[Qgis-user] z factor in aspect calculation?

[Andrew Harfoot]

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