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I am trying to patch two rasters with the raster calculator but I am not finding a way to work with no-data values.

  • Lager@1 has some information I need as the first option, this has no-data -9999 (reading in the properties)

  • Lager@2 has the information I want to complete what is no data in Lager@1

This command does not produce values where Lager@1 is no-data

("Large@1" != -9999) * "Large@1" + ("Large@1" = -9999) * "Lager@2"

I would like to avoid or skip the reclassification because I might have many zeros and I need to perform this with different rasters that might have different no-data formats, for instance, Lager@2 has -99999 instead of -9999.

In GRASS this would be an r.patch task, but I cannot (or I do not know how to) control the order in the r.patch available in the Processing Toolbox.

r.patch input=Lager@1, Lager@2 output=Lager@3

Result of patching of two raster maps containing NULLs using the default settings

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  • I checked the history of the toolbox and I found the command with r.patch and then I copied and edited for my needs (order of raster files) and pasted the processing.run("grass7:r.patch", {'input':... in the console. It worked, but it was not so frinedly as I would love QGIS to be
    – Marco
    Commented Apr 9, 2020 at 10:07
  • Use Fill NoData cells (see docs.qgis.org/3.16/en/docs/user_manual/processing_algs/qgis/…), also possible in batch mode and change nodata values to 0. I don't know of a direct way, see: gis.stackexchange.com/q/420195/88814
    – Babel
    Commented Mar 9, 2022 at 8:31

2 Answers 2

Reset to default
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You can do this with r.patch, or using isnull() in r.mapcalc.simple, or using &&& or ||| operators.

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  • Hi, is ´r.mapcalc` behind the QGIS Calculator? I did not find that information and it would open a door for all its functions and documentation, which is an essential part of a tool. Please confirm my understanding of "r.mapcalc (the QGIS calculator)"
    – Marco
    Commented May 20, 2020 at 7:28
  • Sorry - I was incorrect. You're right, r.mapcalc is not the backend to the QGIS calculator. I've updated the suggestion. You can use r.mapcalc.simple instead of the QGIS calculator.
    – mankoff
    Commented May 20, 2020 at 14:43
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Solution 1: Replace NoData with fixed, chosen value

You can use the QGIS native algorithm Fill NoData cells to assign a "neutral", numerical value instead of nodata, e.g. 0 (if you add the values afterwards) or 1 (if you multiply). In the tool, you can define the Fill value.

Solution 2: Replace NoData with interpolated value

QGIS also offers access to the GDAL tool Fill NoData that also replaces NoData with a value, but this time not by a fixed, chosen value, but based on an interpolation:

The values for the NoData regions are calculated by the surrounding pixel values using inverse distance weighting. From Documentation

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