osm_to_spatialpolygons.r

## This package is used to retrieve data from openstreetmap.org:
require(osmar)
## This pacakge is used to process the spatial data:
require(rgeos)
## This package is used to convert the data from openstreetmap into something workable:
require(maptools)

## function to retrieve a relation from openstreetmap and convert it
## into a SpatialPolygons object:
OSM_relation_to_SP <- function(rel) {
  ## get the data from openstreetmap.org:
  sp_dat <- get_osm(relation(rel), source = osmsource_api(), full = T)

  ## convert the data to a SpatialLines object:
  sp_dat <- as_sp(sp_dat, "lines")

  ## Assuming that the lines form an outline of something,
  ## join the connected lines (this was making me pull my
  ## hair out before including this step, without this
  ## step the SpatialPolygons object will be messed up):
  sp_dat <- gLineMerge(sp_dat)

  ## These two steps convert the SpatialLines object
  ## into a SpatialPolygons object:
  sp_dat <- SpatialLines2PolySet(sp_dat)
  sp_dat <- PolySet2SpatialPolygons(sp_dat)
}

## Create a SpatialPolygons object for Lake IJssel:
lake_ijssel <- OSM_relation_to_SP(945096)

## plot it:
plot(lake_ijssel, col = "lightblue", axes = T)