Sweep a calibrated angle gauge. Count the trees whose crowns exceed it. Multiply by the basal area factor. That's your basal area estimate — per hectare, from wherever you happen to stand.
Each tree casts a counting zone into the space of possible vantage points: the set of all positions from which it would be counted. Zone radius: R = DBH / (2√BAF)
The heatmap is that invisible landscape, made visible. High spread means the forest has a strong opinion about where you stood.