At the risk of stating the obvious, most trees are decidedly asymmetrical, not perfect circles, and the drip line will be an irregular area at best. Plus the roots will extend sometimes several feet beyond the drip line so if you're calculating the area for a tree to be watered, take that into account! Also, quirky plants that they are, if they have a long branch extending out one way, chances are they have a long, strong root out in the opposite direction to keep from falling over.
Mother Nature's a pretty awesome engineer, but straight lines, right angles and perfect circles aren't on her list of specs.
