[Qgis-user] z factor in aspect calculation?

[Nicolas Cadieux]

Hi,

I use a z factor in slope and hill shade calculation when my DEMs are in 
Long/Lat and the altitudes in metres (like SRTM, ASTER GDEM...).  In 
GdalDem , there is not a z factor or a scale option for Aspect (0-360D 
azimuth) but in the Qgis terrain analysis, there is. (It does not appear 
to work in Qgis 2.18.24).

In Qgis 3.2.3, when using the QGIS Terrain analysis Aspect algorithm 
with a z factor of 1, I get identical results as in gdaldem and results 
look and feel good.  I figure it's probably the same algorithm running 
in the background.  What more,   the aspect file is similar to result 
done on a reprojected DEM in a local UTM zone (where no z factor is 
needed as everything is in metres).  Reprojecting is not an option for 
my project.

In the QGIS Terrain analysis Aspect algorithm, if I use 0.0000126 as the 
correct z factor for the area (1 / The length of a Degree of Longitude 
in metres at a latitude of 45N) (or 1/78847=0.0000126)  my results are 
very wrong.

I wonder if using a z factor (0.0000126) is not needed for the aspect 
calculation and that this should be removed from the QGIS Terrain 
Analysis aspect options.  Any thoughts?

Nicolas