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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
- Do not 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 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
- 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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