This is the normal behavior (see image here<http://imagepaste.nullnetwork.net/viewimage.php?id=837>) when I use option A-ish (calculating distance to a single point).
Intuitively, however, I would expect the plot to look something like this (crudely drawn for your amusement<http://imagepaste.nullnetwork.net/viewimage.php?id=838>). Can someone explain the logic behind the "tiled" pattern? They say a picture is worth a thousand words - hopefully, this is the case here and my question is clear. Cheers, Roman On Tue, Apr 20, 2010 at 6:55 PM, Robert J. Hijmans <r.hijm...@gmail.com>wrote: > 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 Luðtrik <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 > > > -- In God we trust, all others bring data. [[alternative HTML version deleted]]
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