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

Hamish hamish_b at yahoo.com
Fri Jun 13 07:50:31 EDT 2008

> 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...)

maybe try "r.series method=sum"? (without the -n flag)

with r.mapcalc you can create something with isnull(), but it gets complicated. See the large "NULL support" section in the r.mapcalc help page for examples.



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