Using the MSU Basal Area Angle Gauge | Mississippi State University Extension Service
Basal area (BA) is commonly used by foresters as a simple and easily measured estimate of the density of a stand of trees. BA is simply a measure of the cross-sectional area of a stand of trees at 4.5 feet aboveground and is expressed in square feet per acre (square feet per acre). In other words, think of BA in the context of slicing all trees on a given acre at 4.5 feet
Source: msstate.edu