An angle gauge is a tool used by

foresters A forester is a person who practises forestry, the science, art, and profession of managing forests. Foresters engage in a broad range of activities including ecological restoration and management of protected areas. Foresters manage forests to ...

to determine which trees to measure when using a variable radius plot design in

forest inventory Forest inventory is the systematic collection of data and forest information for assessment or analysis. An estimate of the value and possible uses of timber is an important part of the broader information required to sustain ecosystems. When taki ...

. Using this tool a forester can quickly measure the trees that are in or out of the plot. An angle gauge is similar to a

wedge prism The wedge prism is a prism with a shallow angle between its input and output surfaces. This angle is usually 3 degrees or less. Refraction at the surfaces causes the prism to deflect light by a fixed angle. When viewing a scene through such a pris ...

though it must be held a fixed distance from the eye to work properly. Unlike the wedge prism, which is held over the plot center, the surveyor's eye is kept over plot-center when using an angle gauge.

Use

When using an angle gauge the user must count trees that are larger than the width of the angle gauge, as viewed from the center of the plot. The angle gauge is held a set distance from the eye of the surveyor. Most angle gauges have a string or chain that lets the user know the set distance. Each angle gauge is set at a certain basal area factor (BAF). Each tree that is in the plot represents this number, the BAF, of square footage. It is multiplied by the number of trees on the plot to give basal area per acre. In the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

BAF is measured in units of

square feet The square foot (plural square feet; abbreviated sq. ft, sf, or ft2; also denoted by '2) is an imperial unit and U.S. customary unit (non- SI, non-metric) of area, used mainly in the United States and partially in Canada, the United Kingdom, Bang ...

. For example, using a BAF 10 angle gauge a forester measures 12 trees that are ''in'' trees. Therefore, this plot represents .

Borderline trees

Some trees are on the borderline of the plot and must be checked to be sure they are to be included. The limiting distance must be calculated to see if the trees are ''in'' or ''out''. The equation to calculate the limiting distance in feet is

diameter at breast height Diameter at breast height, or DBH, is a standard method of expressing the diameter of the trunk or bole of a standing tree. DBH is one of the most common dendrometric measurements. Tree trunks are measured at the height of an adult's breast ...

, DBH, in inches times plot radius factor, PRF, in feet/area D_lim = DBH×PRF. For example, using a BAF 10 angle gauge a forester needs to know the limiting distance for a tree. The PRF using a 10 BAF angle gauge is for every inch of tree diameter. Therefore, 20 inches × 2.75 ft/inch = . This means that a tree must be over away from the center of the plot using a BAF 10 angle gauge to be out of the plot.Introduction to Forest Measurements

Use

Borderline trees

References

Further reading