Split polygon parts of a single SpatialPolygons Object

Question

In R, I have single SpatialPolygons object (i.e. multi-polygons) containing several hundred polygons. I would like to split this SpatialPolygons object

Answer 1

If your SpatialPolygons object is called mysp...

out <- lapply( mysp@polygons , slot , "Polygons" )

Answer 2

This will return a list of SpatialPolygons instead of ordinary Polygons (which some of the answers do).

SpP %>% split(1:length(.))

Answer 3

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)

Answer 4

As I understand it, the OP wants to convert a SpatialPolygons object into a list of Polygons, preserving holes if present.

The SpP object created by the OP contains three polygons, the third of which has an associated hole.

You can use lapply to cycle through each polygon in SpP, returning a list of SpatialPolygons. The difference between a Polygons and SpatialPolygons object is the addition of plot order information. Since each resulting SpatialPolygons is of length = 1, however, this information is superfluous.

n_poly <- length(SpP)

out <- lapply(1:n_poly, function(i) SpP[i, ])

lapply(out, class)

> lapply(out, class)
   [[1]]
   [1] "SpatialPolygons"
   attr(,"package")
   [1] "sp"

   [[2]]
   [1] "SpatialPolygons"
   attr(,"package")
   [1] "sp"

   [[3]]
   [1] "SpatialPolygons"
   attr(,"package")
   [1] "sp"

plot(out[[3]]) # Hole preserved

If a list of Polygons is needed, simply pull the appropriate slot from the SpatialPolygons object:

out <- lapply(1:n_poly, function(i) SpP[i, ]@polygons[[1]])

lapply(out, class)

> lapply(out, class)
[[1]]
[1] "Polygons"
attr(,"package")
[1] "sp"

[[2]]
[1] "Polygons"
attr(,"package")
[1] "sp"

[[3]]
[1] "Polygons"
attr(,"package")
[1] "sp"