What is the Approximate
VBAR? What is its CV?
by Kim Iles
Sometimes you need a reasonable average VBAR without doing a cruise. If your computation system does not show you the individual VBARs on former cruises, you can at least get the average VBAR by dividing the stand volume by the stand basal area . You can then compare that to the average stand height.
I had my friend Jim Wilson compare the VBARs of about a hundred stands to the stand height in cubic volume. What he found was that the cubic VBAR was pretty consistently 30% of the stand height. What that means is that if you count trees with your thumb to get basal area, you can use about 30% of the stand height for the VBAR to calculate rough stand volume.
(Tree Count * BAF) * approximate VBAR = approximate volume
Try it once or twice (perhaps where you already have a cruise result) and you can see how accurate you can get with this simple approach. US cruisers can compare the VBAR in board feet vs. stand height in the same way (BF VBAR is about twice stand height)
When planning a cruise, you often want to know how variable the Tree Count and VBAR will be. It is easy to get the variability of tree counts with a hand calculator in the field, but how do you get the variability of VBARs (since you cannot easily calculate those in the woods)? At the 2017 OSU short course, I mentioned that the CV for VBARs is about 1.3 times more variable than the CV of tree heights chosen with a prism.
Just to check, I had Jim compare the CV of the VBARs to the CV of the tree heights. For gross volume, it was about (1.1*CV of heights), and for net volume (1.2*CV of heights) in cubic measure. It is pretty easy to calculate the CV of a few tree heights in the field.
Jim’s comparisons used total tree height, so the numbers for the way you record tree height might vary. Check it out, so you have a quick way to get VBAR and CV of VBAR for stands you take a look at without a previous cruise.
Originally published May 2017
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