I'm performing terrain analysis due to photovoltaic installation on QGIS with r.slope.aspect plugin. I'm interested in evaluate differently N-S slope and E-W slope based on the PV system I would install. Processing dx and dy tool I don't understand in which format the output raster is formatted. Choosing degrees or percent the results are the same. For example the output can be -1.17 NS (percent) what does it mean -1.17? 117%.


I believe that r.slope.aspect does not use degrees or percent rise for calculating the dx and dy values. These are simply derivatives. So dx is change in elevation divided by change in horizontal distance (in EW direction). A value near 0 means a flat area (again in EW direction). Positive values mean going uphill (EW), negative values mean going downhill. A value near 1 or -1 would indicates a 45 degree slope.

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