Understanding aspect units in QGIS

Question

I created an aspect raster form a DEM raster in QGIS 2.14, and got this results:

I know that the results refer to degree units , but still i don't understand how to read the map result- i mean to which direction the aspect turn.

In ESRI site http://desktop.arcgis.com/en/arcmap/10.3/tools/spatial-analyst-toolbox/how-aspect-works.htm , the spatial analyst doucumentation explain the aspect with this schem:

i seeked the same explanation in https://docs.qgis.org/2.2/en/docs/training_manual/rasters/terrain_analysis.html?highlight=aspect but didn't found anything like it.

How can i read and understand the aspect results?

Answer 1

It depends on which of the available aspect calculators you used as QGIS is beter thought of as a 'meta-GIS' rather than a single GIS as it provides a GUI for multiple GIS technologies (e.g. GDAL, GRASS, SAGA to name a few). If you use the GDAL Aspect calculation from Raster->Terrain Analysis->Aspect then the values are the same as ESRI (degrees from North). See here for more discussion. I believe that SAGA's slope calculator also follows the convention of 0 clockwise from north (confirmed by a quick test) though I can't point you at any documentation.

If however, you used the GRASS r.slope.aspect, then it computes the values as degrees counterclockwise from East.