Learn what Forester Kathy Mattson is doing:

Forester Kathy Mattson is using a relascope to measure trees in the forest.
To use the relascope (relaskop) to measure tree height, diameter or basal area, you look through a hole in the front of the instrument. You'll see several scales for different measurements on the bottom half of your view, and the tree you're looking at will show in the top half of your view.
The different scales are for height and diameter of a tree, and basal area of a group of trees. Basal area is the amount of land (per acre or per hectare) that is covered by tree trunks at 4.5 feet above the ground.

The relascope combines the jobs done by other popular forestry tools, including the clinometer and the diameter tape.

A clinometer is used by foresters and other natural resource professionals to measure the slope of land, roads or streams and to get an approximate measure of tree height.

Using right-triangle trigonometry, you could get the same information by measuring your distance from the tree and the angle between your eye and the tree and your eye and the ground, and using the tangent to calculate the tree's height.
The clinometer does some of the calculations for you. (You'll still need to measure your distance from the tree.)

A diameter tape (D-tape) is used to measure the diameter (distance across the trunk) of a tree. Since trees are swelled at the base, 4.5 feet above the ground is used as a standard measuring site. (This is called diameter at breast height-dbh.)
The D-tape has units that are 3.14 inches apart-the measurement of pi!
You could measure the circumference of a tree with a measuring tape, and then divide by pi (3.14) to find the diameter. D = C/pi.
Since it is divided into units of pi already, the D-tape has done the calculations for you.

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Idaho Forest Products Commission
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