Which of a set of points lie within a polygon defined region

Tests whether each of a set of points lie within a region defined by one or more (possibly nested) polygons. Points count as `inside' if they are interior to an odd number of polygons.

in.out(bnd,x)

Arguments

bnd

A two column matrix, the rows of which define the vertices of polygons defining the boundary of a region. Different polygons should be separated by an NA row, and the polygons are assumed closed. Alternatively can be a lists where bnd[[i]][[1]], bnd[[i]][[2]] defines the ith boundary loop.

x

A two column matrix. Each row is a point to test for inclusion in the region defined by bnd. Can also be a 2-vector, defining a single point.

Value

A logical vector of length nrow(x). TRUE if the corresponding row of x is inside the boundary and FALSE otherwise.

Details

The algorithm works by counting boundary crossings (using compiled C code).

Author

Simon N. Wood simon.wood@r-project.org

Examples

library(mgcv)
data(columb.polys)
bnd <- columb.polys[[2]]
plot(bnd,type="n")
polygon(bnd)
x <- seq(7.9,8.7,length=20)
y <- seq(13.7,14.3,length=20)
gr <- as.matrix(expand.grid(x,y))
inside <- in.out(bnd,gr)
points(gr,col=as.numeric(inside)+1)