I have used the formula from the biltmore stuck tech report in an excell spreadsheet to create biltmore sticks for my students based on their unique arm reach. However, I've noticed that the studnets with a reach of 25" (the same as a standard biltmore stick) arrive at measurements that are very different from a standard Biltmore stick that comes from Forestry Suppliers Inc. For example, the FSI "tree" scale stick has the following mesurements 24 DBH = 17.25", 30 DBH = 20.3", 36 DBH = 23.1. Compared to measurements using the formula; 24 DBH = 18.1, 30 DBH = 21.8, 36 DBH = 25.4. Can anyone suggest why such a large descrepancy?

Hmmm. . . An interesting question.

I reviewed the formula that I posted (several years ago). One of the conversions appears to be in error in the ATAN step, yielding the wrong result. This surprises me, because I went over it many times before posting, and had asked a math teacher to also review it. So, mea culpa. I apologize if this has created any problems.

What I did was to re-work the formulas, converting the trig identities first, instead of using them directly. The following formula yields the same results as the Forestry Handbook tables and the FSI stick.

S = stick measurement (unknown)
r = radius = diameter/2
a = reach (arm’s length of user)
[^2 = square, SQRT = square root]

S =2*a*((r/(r+a))/(SQRT(1-((r/(r+a))^2))))

Use this formula in an Excel spreadsheet. Substitute a field that holds the radius for r, and substitute a field that holds the reach of a.

If you want to communicate directly, send a message to
r carlson AT tree-tech DOT com
with no spaces, “@” sign for AT, and period for DOT. I’d be happy to send an excel file with this correction.

Thanks for pointing out this error.