Provided by: grass-doc_8.4.1-1_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, solar, sun position, shadow
SYNOPSIS
r.sunmask
r.sunmask --help
r.sunmask [-zsg] 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] [--help] [--verbose] [--quiet]
[--ui]
Flags:
-z
Do not ignore zero elevation
-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
--help
Print usage summary
--verbose
Verbose module output
--quiet
Quiet module output
--ui
Force launching GUI dialog
Parameters:
elevation=name [required]
Name of input elevation raster map
output=name
Name for output raster map
altitude=float
Altitude of the sun in degrees above the horizon (A)
Options: 0-89.999
azimuth=float
Azimuth of the sun in degrees from north (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
Default: 0
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 position of
the sun. The output map layer contains the cast shadow areas resulting from sunlight and elevation. The
user can either specify the sun position directly or the module calculates it from given location and
date/time parameters using the SOLPOS (Solar and Moon Position Algorithm) developed by the National
Renewable Energy Laboratory (NREL). SOLPOS operates in two modes, either
• (A) parameters to specify the exact known position of the sun, or
• (B) parameters to specify the date/time for the sun position calculation by r.sunmask itself
must 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. Elevation
is not taken into account for sunset/sunrise calculations.
The solar zenith angle ("sun angle above horizon") is defined as the angle between the horizon and the
vertical (directly overhead or zenith). Its values can range from 90°, when the sun is directly overhead,
to 0°, when the sun is on the horizon. Values lower than 0° indicate that the sun is below the horizon.
The solar azimuth angle ("sun azimuth") defines the direction of the sun. It is the angle between north
and the projection of the sun’s rays onto the horizontal plane. This angle is measured in a clockwise
direction and can vary between 0° and 360°. Specifically, an azimuth of 0° means the sun is in the north,
90° in the east, 180° in the south and 270° in the west.
NOTES
r.sunmask and daylight saving time: Instead of converting the local time to GMT, the SOLPOS algorithm
uses what is known as Local Standard Time, which is generally defined as an offset from GMT. So the key
is the offset from GMT, which is the solpos Time Zone parameter. If the user specifies clock time
(different for winter and summer), s/he would have to change the Time Zone parameter in r.sunmask
(timezone parameter) seasonally. See also Daylight saving time by region and country.
Note: In latitude/longitude projects the position coordinates pair (east/west) has to be specified in
decimal degree (not DD:MM:SS). If not specified, the map center’s coordinates will be used. Also
g.region -l displays the map center’s coordinates in latitude/longitude (or g.region -c in the actual
coordinate system).
Note for module usage with the -g flag, when performing 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 it 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 raster=elev_lid792_1m -p
# compute only sun position and no output map
r.sunmask -s elev_lid792_1m year=2012 month=2 \
day=22 hour=10 minute=30 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: 143.006409, sun angle above horz. (refraction corrected): 36.233879
Sunrise time (without refraction): 06:58:11
Sunset time (without refraction): 17:58:47
# with -g flag, useful for eval() shell function
r.sunmask -s -g elev_lid792_1m year=2012 month=2 \
day=22 hour=10 minute=30 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=143.006409
sunangleabovehorizon=36.233879
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.sunhours, r.slope.aspect
AUTHORS
Janne Soimasuo, Finland, 1994
update to FP by Huidae Cho, 2001
SOLPOS algorithm feature added by Markus Neteler, 2001
SOURCE CODE
Available at: r.sunmask source code (history)
Accessed: Friday Apr 04 01:19:37 2025
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© 2003-2025 GRASS Development Team, GRASS GIS 8.4.1 Reference Manual
GRASS 8.4.1 r.sunmask(1grass)