Adjust for Edge Effects

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Description

Shrink a point pattern, or expand it through replication.

Usage

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edges(plants, width)

core(plants, distance)

Arguments

plants

A spatstat point pattern object (class ppp). It normally contains the plants coordinates, and marks with the plant size and possibly other attributes.

width

Distance from the edges to shrink, if negative, or to expand, if positive.

distance

Distance from the edges.

Details

When computing assimilation or competition indices, those near the edges of the study region are distorted because the outside is empty. Common solutions to this problem are not to use indices computed for plants near the edges, or (with rectangular regions) to attach translated copies, thus changing the topology into a torus. This function implements both strategies. When expanding, the part of the copies to be used can be specified to avoid unnecessary computation.

Typically, in the first case the indices are computed for the full pattern, and then the edges are discarded using edges with a negative width. In the second case, the point pattern is first expanded with edges(plants, width), the indices are computed for the expanded pattern, and then the result is restricted to the original size with edges(result, -width).

core returns a logical vector indicating which plants are at more than the given distance from the edges. With a negative width, edges(plants, width) is the same as plants[core(plants, -width)].

Value

edges returns a point pattern with the same structure as plants.

If width is negative, the parts of the pattern that are at a distance less than -width from an edge are discarded.

If width is positive, the pattern is first expanded by surrounding it with 8 shifted copies (the window must be rectangular). Then, the parts of the pattern that are at a distance less than width from an edge of the enlarged pattern are discarded.

If width = 0, plants is returned unchanged.

core returns a logical vector with TRUE for the plants that are at more than the given distance from the edges, and FALSE otherwise.

Note

Requires the spatstat package.

Author(s)

Oscar GarcĂ­a.

References

http://forestgrowth.unbc.ca/siplab

See Also

assimilation, pairwise

Examples

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finpines
edges(finpines, 3)
edges(finpines, -3)