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

Barbosa, A. Marcia a.m.barbosa at imperial.ac.uk
Fri Jun 13 06:46:29 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...)

I'd appreciate some help. Cheers,

- - -
A. Márcia Barbosa
Dept. Biological Sciences, North 2.1
Imperial College at Silwood Park
Ascot, Berkshire SL5 7PY
United Kingdom

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