I have calculated slope and aspect using the DEM (Terrain models) GUI in QGIS Essen. I was also able to calculate the profile and tangential curvature using the toolbox r.slope.aspect. I was wondering if anyone can point me in the direction of documentation that describes the algorithm used to calculate these terrain parameters. I would like to know HOW they are calculated not how to use the plugins. For example, I would like to now how large of an averaging window the program uses, how it choses direction, what it does on the edges of a raster, etc.
There is good documentation on all aspects of GRASS internals, including references to literature. So see the documentation on r.slope.aspect. Unfortunately I find it difficult to get this level of documentation for the methods exposed in the DEM/Terrain Models QGIS GUI. I'd recommend sticking entirely to GRASS if these implementation details are important to you; but perhaps someone else has a link to the QGIS implementation, it's probably all documented in GDAL.