Provided by: grass-doc_6.4.3-3_all 
NAME
r.sunmask - Calculates cast shadow areas from sun position and elevation raster map.
Either exact sun position (A) is specified, or date/time to calculate the sun position (B) by r.sunmask
itself.
KEYWORDS
raster, sun position
SYNOPSIS
r.sunmask
r.sunmask help
r.sunmask [-zvsg] elevation=name [output=name] [altitude=float] [azimuth=float] [year=integer]
[month=integer] [day=integer] [hour=integer] [minute=integer] [second=integer]
[timezone=integer] [east=value] [north=value] [--overwrite] [--verbose] [--quiet]
Flags:
-z
Don't ignore zero elevation
-v
Verbose output (also print out sun position etc.)
-s
Calculate sun position only and exit
-g
Print the sun position output in shell script style
--overwrite
Allow output files to overwrite existing files
--verbose
Verbose module output
--quiet
Quiet module output
Parameters:
elevation=name
Name of elevation raster map
output=name
Name for output raster map
altitude=float
Altitude of the sun above horizon, degrees (A)
Options: 0-89.999
azimuth=float
Azimuth of the sun from the north, degrees (A)
Options: 0-360
year=integer
Year (B)
Options: 1950-2050
month=integer
Month (B)
Options: 0-12
day=integer
Day (B)
Options: 0-31
hour=integer
Hour (B)
Options: 0-24
minute=integer
Minutes (B)
Options: 0-60
second=integer
Seconds (B)
Options: 0-60
timezone=integer
Timezone
East positive, offset from GMT, also use to adjust daylight savings
east=value
Easting coordinate (point of interest)
Default: map center
north=value
Northing coordinate (point of interest)
Default: map center
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: 2012-03-29 01:32:53 -0700 (Thu, 29 Mar 2012) $
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© 2003-2013 GRASS Development Team
GRASS 6.4.3 r.sunmask(1grass)