Split polygon parts of a single SpatialPolygons Object
In R, I have single SpatialPolygons object (i.e. multi-polygons) containing several hundred polygons. I would like to split this SpatialPolygons object into a list of Polygons (i.e. holes should remain attached to the parent polygon).
Any idea how to do this?
EDITED:
Using the following example provided in the sp package:
# simple example, from vignette("sp"):
Sr1 = Polygon(cbind(c(2,4,4,1,2),c(2,3,5,4,2)))
Sr2 = Polygon(cbind(c(5,4,2,5),c(2,3,2,2)))
Sr3 = Polygon(cbind(c(4,4,5,10,4),c(5,3,2,5,5)))
Sr4 = Polygon(cbind(c(5,6,6,5,5),c(4,4,3,3,4)), hole = TRUE)
Srs1 = Polygons(list(Sr1), "s1")
Srs2 = Polygons(list(Sr2), "s2")
Srs3 = Polygons(list(Sr3, Sr4), "s3/4")
SpP = SpatialPolygons(list(Srs1,Srs2,Srs3), 1:3)
Then running out = lapply(SpP@polygons, slot, "Polygons"). I get a list of three Polygons (i.e. Srs1, Srs2, Srs3).
However, the case I am trying to solve is a bit different from this example. The SpatialPolygons object I am trying to split is the result of a geometric union done with the gUnaryUnion function (in the RGEOS package). If I apply out <- lapply(merged.polygons@polygons, slot, "Polygons"), I get a unique list of Polygon objects (n.b. not a list of Polygons objects). In other words, each polygon is separated from its hole(s).
Running topol <- sapply(unlist(out), function(x) x@hole)
I get:
> length(topol)
[1] 4996
> sum(topol, na.rm=TRUE)
[1] 469
According to the RGEOS v0.3-2 manual (http://cran.r-project.org/web/packages/rgeos/rgeos.pdf):
In order for rgeos to function properly it is necessary that all holes within a given POLYGON or MULTIPOLYGON geometry must belong to a specific polygon. The SpatialPolygons class implementation does not currently include this information. To work around this limitation rgeos uses an additional comment attribute on the Polygons class that indicates which hole belongs to which polygon. Under the current implementation this comment is a text string of numbers separated by spaces where the order of the numbers corresponds to the order of the Polygon objects in the Polygons slot of the Polygons object. A 0 implies the Polygon object is a polygon, a non-zero number implies that the Polygon object is a hole with the value indicating the index of the Polygon that “owns” the hole.
So the createSPComment() function in RGEOS is likely to be a workaround to reaggregate Polygons and holes.
Answer
To separate multipolygon objects into single polygons (with holes if present) you can do
d <- disaggregate(p)
Where p is a SpatialPolygons object. After that, you can use d@polygons.
For example
library(sp)
library(raster)
### example data
p1 <- rbind(c(-180,-20), c(-140,55), c(10, 0), c(-140,-60), c(-180,-20))
hole <- rbind(c(-150,-20), c(-100,-10), c(-110,20), c(-150,-20))
p1 <- list(p1, hole)
p2 <- rbind(c(-10,0), c(140,60), c(160,0), c(140,-55), c(-10,0))
p3 <- rbind(c(-125,0), c(0,60), c(40,5), c(15,-45), c(-125,0))
pols <- spPolygons(p1, p2, p3)
###
a <- aggregate(pols, dissolve=FALSE)
d <- disaggregate(a)