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 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
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 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 : “ 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*