Sampling for Tree Form
It doesnt matter how the taper equation was computed, or what equation was used. It is sure to be wrong. The only question is "by how much?" Most of us just hope that the answer is "not much."
No matter how the trees were chosen to build a volume table or taper equation, the trees you use it for are from a larger, smaller or different population. Since this is the case, the system cannot be unbiased. Everyone would probably feel better if there was a way of verifying that the answers were on track for measurements in their stands.
Some modern systems use a "third diameter" to fine-tune the determination for taper. That really helps, but in the end there is always the question of how good the answer is for the specific inventory you are conducting this time.
The solution is to measure some trees. In some situations, trees are felled as part of the inventory system (which has the additional advantage of checking the net volume and inside diameter as well as the gross volume). More often, the gross volume of the tree is determined directly by measuring the outside diameters at several points up the stem.
The Relascope is a good instrument for this purpose. Choose a point where you can see the tree well. A good Relascope scale for this is the "wide angle" scale, and you might consider purchasing this scale the next time you buy a Relascope. You can also use any other scale, since the band widths are all equivalent to certain "percents" of the distance to the tree. It is particularly handy to use bands that are the same width, and perhaps the small equal-sized bands would be a convenient choice in most cases.
Rather than trying to read the diameters at different heights, you can measure the distance between points where the tree exactly fits the Relascope bands. The stems of real trees are surprisingly "bumpy," especially around branch whorls. You may find that you need to pick a point after looking up and down the stem several feet in each direction and perhaps ignoring the branch swellings. A staff to steady the Relascope will come in very handy.
With this technique, you can at least check the gross volume (outside bark) against the taper equations or volume tables that you normally use. If the measurements are very close, then so much the better. If there are differences, you have some idea how large they are. The trees should be chosen randomly or systematically from actual cruise trees, if possible.
Many years ago, Lew Grosenbaugh developed a technique called "height accumulation" which used this "bar matching" technique, along with a simple way to manipulate the readings quickly on a 2 register adding machine. Those days are long gone, and a programmable calculator or field recorder application will do this work far better.
The concern with taper or volume determination, however, is just as pertinent as when Grosenbaugh first worked out his system. The solution is the same. Measure some trees and get the answer directly. The only way to eliminate a whole set of assumptions and possible errors will always be to make direct measurements on a sample of the cruise trees. Even a few trees per cruise, accumulated over a few months effort should yield interesting results.
If some of you do this at regular intervals (or use some other instrument) and have any practical advice for our readers then please e-mail us with your suggestions or observations.
They also have a very extensive set of publications on the Relascope and research into Variable Plot Sampling (Angle Count Sampling is their term).
If you need a Relascope manual, or are interested in their technical publications, write to them at:
Originally published July 1998
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