Dear Roman, I think it depends on your data. Below are two examples (both assuming your polygons do not overlap).
A) works if each cell in a polygon is closest to its own centroid (and not that of another polygon). It makes a distance surface with distanceFromPoints() and then uses mask() to mask out the areas not covered by polygons. B) uses linesToRaster() to set up a raster with impenetrable boundaries (the polygon outlines) and then use gridDistance() (i.e. the distance when only allowed to traverse certain cells). This method would fail if you have a polygon representing several islands; or a polygon with a hole with the centroid inside that hole. Also, you do not have values for the outer cells of a polygon. This is perhaps OK when polygons are adjacent (whose cell is it anyway?). If they are not adjacent yo could use edge() with 'outer' argument on the rasterized polygons instead of linesToRaster library(raster) p1 <- rbind(c(-180,-20), c(-140,55), c(-20, 0), c(-140,-60), c(-180,-20)) hole <- rbind(c(-150,-20), c(-100,-10), c(-110,20), c(-150,-20)) p2 <- rbind(c(-10,0), c(140,60), c(160,0), c(140,-55), c(-10,0)) pols <- SpatialPolygons( list( Polygons(list(Polygon(p1)), 1), Polygons(list(Polygon(p2)), 2))) xy <- coordinates(pols) #A r <- raster(nrow=18, ncol=36) r <- polygonsToRaster(pols, r) d <- distanceFromPoints(r, xy) x <- mask(d, r) plot(x) #B r <- raster(nrow=18, ncol=36) r <- polygonsToRaster(pols, r) l <- linesToRaster(pols, r) l[l>0] <- 0 p <- pointsToRaster(r, xy) pp <- cover(p, l) gd <- gridDistance(pp) gd[!is.na(l)] <- NA plot(gd) Robert On Tue, Apr 20, 2010 at 12:23 AM, Roman Luštrik <roman.lust...@gmail.com> wrote: > Dear list, > > I have a polygon that I converted to raster (kudos to polygonsToRaster()). I > would like to assign values (through a custom function) to cells within this > polygon (now as raster). Roughly speaking, cell value will depend on its > distance from the central cell (polygon centroid). How would you go about > doing this? Is there a more efficient way of doing this, or will I have to > hard code it, cell by cell? > > Cheers, > Roman > > [[alternative HTML version deleted]] > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@stat.math.ethz.ch > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
