Today I returned to remeasure the Broad Brook Double Pine which Monica has renamed the Broad Brook Grandmother Pine. Fine by me. I had done one photo measurement of the trunk diameter, and my instinct told me that I was pretty lucky with the result I got because the shape of the trunk is definitely not circular. Plus, I got sloppy and didn't take into account the difference in distance between the reference object and target. I also mis-measured the girth by 0.1 feet at the point I placed the marker because my tape wasn't level going around the tree. When I eliminated the problems, the difference between the computed girth based on the photo measurement and the taped measurement became 5.15 inches. This is to be expected given the shape of the trunk, which is much more elliptical than circular. So, my plan was to measure the trunk's major axis and its minor axis and using them compute the perimeter of the ellipse and then compare it to a tape-measured result. The two images that follow tell the story. The first images show the measurement of the major axis and the computed girth I get using it though it were the diameter of a circular trunk. The second images shows the comparable process for the minor axis, plus the computation of the perimeter of the ellipse from the photo-measured major and minor axes. The girth of Grandmother is 15.2 feet at the center of the reference object. The computed perimeter of the ellipse is 15.04 feet. The difference between the taped result and the perimeter of the ellipse is 1.3%. I think that's remarkable. I probably got lucky, but only to an extent. The photo method is proving itself in these tests.
Robert T. Leverett
Co-founder and Executive Director
Native Native Tree Society
Co-founder and President
Friends of Mohawk Trail State Forest