Relascope

The Relascope is a forestry instrument designed to quickly estimate basal area per unit area in a forest stand. It operates based on the principle of angle count sampling, also known as Bitterlich sampling. The relascope allows a forester to efficiently determine the number of trees that are "in" or "out" from a given point, which is then used to calculate the stand's basal area.

The instrument typically consists of a sighting device with a fixed angle. The user stands at a sample point and rotates the relascope 360 degrees, sighting each tree at breast height (usually 1.3 meters). If the tree's diameter at breast height appears wider than the fixed angle of the relascope when viewed from the sample point, the tree is counted as "in". Conversely, if the tree appears narrower, it is "out". Trees where the diameter exactly matches the angle are considered borderline cases and a decision is made to either count them as "in" or "out" based on a pre-determined rule (often alternating between inclusion and exclusion for successive borderline trees).

The count of "in" trees is then multiplied by a basal area factor (BAF), which is a constant specific to the relascope's design. This calculation provides an estimate of the basal area per unit area (e.g., square meters per hectare or square feet per acre). Different relascopes have different BAFs, allowing foresters to select an instrument appropriate for the density of the forest being surveyed.

The relascope offers a relatively rapid and straightforward method for estimating basal area, but its accuracy depends on factors such as the careful selection of sample points and the consistent application of the sighting technique. Slope correction might be necessary on steeper terrain to maintain accuracy.