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I have a world DEM raster in QGIS in EPSG:4326 with elevation units in meters. I'm trying to get a raster of the slope of the terrain, and I'm doing so using the "Slope" tool under "Raster terrain analysis" inside of the Processing toolbox on the DEM.

At first I tried it with the default settings (z = 1), and it didn't work at all (areas either had a slope of 0 or a slope of around 89). Looking through this stackexchange, I found another similar question posted from before (Getting incorrect slope values from an ASTER DEM in QGIS?), where the solution (to my understanding) was to basically set z equal to 111120 or warp the raster to a projection that has elevation in meters already.

Setting the z equal to 111120 still hasn't worked for me, I'm assuming because the person in that question had a raster of a very specific location where setting z equal to 111120 is appropriate, and my raster is of the world and you can't do that.

I think what I need to do is to warp my raster into a projection that has elevation in meters, is that correct?

And if so, what projection should I choose and how should I do so?

The projections suggested in the answers to the previous question seemed to be specific to that specific location.

I'm also slightly suspicious that perhaps the elevation data in the DEM is somehow formatted wrong or something, as this is my first time working with DEMs (if that wasn't obvious already lol). Generally when working with raster analysis tools (Zonal Statistics, etc.) in the past, I have the option to choose which band of the raster to use, even if there's just one band, and I still see the option now to choose the elevation band when looking through the other raster tools.

However, I'm not seeing an option to select a band when using the slope tool- is this normal?

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    Did you try using another processing algorithm like the one from GRASS "r.slope.aspect" ?
    – JULESG
    Oct 7 '20 at 7:55
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Yes, you are correct. It is better or let's say correct to use a projection when calculating a slope or any other DEM derivatives. Don't use WGS84.

The best projection would be any area preserving projection (equal area projection). You could for example use lambert azimuthal equal-area projection. As I don't know your DEM's area, it is hard to give a good advice. If the area is small, you should use a local equal-area projection.

Regarding the band selection. The tool assumes to be provided a single band raster. If this is not the case, simply make a band subset of your DEM. You could use QGIS raster calculator for this. Lets say your DEM is called DEM and the elevation data is in band 1 the expression would be.

DEM@1

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