Buffer Geometry

Expands the given geometry to include the area within the specified width with specific styling options.

gBuffer(spgeom, byid=FALSE, id=NULL, width=1.0, quadsegs=5, capStyle="ROUND",
 joinStyle="ROUND", mitreLimit=1.0)

Arguments

spgeom

sp object as defined in package sp

byid

Logical determining if the function should be applied across subgeometries (TRUE) or the entire object (FALSE)

id

Character vector defining id labels for the resulting geometries, if unspecified returned geometries will be labeled based on their parent geometries' labels.

width

Distance from original geometry to include in the new geometry. Negative values are allowed. Either a numeric vector of length 1 when byid is FALSE; if byid is TRUE: of length 1 replicated to the number of input geometries, or of length equal to the number of input geometries

quadsegs

Number of line segments to use to approximate a quarter circle.

capStyle

Style of cap to use at the ends of the geometry. Allowed values: ROUND,FLAT,SQUARE

joinStyle

Style to use for joints in the geometry. Allowed values: ROUND,MITRE,BEVEL

mitreLimit

Numerical value that specifies how far a joint can extend if a mitre join style is used.

Value

SpatialPolygons (or a SpatialPolygonsDataFrame if byid=TRUE and spgeom has a data.frame); if negative width(s) lead the object to disappear, NULL is returned for byid FALSE, and component Polygons objects are dropped if empty for byid TRUE; the SpatialPolygonsDataFrame is subsetted by row.names or id if given to retain non-empty geometry rows

Author

Roger Bivand & Colin Rundel

Examples

p1 = readWKT("POLYGON((0 1,0.95 0.31,0.59 -0.81,-0.59 -0.81,-0.95 0.31,0 1))")
p2 = readWKT("POLYGON((2 2,-2 2,-2 -2,2 -2,2 2),(1 1,-1 1,-1 -1,1 -1,1 1))")

par(mfrow=c(2,3))
plot(gBuffer(p1,width=-0.2),col='black',xlim=c(-1.5,1.5),ylim=c(-1.5,1.5))
plot(p1,border='blue',lwd=2,add=TRUE);title("width: -0.2")
plot(gBuffer(p1,width=0),col='black',xlim=c(-1.5,1.5),ylim=c(-1.5,1.5))
plot(p1,border='blue',lwd=2,add=TRUE);title("width: 0")
plot(gBuffer(p1,width=0.2),col='black',xlim=c(-1.5,1.5),ylim=c(-1.5,1.5))
plot(p1,border='blue',lwd=2,add=TRUE);title("width: 0.2")

plot(gBuffer(p2,width=-0.2),col='black',pbg='white',xlim=c(-2.5,2.5),ylim=c(-2.5,2.5))
plot(p2,border='blue',lwd=2,add=TRUE);title("width: -0.2")
plot(gBuffer(p2,width=0),col='black',pbg='white',xlim=c(-2.5,2.5),ylim=c(-2.5,2.5))
plot(p2,border='blue',lwd=2,add=TRUE);title("width: 0")
plot(gBuffer(p2,width=0.2),col='black',pbg='white',xlim=c(-2.5,2.5),ylim=c(-2.5,2.5))
plot(p2,border='blue',lwd=2,add=TRUE);title("width: 0.2")


p3 <- readWKT(paste("GEOMETRYCOLLECTION(",
 "POLYGON((0 1,0.95 0.31,0.59 -0.81,-0.59 -0.81,-0.95 0.31,0 1)),",
 "POLYGON((2 2,-2 2,-2 -2,2 -2,2 2),(1 1,-1 1,-1 -1,1 -1,1 1)))"))

par(mfrow=c(1,1))
plot(gBuffer(p3, byid=TRUE, width=c(-0.2, -0.1)),col='black',pbg='white',
xlim=c(-2.5,2.5),ylim=c(-2.5,2.5))
plot(p3,border=c('blue', 'red'),lwd=2,add=TRUE);title("width: -0.2, -0.1")

library(sp)
p3df <- SpatialPolygonsDataFrame(p3, data=data.frame(i=1:length(p3),
 row.names=row.names(p3)))
dim(p3df)
#> [1] 2 1
row.names(p3df)
#> [1] "1" "2"
dropEmpty = gBuffer(p3df, byid=TRUE, id=letters[1:nrow(p3df)], width=c(-1, 0))
dim(dropEmpty)
#> [1] 1 1
row.names(dropEmpty)
#> [1] "b"
row.names(slot(dropEmpty, "data"))
#> [1] "b"
plot(dropEmpty, col='black', pbg='white', xlim=c(-2.5,2.5),ylim=c(-2.5,2.5))
plot(p3df,border=c('blue'),lwd=2,add=TRUE);title("width: -1, 0")

par(mfrow=c(2,3))


#Style options
l1 = readWKT("LINESTRING(0 0,1 5,4 5,5 2,8 2,9 4,4 6.5)")
par(mfrow=c(2,3))
plot(gBuffer(l1,capStyle="ROUND"));plot(l1,col='blue',add=TRUE);title("capStyle: ROUND")
plot(gBuffer(l1,capStyle="FLAT"));plot(l1,col='blue',add=TRUE);title("capStyle: FLAT")
plot(gBuffer(l1,capStyle="SQUARE"));plot(l1,col='blue',add=TRUE);title("capStyle: SQUARE")

plot(gBuffer(l1,quadsegs=1));plot(l1,col='blue',add=TRUE);title("quadsegs: 1")
plot(gBuffer(l1,quadsegs=2));plot(l1,col='blue',add=TRUE);title("quadsegs: 2")
plot(gBuffer(l1,quadsegs=5));plot(l1,col='blue',add=TRUE);title("quadsegs: 5")




l2 = readWKT("LINESTRING(0 0,1 5,3 2)")
par(mfrow=c(2,3))
plot(gBuffer(l2,joinStyle="ROUND"));plot(l2,col='blue',add=TRUE);title("joinStyle: ROUND")
plot(gBuffer(l2,joinStyle="MITRE"));plot(l2,col='blue',add=TRUE);title("joinStyle: MITRE")
plot(gBuffer(l2,joinStyle="BEVEL"));plot(l2,col='blue',add=TRUE);title("joinStyle: BEVEL")

plot(gBuffer(l2,joinStyle="MITRE",mitreLimit=0.5));plot(l2,col='blue',add=TRUE)
 title("mitreLimit: 0.5")
plot(gBuffer(l2,joinStyle="MITRE",mitreLimit=1));plot(l2,col='blue',add=TRUE)
 title("mitreLimit: 1")
plot(gBuffer(l2,joinStyle="MITRE",mitreLimit=3));plot(l2,col='blue',add=TRUE)
 title("mitreLimit: 3")