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Set up for slope analysis using a percentageI am performing a slope analysis (Raster>Analysis>Slope) of a US Geological Survey DEM.

I am calculating the slope of a raster DEM saved as a TIF. I set up the slope analysis as per the picture below in EPSG 4326. I tick on calculate percentage and compute edges and leave all other variables as standard. When I calculate the slope in percentage I get really odd values being e to the 6 as shown in the layer panels in the second item down, rather than between 0 and 90%. In the map the layer looks correct though and this is a hilly area. How do I make it so the values appear between 0 and 90?

Similarly when slope analysis is performed in degrees (not ticking the percentage option it returns nearly all cells as 90 degrees which isn't correct (layer 2 in layers panel).

How do I solve these problems?

Map showing slope analysis output, 1st layers panel item is calculated using degrees and second is slope

Catocala nupta
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  • Please add more details on the exact settings you are using when conducting the slope analysis. Also please add single screenshots of all supposedly faulty layers. – Erik Jul 30 '20 at 09:00
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    Could it be that you are working in degree units CRS? See for example https://gis.stackexchange.com/questions/143139/getting-incorrect-slope-values-from-an-aster-dem-in-qgis – BERA Jul 30 '20 at 09:11
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    Thanks that has worked, I multiplied by 111120 and I now have a sensible percentage. Thanks for your help – Catocala nupta Jul 30 '20 at 09:28
  • yes I think so. Just to check to calculate the slope as a % in m, I need to do the cos (lat)x 111320 for the hor to vertical factor, not just times by 111120 – Catocala nupta Jul 30 '20 at 09:49

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