Method 1:

Hold a ruler of fixed length (one, two or three feet) at an arm's length in front of you, and step backwards from the tree until the line of sight through bottom of the ruler touches the tree's base, and the line of sight over the top of the ruler touches the topmost twig of the tree. Ideally, this sighting should be taken with your eyes as close to the ground as possible to assure the sighting triangle is a right triangle.

Measure the distance (in feet) from your eyes (O see diagram, which can be viewed in a larger format by clicking on it) to the ruler (B in diagram), and from your eyes to the base of the tree (C in diagram). The height of the tree is then distance C multiplied by distance B, and divided by distance A. For simplicity, if the ruler B is one foot, then the height of the tree is simply C divided by A.

Method 2:

Make a clearly visible mark on the tree trunk at a fixed, measured height above ground level—say, four feet, or six feet. Hold a ruler in front of you and step backwards until the one inch measure on the ruler matches the distance from the ground to the mark on the trunk. Notice how many inches on the ruler match the entire height of the tree. The height of the tree is then the number of inches multiplied by the height in feet of the mark.