Determines which polygons of a "SpatialPolygons" object are at the border, i.e. have coordinates in common with the spatial union of all polygons (constructed using unionSpatialPolygons).

polyAtBorder(SpP, snap = sqrt(.Machine$double.eps),
             method = "rgeos", ...)



an object of class "SpatialPolygons".


tolerance used to consider coordinates as identical.


method to use for unionSpatialPolygons. Defaults to "rgeos", since polyclip uses integer arithmetic, which causes rounding errors usually requiring tuning of (i.e., increasing) the tolerance parameter snap (see example below).


further arguments passed to the chosen method.


logical vector of the same length as SpP also inheriting its row.names.


Sebastian Meyer


## Load districts of Germany
load(system.file("shapes", "districtsD.RData", package = "surveillance"))

## Determine districts at the border and check the result on the map
if (requireNamespace("rgeos")) {
    atBorder <- polyAtBorder(districtsD, method = "rgeos")
    plot(districtsD, col = atBorder)

## For method = "polyclip", a higher snapping tolerance is required
## to obtain the correct result
if (requireNamespace("polyclip")) {
    atBorder <- polyAtBorder(districtsD, snap = 1e-6, method = "polyclip")
    plot(districtsD, col = atBorder)