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r.sunmask: Calculates cast shadow areas from sun position and elevation raster map. Either exact sun position (A) is specified, or


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.


raster, sun position


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
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.
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). 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.


Acknowledgements: National Renewable Energy Laboratory for their SOLPOS 2.0 sun position algorithm.


g.region, r.sun, r.slope.aspect


Janne Soimasuo, Finland 1994 update to FP by Huidae Cho 2001 added solpos algorithm feature by Markus Neteler 2001 Last changed: $Date: 2008-05-16 21:09:06 +0200 (Fri, 16 May 2008) $ Full index (C) 2003-2010 GRASS Development Team R.SUNMASK(1)

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