- Wedge prism
The wedge prism is an optical element typically made of glass ground at slight angles to refract light passing through it from the smaller width side of the prism to the thicker width side of the prism. By refracting the light, the wedge prism offsets the object being focused upon.
Uses in Forestry
prismis primarily used as an angle gaugein variable-radius plot sampling. The wedge prism can be used, “To rapidly establish fixed-radius plots in the field, common forest survey techniques can be used with a target object calibrated to the desired size of the plot…Wedge prisms used with a 10.2-cm PVC pipe or cylinder were accurate, precise, inexpensive, and easy to use”. [ [http://www.treesearch.fs.fed.us/pubs/24888 Comparison of combinations of sighting devices and target objects for establishing circular plots in the field ] ] In forestry, the wedge prism is used to record and calculate basal area of a stand via a timber cruiseknown as tree inventory. Because the wedge prism refracts light to offset the object of interest (ex. A tree), it can be used to determine whether or not the tree is inside the plot radius based on the diameter at breast heightof the tree and its distance from the plot center. If the tree is inside the plot radius, then the offset section of the tree will overlap the original bole making it an "IN" tree (Figure 1); if the tree is on the border of your plot radius the offset section of the trunk will be aligned with the original bole which is referred to as a borderline tree(Figure 2); if the tree is outside your plot radius then the offset section of the tree will not overlap the original bole which is referred to as an "OUT" tree (Figure 3). Wedge prisms come in different factors (5, 10, 20 BAF) and the factor of the prism one chooses to use is multiplied by the number of trees tallied in each plot as an observation of the basal area in the stand on a per acre basis. Wedge prisms can be difficult to use in wet conditions due to the effect water droplets have on the optical properties of the glass. Wedge prisms come in different colors such as clear or amber. The amber provides the same function as the clear wedge prism, only it reduces glare and is easier to use on overcast or cloudy days. Operating a wedge prism is one technique used in forestrytoday because, the wedge prism is simple, relatively inexpensive, portable, and as accurate as other angle gauges when properly calibrated and used properly. One simply holds the prism directly over the plot center, [ [Avery, Eugene Thomas; Burkhart, E. Harold. 2002.Forest Measurements 5th Edition. McGraw-Hill Higher Education, New York, NY. Copyright 2002.] ] and by focusing on a tree, the refracted light will offset the trunk of the tree. The wedge prism is used to take measurements in both land management and in timber procurement. Other tools often used to accompany the wedge prism in taking forest inventoryare clinometer, Biltmore stickand a Diameter Tape.
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