# R.sun

## Help page

- r.sun manual page

## Tips

The speed of r.sun is much higher if r.horizon is 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. See the example at the end of the r.sun help page.

### Testing

Create an artificial surface containing a Gaussian mound:

```
r.surf.volcano out=gauss method=gaussian kurtosis=1
```

Overlay some 200m contours to show underlying topography:

r.contour in=gauss out=gauss_200m_contours step=200 d.vect gauss_200m_contours color=white

Set map's color table to highlight detail:

r.colors rad_test.day355.beam col=bcyr -e d.legend rad_test.day355.beam range=1300,1500

### Time step

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.

## Seed maps

Using seed maps can greatly speed up processing. This is especially important if you will batch process e.g. for every day of the year.

- aspin= and slopein= options: create slope and aspect maps with the r.slope.aspect module.

*Caution: currently buggy*. See trac #498.- Estimated speedup: ?%.

- horizon= and horizonstep= options: Pre-calculate horizon shadows by creating a series of horizon rasters with the r.horizon module.

- Results not as smooth as without using this option?? See trac #498.
- Estimated speedup: ?%.

- latin= option: Pre-calculate latitudes for each raster cell.

The following script will create a raster containing latitude as the raster value:

```
# create latitude map (WGS84)
g.region rast=elevation.dem
r.mapcalc one=1
r.out.xyz one | \
cut -f1,2 -d'|' | \
m.proj -oed --quiet | \
sed -e 's/[ \t]/|/g' | \
cut -f1,2,4 -d'|' | \
r.in.xyz in=- out=elevation.lat
g.remove one
```

- Estimated speedup: ?%.
- See trac #498.

## ToDo

- Add support for OpenMP parallelization