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enter image description hereI have a raster with slope values and want to calculate a raster, which only shows values under and over a certain threshold for example 50. In the raster calculator I can do this manually by the expression

"Slope@1" >= 50

As I want to do this for several rasters and it is only one step in a chain, I would like to include it as a step in the graphical modeller of QGIS.This provides the GDAL Raster Calculator, the modeler only tool Calculator and the Saga Raster Calculator, but in none of these I can find out, what to type in to get the desired result.

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  • Could be that you need quotes around the band definition and the whole input. Like ""Slope@1>=50"
    – Matte
    Feb 14, 2017 at 13:56

1 Answer 1

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If you choose "GDAL Raster calculator", Input layer A would be Slope (though grayed out in modeller), Number of raster band= 1, and the Calculation in gdalnumeric syntax using... would be A*(A>=50).

Of A*(A>=50) the latter part (A>=50) returns 1 (TRUE) or 0 (FALSE). So the resultant equation A times 1 or 0 (A*1, A*0) is equivalent to setting a threshold value.

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  • Actually this seems like what I am looking for. However when I execute it like you proposed, I get the raster layer in the middle as a result (first image is the raster with the slope values), instead of the expected lower raster image.
    – Moo
    Feb 14, 2017 at 18:00
  • @Moo I am not sure but it seems you are trying to achieve a binary (1, 0) image. Then the equation would be (A>=50) or (A<50) depending on to which color each 1 and 0 is assigned.
    – Kazuhito
    Feb 14, 2017 at 23:27
  • That is the solution I searched for: I simply did not add the braces.
    – Moo
    Feb 19, 2017 at 22:45
  • @Moo I am glad it worked out. Sorry the braces (I tried to highlight them) were misleading.
    – Kazuhito
    Feb 19, 2017 at 23:41

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