Home › Volumes › Trees › Forestry

Measuring Height

This method of estimating the heights of trees is common among German foresters. At a distance just equal to the tree's height, the observer, with his head between his knees, sees the top of the tree and no higher. To get this location is very easy; then there is left nothing to do but to pace off the distance.

The tree's shadow on bright days may be measured, then the shadow of any short object standing erect--a man, a fence post or a sapling. As the man's shadow is to his height, so is the tree's shadow to its height. Suppose a six-foot man casts a ten-foot shadow, and the tree's shadow is seventy feet. The proportion reads:

10:6::70:x; then 6 x 70/10=x. The tree is forty-two feet high.

A third simple method is interesting. Set a perpendicular pole about five feet high in the ground at a distance about equal to the tree's height from the base of it. Between this short pole and the tree, in line with both, set a taller pole, near enough so that, sighting from the top of the short pole to the top of the tree, the line of vision crosses the tall pole. Have this point marked. Now sight the base of the tree, and mark the place where the line of vision crosses the taller pole. Measure now the distance between the poles, the distance between the short pole and the tree, and the distance between the two marks on the tall pole. Suppose the marks on the pole to be six feet apart, the poles five feet apart, and the short pole forty feet from the tree. Then we have two similar triangles and a proportion with three known quantities. The distance between the poles is to the distance from the short pole to the tree as the distance between the marks on the tall pole is to height of the tree. 5 : 40 :: 6 : x.

Solved, 40 x 6/5=48. The tree is forty-eight feet high.

A fourth method involves a right-angled isosceles triangle and a plumb line, but it is extremely simple, and is in common use by men who go out to estimate standing timber in terms of board measure. Take a square of pasteboard or shingle, and cut it in two diagonally. One of these halves is your tool. To the square corner hang a plumb line--a string with a weight attached--to indicate when you hold the triangle so that its sides are exactly vertical and horizontal. Sight along the diagonal, stepping backward or forward until the top of the tree is in line with the diagonal and your eye. Now sight along the horizontal base line of the triangle to get the point on the tree trunk at the height of your eye. The tree's height above this point is equal to your distance from the tree, for it is one base of an isosceles right-angled triangle similar to your tool. Pace the distance to the tree, add your height, and you have the tree's height. In this method of measurement, level ground is necessary to the amateur. The practised eye makes due allowance for inequalities, which must be taken as they come in the woods.

The Faustman "mirror hypsometer" is a clever little instrument by which the observer may get the height of trees by simply pacing the distance from its base to the point where the treetop is in line with an eye piece and a hair line set six inches away. The treetop appears to the observer, a slide is moved up to the figure corresponding to the distance, a plummet swings over a scale, and the figure it covers, reflected by a mirror to the observer's eye, is the tree's height. This convenient tool does away with computations, and enables the user to accomplish much in a short time.