Sampling via polygons question
On Tue, 2 Aug 2011, Jonathan Greenberg wrote:
R-sig-geo'ers: Hoping to get some feedback on this problem. Given a SpatialPolygons object with N polygons, I would like to sample (probably via raster's "extract") a window from a raster() object around *each pixel* falling with *each polygon*. I'm not asking about a straight buffer around a polygon and then sampling via (e.g.) extract -- this is more akin to converting each polygon to a set of points representing cells of the raster object, then sampling via a buffer around each of these points. The window to be extracted around each location is of a fixed size, so each point within a polygon should yield the same size output. Thoughts? What is the most efficient way to convert a polygon to a set of points?
Could you please provide a self-contained code example, possibly with a simple downloadable shapefile of polygons covering your case? Are the polygons intended to touch? are the buffers in fact the pixels (rectangular) or circular? I think I see what you mean, but may well misunderstand your intentions. Roger
--j
Roger Bivand Department of Economics, NHH Norwegian School of Economics, 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