|[Basal area gauge]|
One method of cruising (estimating the timber in) a forest is by finding the basal area of all the trees on an acre. That's easily done with a calibrated prism (which works somewhat like a camera rangefinder) or a gauge like this one. To use it, we'd hold the end of the brass chain in our teeth (to keep the distance from eye to gauge (and thus the angle) constant, and turn around a point, counting the trees that appear bigger than the notch as we go. (If we're using the bottom notch, the tree shown wouldn't be counted; the tree on the right, being bigger and/or closer, would be.) Since the bottom notch is calibrated as having a factor of 20, it can be mathematically shown that the count times 20 will be the average basal area per acre in this area. The basal area in itself may give us useful information about whether the trees need to be thinned or are too sparse, and together with a volume/basal area ("V-BAR") table, we can estimate how much lumber is growing on an acre of this forest.