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?