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I am calculating angle of incidence by the following formula!

cos(aoi) =  cos(slp) cos(zn) + sin(slp) sin(zn) cos(azm_s - azm )

aoi: angle of incidence slp: slope zn: zenith angle azm_s: solar azimuth angle (N = 0, clockwise) azm: surface azimuth (N = 0, clockwise)

For surface azimuth and slope angle I run 'r.slope.aspect' using Grass framework (https://grass.osgeo.org/grass70/manuals/r.slope.aspect.html). I convert the aspect values from E=0 anti-clockwise to N=0 clockwise. is the calculated aspect and surface azimuth similar?

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Note that there is i.topo.corr in GRASS GIS for performing terrain correction.

If you want to calculate it "manually", then you can update the angle computation with r.mapcalc:

# If you want to limit the range to 0..360, use:
r.mapcalc "angle_cw = (450 - angle_ccw) % 360"

# Similarly, if you want -180..180 (again, clockwise from North):
r.mapcalc "angle_cw = (630 - angle_ccw) % 360 - 180"

A few days ago a new -n flag to produce aspect CW from North with flat = -9999 (like gdaldem) was added to r.slope.aspect in trunk (GRASS GIS 7.5.svn) in order to avoid the r.mapcalc step.

  • Thank you so much for your reply! But I have another question. I tried to calculate the angle of incidence (aoi) using both the formula (e.g. range 0 to 360 and range -180 to 180) and I get a different aoi values. why so? then which aoi value I sud consider! – Sukriti Bhattacharya Feb 19 '18 at 9:40

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