DESCRIPTION

r.sunmask creates an output map layer based on an input elevation raster map layer and the sun position. The output map layer contains the cast shadow areas arising from sun shine and elevations. The user can define the sun position either directly or the module calculates it from given location and date/time parameters using the NREL sun position algorithm. So either "A:"-parameters to specify the exact known sun position or "B:-parameters" to specify date/time for sun position calculation by r.sunmask itself have to be used.

The module performs sunset/sunrise checks and refraction correction for sun position calculation. Local coordinate systems are internally transformed to latitude/longitude for the SOLPOS algorithm. The elevation is not considered in the sunset/sunrise calculations.

NOTES

r.sunmask and daylight savings: Rather than convert time to GMT, the solpos algorithm uses what is called Local Standard Time, which is generally defined politically as an offset from GMT. So the key is the offset from GMT, which the solpos Time Zone parameter. If the user specifies clock time (different for winter and summer), he/she would have to change the Time Zone parameter seasonally in r.sunmask (timezone parameter). See also Daylight saving time by region and country.

Note: In latitude/longitude locations the position coordinates pair (east/west) has to be specified in decimal degree (not D:M:S). If not specified, the map center's coordinates will be used. Also g.region -l displays the map center's coordinates.

Note for module usage with -g flag and calculations close to sunset/sunrise: 

 [...]
 sunangleabovehorizont=0.434240
 sunrise=07:59:19
 sunset=16:25:17
 Time (07:59:02) is before sunrise (07:59:19)!
 WARNING: Nothing to calculate. Please verify settings.
 No map calculation requested. Finished.
In above calculation appears to be a mistake as the program indicates that we are before sunrise while the sun angle above horizon is already positive. The reason is that sun angle above horizon is calculated with correction for atmosphere refraction while sunrise and sunset are calculated without correction for atmosphere refraction. The output without -g flag contains related indications.

EXAMPLE

Example for North Carolina sample data set for the calculation of sun position angles and more: 
# set the region to a place near Raleigh (NC)
g.region rast=elev_lid792_1m -p

# compute the sun position
r.sunmask -s --v elev_lid792_1m out=dummy year=2012 month=2 \
          day=22 hour=10 minute=30 sec=0 timezone=-5
Using map center coordinates: 638650.000000 220375.000000
Calculating sun position... (using solpos (V. 11 April 2001) from NREL)
2012/02/22, daynum: 53, time: 10:30:00 (decimal time: 10.500000)
long: -78.678856, lat: 35.736160, timezone: -5.000000
Solar position: sun azimuth: 143.006424, sun angle above horz. (refraction corrected): 36.233883
Sunrise time (without refraction): 06:58:11
Sunset time  (without refraction): 17:58:47

# with -g flag
r.sunmask -s -g --v elev_lid792_1m out=dummy year=2012 month=2 \
          day=22 hour=10 minute=30 sec=0 timezone=-5
Using map center coordinates: 638650.000000 220375.000000
Calculating sun position... (using solpos (V. 11 April 2001) from NREL)
date=2012/02/22
daynum=53
time=10:30:00
decimaltime=10.500000
longitudine=-78.678856
latitude=35.736160
timezone=-5.000000
sunazimuth=143.006424
sunangleabovehorizon=36.233883
sunrise=06:58:11
sunset=17:58:47

Acknowledgements

Acknowledgements: National Renewable Energy Laboratory for their SOLPOS 2.0 sun position algorithm.

SEE ALSO

g.region, r.sun, r.slope.aspect

AUTHOR

Janne Soimasuo, Finland 1994
update to FP by Huidae Cho 2001
added solpos algorithm feature by Markus Neteler 2001

Last changed: $Date$