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I have a NRCS 2M DEM Bare Earth file for which I have already successfully applied the Hillshade and Aspect tools in my QGIS v3.34.1 project (see below).

Project area of interest w/ Hillshade and Aspect correctly output from the DEM

Now I am attempting to determine slope using the same DEM file, but the output has values that are not realistic (see below).

QGIS Slope tool output raster

The values for the resulting Slope output range from 89.10688 to 89.99926. As you can see from the topography of the area of interest slope should vary the full range from 0% to 100%. So far I have tried using the default QGIS Raster terrain analysis Slope tool, the GDAL Raster analysis Slope tool, and the GRASS Raster (r.*) r.slope.aspect tool, all with similar results.

The Project CRS is EPSG:3857 - WGS 84 / Pseudo-Mercator (see below), and the DEM file I am using is in EPSG:4269 - NAD83--does that create a conflict for the operation I am attempting?

Project Properties & CRS

Edit I reprojected the DEM CRS from EPSG:4269 to EPSG:3857 - WGS84 and then ran the QGIS slope tool. It was able to output slope in degrees, but the tool does not provide the option to calculate slope in percent, which is what I need. I tried using the GDAL slope tool with the reprojected DEM layer, but the result is just a black square with a white outline of the property I am analyzing.

Edit 2 After reprojecting the DEM raster to EPSG:3857 I used the GDAL slope tool selecting output as percent instead of degrees. The output was still a black square covering the view extent with a white outline of the area of interest. I changed symbology to pseudocolor and made the value range 0 to 200 and clipped out of range values. This seems to have worked, as now comparing the outputs of the degree slope and percent slope processes correspond (i.e. where the layer with degree output had a cell with 22.38 the corresponding cell in the layer with percentage was 41.18). There seems to be a problem with the boundary of the area of interest, as the % slope values were in excess of 1 million and were skewing the symbology colors heavily. If I can get this corrected then my last task is to assign a slope and aspect for the area of interest's polygons.

Linus
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    Which projection is your data using ? Your DEM must be projected using a coordinate system in meters like epsg:3857 – Kasper Jan 20 '24 at 08:27
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    I also bet that your DEM is in EPSG:4326 using degrees as horizontal units. – user30184 Jan 20 '24 at 09:31
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    See for example https://gis.stackexchange.com/questions/184517/slope-tool-gives-wrong-degree-values – user30184 Jan 20 '24 at 11:57
  • Hi user30184, how do I check the horizontal units of the DEM CRS? The DEM file I am using is in EPSG:4269 - NAD83. I'm not really sure if that conflicts in any way with the CRS of my project, which is in EPSG:3857 - WGS 84 / Pseudo-Mercator. – Linus Jan 22 '24 at 14:49
  • Edit: Google tells me that EPSG: 4269 has degree units, NAD83 is in meters, EPSG:3857 is in meters, and WGS 84 is in meters. Idk if I've made a mess of things with the CRSes, any pointers would be helpful. – Linus Jan 22 '24 at 15:16

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