Is it true that ...

Taking strips does not require you to know the area of the stand?

No, but you can pretend it is not a problem. In fact, you are just guessing the area. Let’s say that you take a 1 chain strip every 10 chains, implying a 10% sample. Everything is pretty simple if you are drawing this on a blackboard with a square block, but imagine that you are doing it in an actual stand.

You can get the total volume along the strips you put in and multiply by 10 for the total volume. So far, so good. Does that process mean that you did not "need to know" the area of the stand? No, it just means that you have to assume (pretend that you know) the area of the stand – in other words that it is 10 times the area you covered with your strips. Is that true? Not likely.

This is easy to check. Take a mapped stand which is typical of the ones you cruise. The best way is to just put a pin randomly into one of your maps to select the stand. Now draw lines 10 chains apart in the same way you would run them in the field. Add up the length of the lines. Now repeat the process, with the lines shifted, and add up the total lengths again.

The variability in these line lengths is the variability of your estimate of the area of the stand. You may not see it in the math – but it is there, and it is affecting the answer you are getting. This, of course, is if you are doing everything right. The fact that you might not maintain a straight line, a constant width, etc. are all just additional errors.

The area of the stand is calculated by multiplying your line length by the one chain width * 10 for the spacing between lines.

How would you do the statistics for strips? Well, the most common methods are to break them up into long plots, or to base the statistics on the average volume/acre on the strips (which is more correct). Suppose that the volume/acre was exactly the same on each strip. Would that mean that you had exactly the right answer? No, because the length of the strips would vary by how you place them across the stand. If you do not account for this in the statistics you are underestimating the effect of this random difference.

For those of you who are interested in the math, take a look at the article "Tract acreage estimates from timber cruises", by Oderwald, Southern Journal of Applied Forestry, 1993, pages 103-106, that addresses this point, as well as an earlier article in the JB&A series ("Acreage Estimates from Strip Cruises", Issue 25, January 1994).

The important point here is that this myth of area "not mattering" because you are using a strip method is just plain wrong. You would be far better off going to the trouble of measuring the stand area and multiplying it by the volume/acre calculated from your strips. The strip method, in effect, calculates an area – and it is not very good at doing it. The additional sampling error from this is not always trivial, and you can check out the amount of effect by using this method of summing up the lines across typical stand outlines to quantify this effect.

Pretending that area does not matter because you do not see it (in the way you write down the math) is just like a kid that covers his eyes and thinks that what he does not see is not there. They are wrong.

Originally published April 1999

Return to Home
Back to Contents