An angle gauge is a tool used by foresters to determine which trees to measure when using a variable radius plot design in forest inventory. Using this tool a forester can quickly measure the trees that are in or out of the plot. An angle gauge is a very similar tool to a wedge prism though it must be held a fixed distance from your eye for it 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.