[GRASS-user] Subtraction of raster maps

Markus GRASS markus.metz.giswork at googlemail.com
Wed Jun 24 02:54:53 EDT 2009

Thomas Becker wrote:
> Hi,
> I know it is pretty standard to subtract to maps from each other and I do have the command for r.mapcalc, but it is not doing what I expect.
> I have a landcover map and a map wit buildings. Both are raster maps. Now I want to get rid of the areas in the landcover map that are covered by buildings. To get this done I wrote:
> r.mapcalc "newmap = landcover - buildings"
r.mapcalc "newmap = if (isnull(buildings), landcover, null())"

may do the trick, assuming that the buildings map has nodata where there
are no buildings. The newmap would have nodata where there are buildings
otherwise landcover.

Markus M
> The outcome is a map showing only the buildings and no landcover. It also doesn't matter if I change the order in the equation, the result is the same: Only buildings are delivered.
> Can you help me out with this?
> Thanks,
>     Thomas

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