methods.xypolygon {spatstat.utils}

Calculations for Polygons in the Plane

Description

Compute the area or boundary length of a polygon, determine whether a point falls inside a polygon, compute the area of overlap between two polygons, and related tasks.

Usage

verify.xypolygon(p, fatal = TRUE)
is.hole.xypolygon(polly)
Area.xypolygon(polly)
bdrylength.xypolygon(polly)
reverse.xypolygon(p, adjust=FALSE)
overlap.xypolygon(P, Q)
simplify.xypolygon(p, dmin)
inside.xypolygon(pts, polly, test01, method)

Arguments

p, polly, P, Q

Data representing a polygon. See Details.

dmin

Single numeric value giving the minimum permissible length of an edge in the simplified polygon.

fatal

Logical value indicating whether failure is a fatal error.

pts

Coordinates of points to be tested. A named list with entries x,y which are numeric vectors of coordinates.

adjust

Logical value indicating whether internal data should be adjusted. See Details.

test01, method

For developer use only.

Details

In the spatstat family of packages, a polygon in the Euclidean plane is represented as a named list with the following entries:

x,y

Numeric vectors giving the coordinates of the vertices. The vertices should be traversed in anti-clockwise order (unless the polygon is a hole, when they should be traversed in clockwise order) and the last vertex should not repeat the first vertex.

hole

Optional. A logical value indicating whether the polygon is a hole.

area

Optional. Single numeric value giving the area of the polygon (negative if it is a hole).

The function verify.xypolygon checks whether its argument satisfies this format. If so, it returns TRUE; if not, it returns FALSE or (if fatal=TRUE) generates a fatal error.

The other functions listed here perform basic calculations for polygons using elementary Cartesian analytic geometry in R.

is.hole.xypolygon determines whether a polygon is a hole or not.

Area.xypolygon computes the area of the polygon using the discrete Green's formula.

bdrylength.xypolygon calculates the total length of edges of the polygon.

reverse.xypolygon reverses the order of the coordinate vectors x and y. If adjust=TRUE, the other entries hole and area will be adjusted as well.

overlap.xypolygon computes the area of overlap between two polygons using the discrete Green's formula. It is slow compared to the code in the polyclip package.

simplify.xypolygon removes vertices of the polygon until every edge is longer than dmin.

inside.xypolygon(pts, polly) determines whether each point in pts lies inside the polygon polly and returns a logical vector.

Value

verify.xypolygon and is.hole.xypolygon return a single logical value.

inside.xypolygon returns a logical vector.

Area.xypolygon, bdrylength.xypolygon and overlap.xypolygon return a single numeric value.

reverse.xypolygon and simplify.xypolygon return another polygon object.

Author(s)

Adrian Baddeley [e-mail].

Examples

  p <- list(x=c(0,1,4,2), y=c(0,0,2,3))
  is.hole.xypolygon(p)
  Area.xypolygon(p)
  bdrylength.xypolygon(p)
  overlap.xypolygon(p, list(x=p$x+1, y=p$y+1))
  reverse.xypolygon(p)

  plot(c(0,5),c(0,3),type="n",xlab="x", ylab="y")
  polygon(p)
  polygon(simplify.xypolygon(p, 1.1), lty=3)

  plot(c(0,5),c(0,3),type="n",xlab="x", ylab="y")
  polygon(p)
  xx <- runif(10, max=5)
  yy <- runif(10, max=3)
  points(xx, yy)
  ok <- as.logical(inside.xypolygon(list(x=xx, y=yy), p))
  points(xx[ok], yy[ok], pch=16)