FOREST Plot Measurement Tools
The 10-Factor Wedge Prism
In this exercise, you will practice using a surprisingly simple, yet innovative instrument to determine the basal area on a specific site or sample point. The wedge prism is a small piece of glass that has been ground to refract light rays at a specific offset angle, which creates an optical illusion. In most eastern forests, we use a prism ground to an angle of 104.8 minutes. Most importantly, this means that each tree that is measured or tallied is equal to 10 square feet of basal area, so we are using a wedge prism that has a basal area factor of 10 (BAF = 10).

The optical illusion the wedge prism creates (as a result of the angle it is ground) appears to offset a portion of the tree’s stem or trunk when viewed, preferably at DBH (4.5 feet or 1.3 m). If the offset portion viewed through the prism appears to connect with the main stem of the tree, you will tally that tree as in or countable. If the offset portion appears completely removed from the main stem of the tree, do not tally that tree as it is out or not countable. For trees that appear to be borderline or on the edge, simply count it as half a tree.

After determining the number of in or countable trees, as well as half of every borderline tree, simply multiply that number by 10 to determine your basal area (because you are using a 10 BAF prism). For example, if you tally have 10.5 trees, your basal area is 105, or you have "105 square feet of basal area per acre". If you think about it, an acre (.4 hectares) contains 43,560 square feet, while in this example, only 105 square feet is actually occupied by tree stems, or trunks. The branches and crowns of the trees, as well as other smaller vegetation occupy the remainder of a forested acre.

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