I'd agree, the aspect of a slope does not vary with it's gradient, and
the scale factor is only used to ensure that the gradient calculations
are made using consistent units in XY & Z.
Cheers,
Andy
On 11/10/2018 19:43, Nicolas Cadieux wrote:
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
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--
Andy Harfoot
GeoData Institute
University of Southampton
Southampton
SO17 1BJ
Tel: +44 (0)23 8059 2719
www.geodata.soton.ac.uk
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