Check holes in Polygons objects

Description

The function checks holes in Polygons objects, using an intersection between a gpclib package gpc.poly object with one or more polygon contours and its bounding box to set the hole flag. The function will set single polygon contours to hole=FALSE, and if multiple polygon contours are holes, will set them TRUE.

Usage

checkPolygonsHoles(x)

Arguments

Value

An Polygons object re-created from the input object.

Author(s)

Roger Bivand

Examples

library(sp)
library(gpclib)
nc1 <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1], proj4string=CRS("+proj=longlat +ellps=clrk66"))
pl <- getSpPpolygonsSlot(nc1)
sapply(getPolygonsPolygonsSlot(pl[[4]]), function(x) x@hole)
pl[[4]] <- Polygons(list(getPolygonsPolygonsSlot(pl[[4]])[[1]], Polygon(getPolygonCoordsSlot(getPolygonsPolygonsSlot(pl[[4]])[[2]]), hole=TRUE), getPolygonsPolygonsSlot(pl[[4]])[[3]]), getPolygonsIDSlot(pl[[4]]))
sapply(getPolygonsPolygonsSlot(pl[[4]]), function(x) x@hole)
pl_new <- lapply(pl, checkPolygonsHoles)
sapply(getPolygonsPolygonsSlot(pl_new[[4]]), function(x) x@hole)
srs <- getPolygonCoordsSlot(getPolygonsPolygonsSlot(pl[[1]])[[1]])
hle2 <- structure(c(-81.64093, -81.38380, -81.34165, -81.66833, -81.64093, 
 36.57865, 36.57234, 36.47603, 36.47894, 36.57865), .Dim = as.integer(c(5, 2)))
hle3 <- structure(c(-81.47759, -81.39118, -81.38486, -81.46705, -81.47759, 
 36.56289, 36.55659, 36.49907, 36.50380, 36.56289), .Dim = as.integer(c(5, 2)))
x <- Polygons(list(Polygon(srs), Polygon(hle2), Polygon(hle3)),
 ID=getPolygonsIDSlot(pl[[1]]))
sapply(getPolygonsPolygonsSlot(x), function(x) x@hole)
res <- checkPolygonsHoles(x)
sapply(getPolygonsPolygonsSlot(res), function(x) x@hole)
## Not run: 
opar <- par(mfrow=c(1,2))
plot(SpatialPolygons(list(x)))
text(t(sapply(getPolygonsPolygonsSlot(x), getPolygonLabptSlot)),
 labels=sapply(getPolygonsPolygonsSlot(x), getPolygonHoleSlot), cex=0.6)
title(xlab="Hole slot values before checking")
plot(SpatialPolygons(list(res)))
text(t(sapply(getPolygonsPolygonsSlot(res), getPolygonLabptSlot)),
 labels=sapply(getPolygonsPolygonsSlot(res), getPolygonHoleSlot), cex=0.6)
title(xlab="Hole slot values after checking")
par(opar)
## End(Not run)