Thanks, but there's some problems with that technique. First, I don't have
a tool that would remove a hollow core from these trees (The largest are
3.5-4.0 ft. in diameter). Second, its intrusive and I'd rather not bore
into the tree. Finally, I'd like to survey a hundred trees or so which
would make it impractical.

The "growth factor" method involves a quick and easy measurement with a
tape, and an estimate is all I need, not spot on accuracy.

Thanks again.