Dissolve holes in polygon in R

Question

I am running some geoprocessing tasks in R, in which I am trying to create some polygons for clipping rasters of environmental information. I am buffering somewhat complex polygons, and this leaves small subgeometries that I would like to get rid of. In ArcGIS, I think this would involve converting my polygon from multipart to singlepart (or something along those lines) and then dissolving, but I don't know how to do this in R.

Here's an example that illustrates the problem:

require(maptools)
require(rgeos)

data(wrld_simpl)
wrld_simpl[which(wrld_simpl@data$NAME=='Greece'),]->greece
proj4string(greece)<-CRS('+proj=lonlat +datum=WGS84')
gBuffer(greece,width=0.5)->buf
plot(buf)

What I really want is the outer boundary of the polygon, with nothing else inside. Any ideas?

Answer 1

If you just want to get the one ring that forms the boundary of your buffer, then this:

plot(SpatialPolygons(list(Polygons(list(buf@polygons[[1]]@Polygons[[1]]),ID=1))),lwd=2)

is a very ad-hoc way of doing it (and plotting it) for your case.

What you really really want is to get all the rings with ringDir=1, since the rest will be holes. You need all the rings because your buffer might still be two disconnected islands.

outerRings = Filter(function(f){f@ringDir==1},buf@polygons[[1]]@Polygons)
outerBounds = SpatialPolygons(list(Polygons(outerRings,ID=1)))
plot(outerBounds)

might do the trick... Try it with width=0.1 and you'll see it work with multiple islands, but still removing a hole.