How to extract coordinates values from a shapefile?

On Wednesday 09 June 2010, Hadley Wickham wrote:
   

On Wed, Jun 9, 2010 at 3:24 PM, Edzer Pebesma

<edzer.pebesma at uni-muenster.de>  wrote:
     

The example provided by Matt assumes that each polygon consists of a
single ring, and doesn't have islands, lakes etc. The function below
pasts all coordinates to a single 2-column matrix. For clarity's sake, I
avoided nested sapply's.

library(maptools)
xx<- readShapePoly(system.file("shapes/sids.shp",
package="maptools")[1], IDvar="FIPSNO", proj4string=CRS("+proj=longlat
+ellps=clrk66")) allcoordinates = function(x) {
    ret = NULL
    polys = x at polygons
    for(i in 1:length(polys)) {
        pp = polys[[i]]@Polygons
        for (j in 1:length(pp))
            ret = rbind(ret, coordinates(pp[[j]]))
    }
    ret
}
q = allcoordinates(xx)

# check:
plot(xx)
lines(q, col='blue')
       

And that's basically what fortify does, except it covers a few more cases.

Hadley
     

Not to be nit-picky, but wouldn't it be safer to pre-allocated "ret" instead
of dynamically appending? This would allow the suggested function to scale to
very large geometries.
   

Cheers,
Dylan