On Fri, Jul 20, 2012 at 8:40 PM, C?line 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