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The speed of r.sun is much higher ifis used first and the resulting maps are given as input to r.sun. Background: the horizon needs to be computed only one time before, not in every step within r.sun.
Create an artificial surface containing a Gaussian mound:
out=gauss method=gaussian kurtosis=1
The following three images demonstrate the effects of using different time step parameters.
r.sun -s elevin="gauss" glob_rad="rad.global.30minT" day=180 step=0.5
r.sun -s elevin="gauss" glob_rad="rad.global.15minT" day=180 step=0.25
r.sun -s elevin="gauss" glob_rad="rad.global.03minT" day=180 step=0.05
The 3 minute time step takes roughly ten times as long to run as the 30 minute timestep.