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I'm using a Aster tile DEM to perform some terrain analysis on an area in the French alps (original CRS:EPSG 4326-wgs 84), using QGIS 3.2. My projected coordinate reference system is EPSG:2154-RGF93/Lambert-93.

If I re-project the original tile with the warp tool and then perform a slope analysis in degrees (z factor 1), I obtain an acceptable result from 0 to 89 degrees. If then I try to obtain the slopes in percentages (z factor always 1), I have absurd results in the order of thousands and poor visualization.

Is this a possible correct result or is there an issue?

This is the result on the reprojected and filled DEM, I changed the symbology to blue, is this a possible correct result or is there an issue?

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    Never ever set the CRS of data unless it is obviously wrong and you know which CRS is the correct one. Otherwise always reproject/transform. As to the slope issue: please provide example images.
    – Erik
    Aug 8, 2022 at 11:36
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    – Vince
    Aug 8, 2022 at 11:57
  • What should I do when someone answers my question?
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    May 11 at 13:26

1 Answer 1

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It looks like your slope in percent is reasonable, except for some pixels at the edge at where the NoData border be as you haven't set your nodata correctly when warping.

Just set an output NoData value when you warp.

enter image description here

E.g. with output NoData set:

enter image description here

With output nodata not set:

enter image description here

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  • Hello, thank you for your answer, could you please explain why you chose the Bilinear sampling method and why you set the Nodata value to -9999?
    – Agnese Marcigliano
    Aug 10, 2022 at 9:54
  • Bilinear because your data is continuous, -9999 because unless you're in the Mariana Trench, it's not a valid elevation value.
    – user2856
    Aug 10, 2022 at 10:05

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