OOPS – I used the wrong plot radius factor for the borderline trees -- now what??

by Kim Iles

Recently, a young cruiser made a simple mistake. He used the Plot Radius Factor (PRF) to the center of the tree – but applied it to the face of the tree, thereby including a few too many borderline trees. Now as you all know, the checked distance is similar (and amounts to a distance equal to the radius of the tree). What can he do to salvage the situation?

All of the right borderline trees would have been checked with either factor. After all, he was using the same angle gauge either way. If “close” trees were decided incorrectly, there would be a bias in the number of “in” trees – followed by a bias in basal area, volume, value, etc. If the basal area is off by X%, the rest of the cruise information on a per acre basis would be off by X%.

Even if we don’t believe that the difference is large, how can we fix this problem? It turns out to be quite simple. First, remember that the actual BAF that you are using depends on how you check the borderline trees (not the reverse). Think about that a moment, and you will see the logic here. If you round off the PRF, for instance, you can actually change the BAF that you are using. The prism or Relascope is only a preliminary tool for judging “in or out”, and the exact tree count comes from how you determine any borderline trees.

To calculate the PRF for a 40 sq ft/acre BAF, enter 40 in cell C62 of the STAR_BAR spreadsheet available for download at this website. You get one of two possible values: 1.375 ft/inch of DBH from the center of the tree, or only 1.3333 times DBH to the face of the tree, about a 3% distance difference, but the total effect is about double that, because you are putting a “rind” around the outside of each variable plot.

You can get the appropriate constants for any BAF (Metric or English) using this spreadsheet. So, our cruiser used 1.375, and applied to the face of the tree, and got an incorrect tree count for a 40 BAF angle gauge. We need to fix that.

Well, let’s just ask this question : “What BAF are you assuming when you use a PRF of 1.375 to the face of a tree?”. Use the STAR_BAR program again. Enter 1.375 into cell C43, and you calculate BAF=37.682. That’s the BAF he was actually using. This points out the need to be able to record BAF to several digits in your data processes. I recommend at least 2 or 3 decimals of precision. You can adjust your EXCEL calculations for closer precision in these calculations, but the current settings are probably close enough for any practical work. Now calculate the timber cruise using a BAF=37.682, and you have corrected the cruiser’s field error.

You could also make the new cruiser go back and re-do all the borderline trees, but perhaps there are better ways to train him and use his time ... or maybe not.

Originally published March 2014

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