[R-sig-Geo] Dissolve polygon

[Barry Rowlingson]

On Fri, Jul 20, 2012 at 8:40 PM, Céline Bellard
<bellard.celine at gmail.com> 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