Point sampling - the equipment

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Point sampling is a method of determining the frequency of occurrence of trees throughout the stand using a fixed point. These points can be either random, or you can use a systematic approach. I usually use a system to locate my points.

What you will be measuring is the basal area of a stand. The basal area is the cross-sectional area of a tree at dbh (diameter at breast height). Basal area is usually referred to as the BA. After finding the BA, you can calculate the volume per acre.

The equipment I use for a cruise consists of an angle guage, a compass, a diameter tape, a pencil, a pocket notebook, and a map. You can use anything to measure diameter. Being exact is not necessary. The compass is used to locate your points. An angle guage can be bought, or made.

You can also buy a prism, which is a wedge shaped piece of glass that deflects the image when viewed. When using a prism, it is important to note that the prism is the center of the plot, and not you. With angle guages, you are the center of the plot.

An angle guage is really pretty easy to make. For a BA factor of 10, you need to have a ratio of 1:33. A 1 inch sight could be put at the end of a 33 inch stick. You then eyeball down the stick to determine if a tree is counted or not.

Another alternative is to use a 1 inch sight put at the end of a 33 inch string (kite string would work) or chain (similar to the chain used for pull light switches). You can knot the end of the string or chain at 33 inches to maintain your distance. You can buy one very similar to this and they are fairly inexpensive. Simply put the end of the string or chain between your teeth, and hold the guage out.

I've seen it suggested you could use a dime as an angle guage. True, if you hold it 23 inches to maintain your 1:33 ratio. A quarter held 33 inches is a better alternative.

To use one of these, you simply stand at the cruise point. You simply turn a 360 degree circle at the point and sight the surrounding trees. Some trees will be counted, some not. It is important to keep the guage parallel to the tree. If a tree is growing in an angle, the guage should be tilted to accommodate the angle.

You will look at each tree with your sight held out at the required distance. Some trees will appear larger than your sight, some will be smaller, and some will be equal to your sight. You only want to count those trees that are larger. For those that are equal, you should count every other one.

For most points, you will only be counting 6 to 15 trees. It won't take long to figure that the smaller the tree, the closer it has to be to you. Most plots can be taken in 5 to 10 minutes by 1 person, depending on conditions.

If you are only interested in the BA stocking, then counting each tree and multiplying by 10 will give you your answer. If you want to know volumes, then more data will be needed.

For more information, look to point sampling - the cruise, and point sampling - number crunch.

Ron Wenrich

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