r.sunmask(1grass) GRASS GIS User's Manual r.sunmask(1grass)
NAME
r.sunmask - Calculates cast shadow areas from sun position and eleva-
tion 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=inte-
ger] [east=value] [north=value] [--overwrite] [--help] [--ver-
bose] [--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 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 posi-
tion 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 eleva-
tion is not considered in the sunset/sunrise calculations.
NOTES
r.sunmask and daylight savings: Rather than converting the local time
to GMT, the SOLPOS algorithm uses what is called Local Standard Time,
which is generally politically defined 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), s/he
would have to change the Time Zone parameter seasonally in r.sunmask
(timezone parameter). See also Daylight saving time by region and coun-
try.
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 in latitude/longitude (or g.re-
gion -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 indi-
cates that we are before sunrise while the sun angle above horizon is
already positive. The reason is that sun angle above horizon is calcu-
lated with correction for atmosphere refraction while sunrise and sun-
set 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
AUTHOR
Janne Soimasuo, Finland 1994
update to FP by Huidae Cho 2001
added solpos algorithm feature by Markus Neteler 2001
SOURCE CODE
Available at: r.sunmask source code (history)
Accessed: unknown
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