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I want to calculate a simpliefied mobility model that classifies an terrain map into movement areas. The Mountains shall be like walls and everything else shall be movable terrain. I started to classify areas with a slope of 0-30% as GO areas and areas > 31% as NO-GO areas. As input I have a slope raster file. However, when I do a raster calculation for each cell being 1 when there is a slope of 0-30% and 0 otherwise, I still have some white lines which seem to be ridges of the mountains. How can I include these lines in the NO-GO area? Or is there another way to reach my goal? I am using QGis.

To illustrate my problem:

This is my slope raster input file: enter image description here

This is my result when doing the raster calculation step: enter image description here

As you can see in the second picture, there are white lines indicating mountain ridges. How do I add them to the NO-GO areas?

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    It's just doing exactly what you asked. The slope at the top of a ridge or mountain is less than 30% based on the calculations; if you don't want these ridge/peaks in your data you will need some other way to find them. Oct 17, 2017 at 21:45
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    You can find ridges using hydrology tools, they are cells with zero flow accumulation. However the ridges are often the safest routes, no avalanche, landslide or flood risk.
    – FelixIP
    Oct 17, 2017 at 21:51
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    I don't have time to write a developed answer but this is a typical morphological mathematics problem. You could try to perform a dilation followed by an erosion (on your second picture) -- unfortunately, there are not many tools in QGIS suited to these tasks. You can have a look at this post though: Erode pixel areas in QGIS
    – ArMoraer
    Oct 18, 2017 at 4:45

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