[R-sig-Geo] polygons from pixels.

[Roger Bivand]

On Mon, 29 Sep 2008, milton ruser wrote:

> Dear RGeo-Gurus,
>
> I have a ASC file, and would like that each pixel come to be a polygon.
> Any help are welcome.
>
> require(rgdal)
> require(adehabitat)
>
> (file1 <-  paste(system.file(package = "adehabitat"),
>               "ascfiles/elevation.asc", sep = "/"))
> el <- readGDAL(file1)
> image(el, axes=T)

Something like this is in sp:

el1 <- as(el, "SpatialPixelsDataFrame")
el2 <- as(el1, "SpatialPolygons")
IDs <- sapply(slot(el2, "polygons"), function(x) slot(x, "ID"))
df <- as(el1, "data.frame")
row.names(df) <- IDs
el3 <- SpatialPolygonsDataFrame(el2, data=df)
spplot(el3, "band1", col.regions=heat.colors(20))

The actual coercion is from SpatialPixels to SpatialPolygons, so the data 
need to be put back in.

Roger

>
> best wishes,
>
> miltinho astronauta
> brazil
>
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>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no