Does anyone know of literature about the error rate of Biltmore sticks? We are finding about a 10% discrepancy between Biltmore stick DBHs and D-tape DBHs, and are interested in any formal study of the reliability of the tool.

Reply to post by Jerry Bpmd, on January 31, 2002 at 08:55:01:

Is the 10% discrepancy more or less from D-tape?

Reply to post by Jerry Bpmd, on January 31, 2002 at 08:55:01:

Your sticks might be too short.

Reply to post by Jerry Bpmd, on January 31, 2002 at 08:55:01:

Biltmore sticks are used frequently by foresters in CA. We use them when we need precision to a 2" diameter classification. Any time you need precision finer than that you should use a Diameter Tape. The Biltmore stick is a standard manufactured product so they are all constructed the same length. The accuracy of the reading is predicated on the length between the eye and the stick layed against the tree being measured. The sticks are manufactured so that the correct angle is achieved when the stick/tree is exactly 25" from the eye. Anything shorter or longer than that will affect the indicated diameter. Phil RPF 1671, ISA WC6122.

Reply to post by Philip A Bjorkman, on January 31, 2002 at 08:55:01:

I have posted a Technical Report on this site detailing how to construct a Biltmore Stick. You can vary the length of 'reach,' customizing it to your own needs.

The discrepancies in measurements can come from several sources. Proper use of the stick is first. As Jerry pointed out, the stick must be held at the appropriate distance from the eye. It must also be held so that the stick is perpendicular to the line of sight from eye to the index or '0' mark. Finally, the measurement must be taken by shifting the gaze, not turning of the head by much.

Another error is introduced by the tree. Unless the trunk being measured is perfectly round, you will measure an axis that may not be exactly the diameter. It could be more or less. A D-tape averages this automatically.

Finally, a D-tape is not accurate in all cases. For trees that are not circular, there can be some error. A D-tape really measures circumference, then mathematically computes the diameter based on a circle (C = PI * D).

Reply to post by Jerry Bpmd, on January 31, 2002 at 08:55:01:

Hi Jerry.

I guess another question is how badly you need better precision and at what cost. If there is a simple fix like better training or a simple reach control (remember the pre-measured focal length cord on the classic Minnox?)it might be cheap. If on the other hand it is a repetitive training cost (new volunteers all the time say) or if the other issues Russ describes mean the errors still creep in maybe you accept the error or go to a different method.

If you are not actually calculating bd.ft. of timber or basing payments or revenues on size or size classes does it matter that much? If you are establishing rough size classes within populations or providing a visual description "oh yeah that's the 20" peanut butter and jelly tree over there," does +/- 2" matter?

Scott

Reply to post by Nelson Shaw, on January 31, 2002 at 08:55:01:

Yup. I was actually doing a demo for the Virginia Department of Forestry when I posted the question, hence the less than careful format (like misspelling my own name).
I think I actually have 2 questions: 1) if I send 10 experienced users to measure the same trees with a Biltmore stick, what will the variance be?, and 2) what is the average diff between using a Biltmore stick and a D-tape?
I appreciate the useful comments from everybody. I think I will try to push Davey to get some real numbers the next time we do an inventory, and then I will post the results for anyone interested.