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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?

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6 Answers 6

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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
    Commented Dec 19, 2020 at 14:15
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    @wesleysc352 That shouldnt be the case. I have attached a short GIF to illustrate.
    – MrXsquared
    Commented Dec 19, 2020 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
    Commented Dec 19, 2020 at 15:08
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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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You can use r.series:

r.series input=raster1,raster2 output=raster3 method=sum
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The SAGA tool "Rasters Sum" has a simple option for counting 'no data' values as zero.

SAGA > Raster - Calculus > Rasters Sum

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The SAGA tool "Change a grid's No-Data value" also helps.

In the Processing panel:

SAGA > Raster - Tools > Change a Grid's No-Data Value

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Use Fill NoData cells to remove NoData values from a raster: you can choose a value the No Data cells are replaced with.

Hot to find the tool:

Menu Processing > Toolbox > Fill NoData cells
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