I have two raster (raster1 and raster2) from the same area. I would like to make the sum between the two (raster3 = raster1 + raster2) in QGIS, so that the Digital Number (DN) of raster 3 is the sum between them. However, raster2 has some pixels like "nodata" or "null" without data. When I add them together, the "nodata" pixels do not add up.

For example: raster1 = 2 and raster2 = nodata. When I add these, the result is nodata (2 + nodata = nodata), but I expected it to be = 2, that is, (2 + nodata = 2).

How could I solve this problem?


You start QGIS with GRASS and then run r.null from processing toolbox on your nodata raster and convert all NULL values to 0 values. Then you should be able to build your sum in raster calculator.

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  • hello @MrXsquared, i tried apply you answer, but not the result yet returns pixels with nodata. – wesleysc352 Dec 19 '20 at 14:15
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    @wesleysc352 That shouldnt be the case. I have attached a short GIF to illustrate. – MrXsquared Dec 19 '20 at 14:37
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    hello @MrXsquared,thanks for the immense help. I had filled in the wrong field. Now it worked. – wesleysc352 Dec 19 '20 at 15:08

I would note that the GRASS module r.mapcalc allows you to add rasters, while ignoring NULL cells, without changing the original. For example:

r.mapcalc "raster3 = if(isnull(raster1), raster2, raster1 + raster2)

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