[Qgis-user] trigonometric angle in aspect maps

[Even Rouault]

Le mercredi 22 avril 2015 12:59:53, Larissa Junek a écrit :
> Hello,
> I look for a GIS expert who can give a good description of the
> trigonometric angle that can be produced when using the GDAL aspect
> algorithm in Processing.
> I need this description for the user manual 2.8..

Larissa,

from the GDAL manual: http://gdal.org/gdaldem.html

"""
 aspect

This command outputs a 32-bit float raster with values between 0° and 360° 
representing the azimuth that slopes are facing. The definition of the azimuth 
is such that : 0° means that the slope is facing the North, 90° it's facing 
the East, 180° it's facing the South and 270° it's facing the West (provided 
that the top of your input raster is north oriented). The aspect value -9999 
is used as the nodata value to indicate undefined aspect in flat areas with 
slope=0.

The following specifics options are available :

-trigonometric:
    return trigonometric angle instead of azimuth. Thus 0° means East, 90° 
North, 180° West, 270° South 
-zero_for_flat:
    return 0 for flat areas with slope=0, instead of -9999 

By using those 2 options, the aspect returned by gdaldem aspect should be 
identical to the one of GRASS r.slope.aspect. Otherwise, it's identical to the 
one of Matthew Perry's aspect.cpp utility.
"""

Even

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