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Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.


r.sunhours - Calculates solar elevation, solar azimuth, and sun hours.
Solar elevation: the angle between the direction of the geometric center of the sun's apparent disk and the (idealized) horizon. Solar azimuth: the angle from due north in clockwise direction.


raster, solar, sun energy, sun position


r.sunhours --help
r.sunhours [-ts] [elevation=name] [azimuth=name] [sunhour=name] year=integer [month=integer] day=integer [hour=integer] [minute=integer] [second=integer] [--overwrite] [--help] [--verbose] [--quiet] [--ui]


Time is local sidereal time, not Greenwich standard time
Do not use SOLPOS algorithm of NREL
Allow output files to overwrite existing files
Print usage summary
Verbose module output
Quiet module output
Force launching GUI dialog


Output raster map with solar elevation angle
Name for output raster map
Output raster map with solar azimuth angle
Name for output raster map
Output raster map with sunshine hours
Sunshine hours require SOLPOS use and Greenwich standard time
year=integer [required]
Options: 1950-2050
If not given, day is interpreted as day of the year
Options: 1-12
day=integer [required]
Options: 1-366
Options: 0-24
Default: 12
Options: 0-60
Default: 0
Options: 0-60
Default: 0

Table of contents


r.sunhours calculates sun elevation and sun azimuth angles for the given time of day and each grid cell in the current region. Additionally, the photoperiod (sunshine hours on flat terrain) can be calculated.

Sun elevation, height, height angle, or solar altitude angle is the angle in degrees between the horizon and a line that points from the site towards the centre of the sun.

The sun azimuth angle is here defined as the azimuth angle in degrees of the sun from due north in a clockwise direction.

The time used here is defined such that 12:00 (high noon) is the time when the sun has reached its highest point in the sky at the current site, unless the -t flag is used in which case time is interpreted as Greenwich standard time.

If a sunhour output map is specified, the module calculates sunshine hours for the given day. This option requires both Greenwhich standard time and the use of the SOLPOS algorithm by NREL.


To consider also cast shadow effects of the terrain, r.sun has to be used.


Calculating a sun elevation angle map

Calculate the sun elevation angle map for 2010-10-11 at 14:00h solar time:
# set computational region to North Carolina state extent
g.region n=318500 s=-16000 w=124000 e=963000 res=500 -p
r.sunhours elevation=sun_elev year=2010 month=10 day=11 hour=14 minute=00

# visualize
d.mon wx0
d.rast sun_elev 
d.vect nc_state type=boundary
d.legend sun_elev -s 

Sun angle map (in degree) of NC, USA
Sun angle map (in degree) of North Carolina for the 2010-10-11 at 14:00h solar time

Calculate map of photoperiod (insolation time)

Calculate photoperiod of day-of-year 001 (1st January) of 2012 for the current computational region, ignoring cast shadow effects of the terrain:
g.region -p
r.sunhours sunhour=photoperiod_doy_001 year=2012 day=1


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


g.region, r.sun, r.sunmask


Markus Metz


Available at: r.sunhours source code (history)

Latest change: Monday Nov 18 20:15:32 2019 in commit: 1a1d107e4f6e1b846f9841c2c6fabf015c5f720d

Note: This document is for an older version of GRASS GIS that will be discontinued soon. You should upgrade, and read the current manual page.

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