On Fri, Jul 20, 2012 at 8:40 PM, Céline Bellard
<[email protected]> wrote:
> Hi,
>
> I am working with a SpatialPolygonsDataFrame of many islands. There are a
> lot of polygons (islands) composing my SpatialPolygonsDataFrame.
> I want to extract the elevation of each island.
> I need to separate the different polygons (like dissolve function in
> arcgis), to have the elevation of each island.
>
> Do you have an idea how can I do that ?
>
> dim(hot)
> [1] 10945 5
>
> class(hot)
> [1] "SpatialLinesDataFrame"
> attr(,"package")
> [1] "sp"
Looks like you have 10945 features in your shapefile...
The extract function in the raster package will process pixels under
polygons. In this example, I sum a pollution raster over each lake
("Lake Exposure", about half-way down)
http://www.maths.lancs.ac.uk/~rowlings/Teaching/UseR2012/plume.html
You probably want something like the mean or the max of the pixels
over each island. In your case, you should end up with 10945 values.
Barry
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