Permanent Plot Area

People worry a lot about the area of permanent plots. More than that, they keep trying to get, for instance, exactly acre for an area. Why? There is nothing magic about any particular plot size. All that is necessary is to know the area.

We often see people working hard to calculate the angles at the corners of plots, and trying hard to make them right angles. Why? In some cases, there is a minor advantage in being able to "mirror" the edge trees across the border for some kinds of growth modeling, but this is a pretty small advantage. We think that people are a bit too hung up on right angles and fixed areas.

There is nothing wrong with a plot with 5 sides, and nothing wrong with one of 4 sides that are not right angles. The key here is knowing the area of the plot, not making a perfect rectangle or square.

Lasers are pretty good at measuring distances, not as good at measuring angles. You do not need to measure angles at all. You can calculate the plot area from the length of the sides of triangles (and a programmable calculator can do that for you on the spot).

When you place the stakes for the sides of the plot (referenced to trees, perhaps, and with metal stakes under the ground) try to see two stakes at the same time. This will allow a direct measurement of the distances involved, and allows you to calculate the area of each of the triangles making up the area.


Plot sides have lengths as follows.

We need the areas for triangles 1 and 2.

illustration of computing area by triangles

The triangle area formula is:

triangle area formula

where x,y and z are the lengths of the sides of the triangle.

Area of triangle #1(a-b-c) is:
     643.6474 square feet
Area of triangle #2 (a-c-d) is:
     759.326 square feet
The total plot size is :
     1,402.973 square feet.

This is very close to the area of a square of 34 by 41.5 feet (less than 1% difference), even though the sides are far off what we would try to install. The exact solution only needs the 54 foot distance between the corners.

Permanent Sample Plot databases should be designed to take several digits for plot size in anticipation of plots which are odd areas.

Kim Iles
Iles and Associates

Originally published October 1999

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