rgeos (version 0.3-5)

gUnion: Geometry Union

Description

Functions for joining intersecting geometries.

Usage

gUnion(spgeom1, spgeom2, byid=FALSE, id=NULL, drop_not_poly=FALSE)
gUnionCascaded(spgeom, id = NULL)
gUnaryUnion(spgeom, id = NULL)
gLineMerge(spgeom, byid=FALSE, id = NULL)

Arguments

spgeom1, spgeom2

sp objects as defined in package sp

byid

Logical vector determining if the function should be applied across ids (TRUE) or the entire object (FALSE) for spgeom1 and spgeom2

id

Character vector defining id labels for the resulting geometries, if unspecified returned geometries will be labeled based on their parent geometries' labels; it may contain NA values for input objects not included in the union; it should define the membe

drop_not_poly

default FALSE, if TRUE and spgeom1, spgeom2 both inherit from SpatialPolygons, POINT and LINESTRING objects will be dropped from output GEOMETRYCOLLECTION objects to simplify output.

spgeom

sp Polygon(s) or Line(s) depending on the function used

Details

Returns an sp object with intersecting geometries merged. If geometries do not intersect then both are returned as distinct subgeometries.

gUnionCascaded expects a single sp object of class SpatialPolygons with subgeometries which it unions together. gUnionCascaded can only dissolve MultiPolygon objects, so GeometryCollection objects to be dissolved, here a SpatialPolygons object, must be flattened a Polygons object; if GEOS version 3.3.0 is available, use gUnaryUnion.

gUnaryUnion expects a single sp object of class SpatialPolygons with subgeometries which it unions together; introduced in GEOS version 3.3.0, and handles GeometryCollection objects. If the id argument is used, it should be a character vector defining the memberships of the output Polygons objects, equal in length to the length of the polygons slot of spgeom.

gLineMerge is similar to gUnionCascaded but is written to work with lines, specifically it joins line segments with intersecting end points.

See Also

gDifference gIntersection gSymdifference

Examples

library(maptools)
nc1 <- readShapePoly(system.file("shapes/sids.shp", package="maptools")[1],
 proj4string=CRS("+proj=longlat +datum=NAD27"))
lps <- coordinates(nc1)
ID <- cut(lps[,1], quantile(lps[,1]), include.lowest=TRUE)
if (version_GEOS0() < "3.3.0") {
   reg4 <- gUnionCascaded(nc1, ID)
} else {
   reg4 <- gUnaryUnion(nc1, ID)
}
row.names(reg4)
par(mfrow=c(2,1))
plot(nc1)
plot(reg4)

par(mfrow=c(1,1))
gt <- GridTopology(c(0.05,0.05), c(0.1,0.1), c(2,2))
set.seed(1)
xv <- rnorm(length(coordinates(gt)[,1]))
xvs <- ifelse(xv > 0.2,1,0)
grd <- SpatialGridDataFrame(gt, data.frame(xvs))
spix <- as(grd, "SpatialPixelsDataFrame")
spol <- as(spix, "SpatialPolygonsDataFrame")
image(grd, axes=TRUE)
if (version_GEOS0() < "3.3.0") {
  spol1 <- gUnionCascaded(spol, as.character(spol$xvs))
} else {
  spol1 <- gUnaryUnion(spol, as.character(spol$xvs))
}
plot(spol1, add=TRUE)