## Edge Effect in Forest Sampling (Part II)
Part I of this article covered the historical attempts to solve the
problem of edge effect. Part II covers only
and count or measure any trees that are located inside
the polygon. Ignore any trees that are outside the polygon. So far, of
course, you have the classic problem of "edge effect" caused by
tree circles that fall partly beyond the border of the polygon. One example
of a very large tree circle (near the upper left edge) is shown with a
dashed line.
and
again count or measure any trees inside the polygon that are
"in" with your angle gauge. As before, ignore any trees
outside the polygon. If a sample point falls into any "hole" in
the polygon that is "typed out", it is still an "outside
point." The key is what you do with these "extra" trees.This is straightforward. Once the data is added to the inside points, just process
the data in the normal way. Use the same procedure for inside and outside
sample points. You can also use a large BAF to choose sample trees, and a
smaller one to count trees as discussed in an earlier article on the "Big
& Little BAF" method. Figures 2, 3 and 4 show an example.
You can visualize the process by considering each of the grid points
inside a square or rectangle. For points outside the polygon, imagine
picking up the exterior part of the rectangle and dropping it over one of
the interior rectangles. When these rectangles are matched, the sample
points will fall on top of each other. All the outside parts of the tree
circle are thereby moved "into" the polygon by this process. When
this has been done for all of the rectangles, the entire area of all the
tree circles is shifted
In addition, of course, you have to visit the extra sample points. There are some things you could do if the outside sample points were impossible to visit. Basically you just have to ask the question "could any of the trees be "in" on a sample point that far from the edge?" If so, you would have to compute the distance from the tree to that outside point. Alternatively, you could use one of the older methods described in the first part of this article for these situations. As far as I am aware, other than a few informal presentations by myself, this method has never been published. It is new, and I think it is a major improvement over previous methods of correcting for edge effect. While it is possible to extend this logic to sample points on large inventory projects, it would require that at least one sample point fell into each forest type. The technique is probably best adapted to cruises that use a grid and that sample individual polygons. |

*Originally published January 2001*