This tool makes it a snap to figure basal area for the government programs that require removal of a certain amount of BA. It measures 5,10,20 and even 40 BAF (basal area factor) to figure BA per acre. I checked this spot and counted 17 trees wider than the 5 spot which is 85 BA. The plan calls for 23 BA average.
I know you can also use one of the coins (can't remember if it's penny, nickel, quarter) to factor basal area. My arms a quite a bit longer than the distance needed to use said coin, so I prefer this tool.