Linestring Polygonizer

Function attempts to polygonize the given list of linestrings. If the linestrings are not noded (coordinates inserted at intersection points), the function may fail. gNode may be tried to insert such missing points

gPolygonize( splist, getCutEdges=FALSE);

Arguments

splist

a list of sp SpatialLines objects

getCutEdges

Logical vector indicating if cut edges should be returned

Value

By default returns polygons generated by polygonizing the given linestrings. If getCutEdges is TRUE then any cut edges are returned.

Details

Polygonization is the process of forming polygons from linestrings which enclose an area. Linestrings are expected to be fully noded, as such they must not cross and must touch only at endpoints. gPolygonize takes a list of fully noded linestrings and forms all the polygons which are enclosed by the lines. Polygonization errors such as dangling lines or cut lines can be identified and reported.

Author

Roger Bivand & Colin Rundel

See also

gNode

Examples

library(sp)
linelist = list(readWKT("LINESTRING (0 0 , 10 10)"),
        readWKT("LINESTRING (185 221, 100 100)"),
        readWKT("LINESTRING (185 221, 88 275, 180 316)"),
        readWKT("LINESTRING (185 221, 292 281, 180 316)"), 
        readWKT("LINESTRING (189 98, 83 187, 185 221)"), 
        readWKT("LINESTRING (189 98, 325 168, 185 221)"))

par(mfrow=c(1,2))
plot(linelist[[1]],xlim=c(0,350),ylim=c(0,350))
title("Linestrings with nodes")
plot(as(linelist[[1]],"SpatialPoints"),pch=16,add=TRUE)

for(i in 2:length(linelist)) {
  plot(linelist[[i]],add=TRUE)
  plot(as(linelist[[i]],"SpatialPoints"),pch=16,add=TRUE)
}

plot(gPolygonize(linelist),xlim=c(0,350),ylim=c(0,350))
title("Polygonized Results")


gPolygonize(linelist,getCutEdges=TRUE) # no cut edges
#> NULL



linelist2 = list(readWKT("LINESTRING(1 3, 3 3, 3 1, 1 1, 1 3)"),
         readWKT("LINESTRING(1 3, 3 3, 3 1, 1 1, 1 3)"))

gPolygonize(linelist2,getCutEdges=FALSE) # NULL
#> NULL
gPolygonize(linelist2,getCutEdges=TRUE) # Contains LineStrings
#> SpatialLines:
#> An object of class "SpatialLines"
#> Slot "lines":
#> [[1]]
#> An object of class "Lines"
#> Slot "Lines":
#> [[1]]
#> An object of class "Line"
#> Slot "coords":
#>      x y
#> [1,] 1 3
#> [2,] 3 3
#> [3,] 3 1
#> [4,] 1 1
#> [5,] 1 3
#> 
#> 
#> 
#> Slot "ID":
#> [1] "1"
#> 
#> 
#> [[2]]
#> An object of class "Lines"
#> Slot "Lines":
#> [[1]]
#> An object of class "Line"
#> Slot "coords":
#>      x y
#> [1,] 1 3
#> [2,] 3 3
#> [3,] 3 1
#> [4,] 1 1
#> [5,] 1 3
#> 
#> 
#> 
#> Slot "ID":
#> [1] "2"
#> 
#> 
#> 
#> Slot "bbox":
#>   min max
#> x   1   3
#> y   1   3
#> 
#> Slot "proj4string":
#> Coordinate Reference System:
#> Deprecated Proj.4 representation: NA 
#> 
#> Coordinate Reference System (CRS) arguments: NA 
# bug fix 130206
LS = list(
readWKT("LINESTRING (425963 576719, 425980 576703)"),
readWKT("LINESTRING (425963 576719, 425882 577073)"),
readWKT("LINESTRING (425980 576703, 426082 577072)"),
readWKT("LINESTRING (425882 577073, 426082 577072)"),
readWKT("LINESTRING (426138 577068, 426082 577072)"),
readWKT("LINESTRING (426138 577068, 426420 577039)"),
readWKT("LINESTRING (426420 577039, 426554 576990)"),
readWKT("LINESTRING (426751 576924, 426776 576823)"),
readWKT("LINESTRING (426751 576924, 426783 576919)"),
readWKT("LINESTRING (426751 576924, 426714 576953)"),
readWKT("LINESTRING (426776 576823, 426783 576919)"),
readWKT("LINESTRING (426658 576966, 426554 576990)"),
readWKT("LINESTRING (426658 576966, 426667 577031)"),
readWKT("LINESTRING (426658 576966, 426714 576953)"),
readWKT("LINESTRING (426667 577031, 426714 576953)")
)
plot(gPolygonize(LS))