake to the tip of the shadow. Assume for this example the stake's shadow is 3 feet in length.
Measure the length of the tree's shadow from the base of the tree to the tip of the shadow. Assume for this example the tree's shadow is 27 feet in length.
Divide the length of the stake's shadow by the height of the stake to find the ratio between an object's shadow and its height. The shadow of the stake is 3 feet and its height is 4 feet long in this example, so the ratio between the stake's height and its shadow is 3 divided by 4, or 0.75.
Divide the length of the tree's shadow by the ratio between an object's shadow and its height to obtain the height of the tree. The tree's shadow is 27 feet long in this example and the ratio between the length of its shadow to its height is 0.75. The height of the tree is therefore 27 divided by 0.75, or 36 feet.