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I'm new to QGIS and I have to calculate the slope of a DEM (I use srtm-data). I already detected the geoalgorithms toolbox and I also produced a raster layer with the slope using the tool "slope, aspect, curvature" (One can find it in Domain specific algorithms - Terrain analysis and geomorphometry). I used the 2. Degree Polynom (Zevenbergen & Thorne), which is the default. A visual check shows that the general location of slopes seems to fit (I compared it to Google Earth).

Now my problem is: I have no clue about the unit of the output. The pixels at the steepest slopes have values around 0.3.... I have never been to that area, but it is located in Southern Germany, in an area that is usually quite hilly. So I strongly doubt that the slope is only 0.3° in the steepest area. I have the assumption that it is just 30°, but I would like to check somewhere to be absolutely sure how to interpret the result (since it could also be percentage values). I find a lot of information about what the tool does, but never any site where the units of the output are clearly stated.

Does anybody know how I can find out the unit of this output?

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I guess you are using the SAGA GIS Terrain Analysis tool? In this case it seems that the output comes in degrees: http://sourceforge.net/p/saga-gis/wiki/Terrain%20Analysis%20-%20Morphometry%20module%20library/

You could also use GRASS (r.slope.aspect) from the toolbox or from the QGIS Raster tools: Raster > Analisis > MDT (sorry, I´m working with the spanish interface, other language versions probably use different terms). These tools allow you to choose between degree or percent.

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  • Thanks for your answer Flo! I only tried the geoalgorithms and did not see the Raster Analysis tools. So now I got a result that makes sense to me.
    – MB_Ronaldo
    Sep 23, 2014 at 9:51

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