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rgeos (version 0.4-2)

gIntersection: Geometry Intersections

Description

Function for determining the intersection between the two given geometries

Usage

gIntersection(spgeom1, spgeom2, byid=FALSE, id=NULL, drop_not_poly,
 drop_lower_td=FALSE, unaryUnion_if_byid_false=TRUE, checkValidity=FALSE)

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.

drop_not_poly

deprecated argument, use drop_lower_td

drop_lower_td

default FALSE; if TRUE, objects will be dropped from output GEOMETRYCOLLECTION objects to simplify output if their topological dimension is less than the minimum topological dinension of the input objects.

unaryUnion_if_byid_false

default TRUE; if byid takes a FALSE in either position, the subgeometries are combined first to avoid possible topology exceptions (change in 0.3-13, previous behaviour did not combine subgeometries, and may be achieved by setting this argument FALSE

checkValidity

default FALSE; error meesages from GEOS do not say clearly which object fails if a topology exception is encountered. If this argument is TRUE, gIsValid is run on each in turn in an environment in which object names are available. If objects are invalid, this is reported and those affected are named

Details

Returns all spatial intersections as sp objects of the appropriate class. If the geometries do not intersect then an empty geometry is returned.

See Also

gDifference gSymdifference gUnion

Examples

Run this code
# NOT RUN {
if (require(maptools)) {
xx <- readShapeSpatial(system.file("shapes/fylk-val-ll.shp", package="maptools")[1],
 proj4string=CRS("+proj=longlat +datum=WGS84"))
bbxx <- bbox(xx)
wdb_lines <- system.file("share/wdb_borders_c.b", package="maptools")
xxx <- Rgshhs(wdb_lines, xlim=bbxx[1,], ylim=bbxx[2,])$SP
res <-gIntersection(xx, xxx)
plot(xx, axes=TRUE)
plot(xxx, lty=2, add=TRUE)
plot(res, add=TRUE, pch=16,col='red')
}
pol <- readWKT(paste("POLYGON((-180 -20, -140 55, 10 0, -140 -60, -180 -20),",
 "(-150 -20, -100 -10, -110 20, -150 -20))"))
library(sp)
GT <- GridTopology(c(-175, -85), c(10, 10), c(36, 18))
gr <- as(as(SpatialGrid(GT), "SpatialPixels"), "SpatialPolygons")
try(res <- gIntersection(pol, gr, byid=TRUE))
res <- gIntersection(pol, gr, byid=TRUE, drop_lower_td=TRUE)
# Robert Hijmans difficult intersection case
load(system.file("test_cases/polys.RData", package="rgeos"))
try(res <- gIntersection(a, b, byid=TRUE))
res <- gIntersection(a, b, byid=TRUE, drop_lower_td=TRUE)
unlist(sapply(slot(res, "polygons"), function(p) sapply(slot(p, "Polygons"), slot, "area")))
# example from Pius Korner 2015-10-25
poly1 <- SpatialPolygons(list(Polygons(list(Polygon(coords=matrix(c(0, 0, 2, 2, 0, 1, 1, 0),
 ncol=2, byrow=FALSE))), ID=c("a")), Polygons(list(Polygon(coords=matrix(c(0, 0, 2, 2, 2, 3, 3, 2),
 ncol=2, byrow=FALSE))), ID=c("b"))))
poly2 <- SpatialPolygons(list(Polygons(list(Polygon(coords=matrix(c(0, 0, 2, 2,
 1, 1, 1, 3, 3, 0, 0, 2), ncol=2, byrow=FALSE))), ID=c("c"))))
plot(poly1, border="orange")
plot(poly2, border="blue", add=TRUE, lty=2, density=8, angle=30, col="blue")
gI <- gIntersection(poly1, poly2, byid=TRUE, drop_lower_td=TRUE)
plot(gI, add=TRUE, border="red", lwd=3)
oT <- get_RGEOS_polyThreshold()
oW <- get_RGEOS_warnSlivers()
oD <- get_RGEOS_dropSlivers()
set_RGEOS_polyThreshold(1e-3)
set_RGEOS_warnSlivers(TRUE)
res1 <- gIntersection(a, b, byid=TRUE, drop_lower_td=TRUE)
unlist(sapply(slot(res1, "polygons"), function(p) sapply(slot(p, "Polygons"), slot, "area")))
set_RGEOS_dropSlivers(TRUE)
res2 <- gIntersection(a, b, byid=TRUE, drop_lower_td=TRUE)
unlist(sapply(slot(res2, "polygons"), function(p) sapply(slot(p, "Polygons"), slot, "area")))
set_RGEOS_dropSlivers(FALSE)
oo <- gUnaryUnion(res1, c(rep("1", 3), "2", "3", "4"))
unlist(sapply(slot(oo, "polygons"), function(p) sapply(slot(p, "Polygons"), slot, "area")))
ooo <- gIntersection(b, oo, byid=TRUE)
gArea(ooo, byid=TRUE)
unlist(sapply(slot(ooo, "polygons"), function(p) sapply(slot(p, "Polygons"), slot, "area")))
set_RGEOS_dropSlivers(TRUE)
ooo <- gIntersection(b, oo, byid=TRUE)
gArea(ooo, byid=TRUE)
unlist(sapply(slot(ooo, "polygons"), function(p) sapply(slot(p, "Polygons"), slot, "area")))
set_RGEOS_polyThreshold(oT)
set_RGEOS_warnSlivers(oW)
set_RGEOS_dropSlivers(oD)
# see thread https://stat.ethz.ch/pipermail/r-sig-geo/2015-September/023468.html
Pol1=rbind(c(0,0),c(0,10),c(10,10),c(10,0),c(0,0))
Pol2=rbind(c(0,0),c(10,0),c(10,-10),c(0,-10),c(0,0))
library(sp)
Pols1=Polygons(list(Polygon(Pol1)),"Pols1")
Pols2=Polygons(list(Polygon(Pol2)),"Pols2")
MyLay=SpatialPolygons(list(Pols1,Pols2))
Pol1l=Pol1+0.5
Pol2l=Pol2+0.5
Pols1l=Polygons(list(Polygon(Pol1l)),"Pols1l")
Pols2l=Polygons(list(Polygon(Pol2l)),"Pols2l")
MyLayl=SpatialPolygons(list(Pols1l,Pols2l))
inter=gIntersection(MyLay, MyLayl)
plot(MyLay)
plot(MyLayl,add=TRUE)
plot(inter, add=TRUE, border="green")
try(gIntersection(MyLay, MyLayl, unaryUnion_if_byid_false=FALSE))
Pol1=rbind(c(0,0),c(0,1),c(1,1),c(1,0),c(0,0))
Pol2=rbind(c(0,0),c(1,0),c(1,-1),c(0,-1),c(0,0))
Pols1=Polygons(list(Polygon(Pol1)),"Pols1")
Pols2=Polygons(list(Polygon(Pol2)),"Pols2")
MyLay=SpatialPolygons(list(Pols1,Pols2))
Pol1l=Pol1+0.1
Pol2l=Pol2+0.1
Pols1l=Polygons(list(Polygon(Pol1l)),"Pols1l")
Pols2l=Polygons(list(Polygon(Pol2l)),"Pols2l")
MyLayl=SpatialPolygons(list(Pols1l,Pols2l))
inter=gIntersection(MyLay, MyLayl, unaryUnion_if_byid_false=FALSE)
gEquals(inter, MyLay)
inter1=gIntersection(MyLay, MyLayl, unaryUnion_if_byid_false=TRUE)
gEquals(inter1, MyLay)
gEquals(inter, inter1)
plot(MyLay, ylim=c(-1, 1.1))
plot(MyLayl, add=TRUE)
plot(inter, angle=45, density=10, add=TRUE)
plot(inter1, angle=135, density=10, add=TRUE)
inter2=gIntersection(MyLay, MyLayl)
gEquals(inter2, MyLay)
gEquals(inter1, inter2)
# }

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