Measuring Board Feet

My son and I went to Ritter Park to measure board length in a tree.  In the picture I was measuring the circumference(182inches) of the tree (not hugging the tree).  I then paced off 66 feet in my case that is 29 paces (a pace is a right step and a left step). Next I used the clinometer to find the angle up to the top of usable wood and down to the ground (63%). All that was left was to do a little math to find that the trree had 18 board ft.