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.
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.
# 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
Last changed: $Date$