assigning raster cell values based on predefined criteria

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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.hijmans at 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 Lu?trik <roman.lustrik at 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

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--
In God we trust, all others bring data.

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