r.sunmask - GRASS manual
Author: GRASS Development Team
Description: r.sunmask: 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

## 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)

Latest change: Friday Feb 07 19:16:09 2025 in commit: a82a39f673b97fafcfce1604dc3e6015cb96487a

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