[R-sig-Geo] (no subject)

[Roger Bivand]

On Tue, 5 Jan 2010, laure velez wrote:

>
> Dear list member,
>
> Although the answer to this might be straightforward I'm struggling with this issue for weeks now :
>
> I have a shapefile (only one polygon) and a raster (.tif). I would like to know the value of each pixel on the raster which overlap with the polygon in the shapefile.
> I had various tries but none did work :
>
> library(rgdal)
>
> pol = readOGR(?shape.shp?, layer = ?shape?)
> grid = readGDAL(?raster.tif?)
> overlay(grid,pol)
>
> With this I obtain ?NA? for all pixels
>
> Could you help me ?

Please try:

image(grid, axes=TRUE)
plot(pol, add=TRUE)

If the polygon doesn't show up in the correct place, do examine summary() 
output for both objects. I suspect that the coordinate values of the 
objects - seem in the bounding boxes in the summary output - do not 
overlap, probably because the coordinate reference systems differ.

Roger

>
> Thanks in advance,
>
> Laure
>
>
> Please find attached the shape and the raster
>
>
>
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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