[R-sig-Geo] How to extract coordinates values from a shapefile?

[Dylan Beaudette]

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



-- 
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341