Aw: [GRASS-user] map algebra: sums including null values

tim.ritter at tim.ritter at
Fri Jun 13 08:02:27 EDT 2008

Hi Marcia,

it should work with:

X=if(isnull(A), 0, A) + if(isnull(B), 0, B) + if(isnull(C), 0, C) ...

Hope it'll help you,


----- Original Nachricht ----
Von:     "Barbosa, A.  Marcia" <a.m.barbosa at>
An:      grass-user at
Datum:   13.06.2008 12:46
Betreff: [GRASS-user] map algebra: sums including null values

> Hi,
> I've just started using GRASS and QGIS. I'm using r.mapcalculator to sum
> groups of partially overlapping rasterized polygons, with value 1 inside and
> null outside the polygons. I want each pixel in the output (sum) map to have
> the value of the number of polygons overlapping at that point, so I'm just
> summing them up (A+B+C...). The problem is that the sum is only performed
> where all polygons overlap: if there is a null value at one site, the
> resulting sum is null... Is there a way to tell GRASS to consider null
> values as zeroes when performing the sum? Or do I really have to transform
> null cells into zero-value cells in all rasters before summing them up?
> (It's a lot of maps...)
> I'd appreciate some help. Cheers,
> Marcia
> - - -
> A. Márcia Barbosa
> Dept. Biological Sciences, North 2.1
> Imperial College at Silwood Park
> Ascot, Berkshire SL5 7PY
> United Kingdom
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