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I'm using QGIS and I'm trying to calculate slope < 20% from a file I have but when I use the raster calculation SLOPE < 20 the result is all black. What am I doing wrong? The data is already in percent.

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    Check the output raster properties and see what its range is. Check the min/max and scaling. Check the histogram to see if has 1s and 0s.
    – Spacedman
    Nov 21 '15 at 14:03
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The result is all black because all values are 1. To get the true values you have to do (with the raster calculator nomenclature):

("SLOPE@1" < 20) * "SLOPE@1"
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  • That will give you zero where the slope is greater than 20, and also zero where the slope is zero. I'm not sure that's what's wanted...
    – Spacedman
    Nov 21 '15 at 17:26

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