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I am working with QGIs and am converting a slope raster with values in degrees to a slope raster with values in percent using the raster calculator and the following formula:

(arctan(slope_raster))*100

My original slope raster in degrees has no values outside of the 0-90 degree range which is expected but the percent slope raster it generates gives me a low of -62000 which is pretty silly.

I guess my question is what angular units does the QGIS raster calculator use? Degrees? Radians? Gradians?

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The raster calculator uses radians, so you'll have to convert your angular units.

To get from angles to percent slope, you need to use tan, not arctan, though, since tan(angle) = opposite / adjacent.

The following raster calculator expression should do the job:

tan("slope_raster" * 3.14159 / 180) * 100
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  • I would like to help those who want to get degrees out of slope percentage in the raster or field calculator: atan("Naklon_pro" /100) /3.14159*180
    – Jost Hobic
    Jan 13, 2020 at 14:33

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