.TH r.sunmask 1grass "" "GRASS 8.3.1" "GRASS GIS User's Manual"
.SH NAME
\fI\fBr.sunmask\fR\fR  \- Calculates cast shadow areas from sun position and elevation raster map.
.br
Either exact sun position (A) is specified, or date/time to calculate the sun position (B) by r.sunmask itself.
.SH KEYWORDS
raster, solar, sun position, shadow
.SH SYNOPSIS
\fBr.sunmask\fR
.br
\fBr.sunmask \-\-help\fR
.br
\fBr.sunmask\fR [\-\fBzsg\fR] \fBelevation\fR=\fIname\fR  [\fBoutput\fR=\fIname\fR]   [\fBaltitude\fR=\fIfloat\fR]   [\fBazimuth\fR=\fIfloat\fR]   [\fByear\fR=\fIinteger\fR]   [\fBmonth\fR=\fIinteger\fR]   [\fBday\fR=\fIinteger\fR]   [\fBhour\fR=\fIinteger\fR]   [\fBminute\fR=\fIinteger\fR]   [\fBsecond\fR=\fIinteger\fR]   [\fBtimezone\fR=\fIinteger\fR]   [\fBeast\fR=\fIvalue\fR]   [\fBnorth\fR=\fIvalue\fR]   [\-\-\fBoverwrite\fR]  [\-\-\fBhelp\fR]  [\-\-\fBverbose\fR]  [\-\-\fBquiet\fR]  [\-\-\fBui\fR]
.SS Flags:
.IP "\fB\-z\fR" 4m
.br
Do not ignore zero elevation
.IP "\fB\-s\fR" 4m
.br
Calculate sun position only and exit
.IP "\fB\-g\fR" 4m
.br
Print the sun position output in shell script style
.IP "\fB\-\-overwrite\fR" 4m
.br
Allow output files to overwrite existing files
.IP "\fB\-\-help\fR" 4m
.br
Print usage summary
.IP "\fB\-\-verbose\fR" 4m
.br
Verbose module output
.IP "\fB\-\-quiet\fR" 4m
.br
Quiet module output
.IP "\fB\-\-ui\fR" 4m
.br
Force launching GUI dialog
.SS Parameters:
.IP "\fBelevation\fR=\fIname\fR \fB[required]\fR" 4m
.br
Name of input elevation raster map
.IP "\fBoutput\fR=\fIname\fR" 4m
.br
Name for output raster map
.IP "\fBaltitude\fR=\fIfloat\fR" 4m
.br
Altitude of the sun in degrees above the horizon (A)
.br
Options: \fI0\-89.999\fR
.IP "\fBazimuth\fR=\fIfloat\fR" 4m
.br
Azimuth of the sun in degrees from north (A)
.br
Options: \fI0\-360\fR
.IP "\fByear\fR=\fIinteger\fR" 4m
.br
Year (B)
.br
Options: \fI1950\-2050\fR
.IP "\fBmonth\fR=\fIinteger\fR" 4m
.br
Month (B)
.br
Options: \fI0\-12\fR
.IP "\fBday\fR=\fIinteger\fR" 4m
.br
Day (B)
.br
Options: \fI0\-31\fR
.IP "\fBhour\fR=\fIinteger\fR" 4m
.br
Hour (B)
.br
Options: \fI0\-24\fR
.IP "\fBminute\fR=\fIinteger\fR" 4m
.br
Minutes (B)
.br
Options: \fI0\-60\fR
.IP "\fBsecond\fR=\fIinteger\fR" 4m
.br
Seconds (B)
.br
Options: \fI0\-60\fR
.br
Default: \fI0\fR
.IP "\fBtimezone\fR=\fIinteger\fR" 4m
.br
Timezone
.br
East positive, offset from GMT, also use to adjust daylight savings
.IP "\fBeast\fR=\fIvalue\fR" 4m
.br
Easting coordinate (point of interest)
.br
Default: map center
.IP "\fBnorth\fR=\fIvalue\fR" 4m
.br
Northing coordinate (point of interest)
.br
Default: map center
.SH DESCRIPTION
\fIr.sunmask\fR 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
.RS 4n
.IP \(bu 4n
(A) parameters to specify the exact known position of the sun, or
.IP \(bu 4n
(B) parameters to specify the date/time for the sun position calculation
by \fIr.sunmask\fR itself
.RE
must be used.
.PP
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.
.PP
The solar zenith angle (\(dqsun angle above horizon\(dq) 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.
.PP
The solar azimuth angle (\(dqsun azimuth\(dq) defines the direction of the sun.
It is the angle between north and the projection of the sun\(cqs 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.
.SH NOTES
\fIr.sunmask\fR 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
\fIr.sunmask\fR (\fBtimezone\fR parameter) seasonally. See also
Daylight saving time by region and country.
.PP
.PP
Note: In latitude/longitude locations the position coordinates pair
(east/west) has to be specified in decimal degree (not DD:MM:SS). If
not specified, the map center\(cqs coordinates will be used.
Also \fIg.region \-l\fR displays the map center\(cqs coordinates in
latitude/longitude (or \fIg.region \-c\fR in the actual coordinate
system).
.PP
Note for module usage with the \fI\-g\fR flag, when performing calculations
close to sunset/sunrise:
.br
.nf
\fC
 [...]
 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.
\fR
.fi
In above calculation it appears to be a mistake as
the program indicates that we are before sunrise while
the \fIsun angle above horizon\fR is already positive.
The reason is that \fIsun angle above horizon\fR is
calculated with correction for atmosphere refraction while
\fIsunrise\fR and \fIsunset\fR are calculated \fBwithout\fR
correction for atmosphere refraction. The output without
\fI\-g\fR flag contains related indications.
.SH EXAMPLE
Example for North Carolina sample data set for the calculation
of sun position angles and more:
.br
.nf
\fC
# 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 \(rs
          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 \(rs
          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
\fR
.fi
.SH Acknowledgements
Acknowledgements: National Renewable Energy Laboratory for their
SOLPOS 2.0 sun position
algorithm.
.SH SEE ALSO
\fI
g.region,
r.sun,
r.sunhours,
r.slope.aspect
\fR
.SH AUTHORS
Janne Soimasuo, Finland, 1994
.br
update to FP by Huidae Cho, 2001
.br
SOLPOS algorithm feature added by Markus Neteler, 2001
.SH SOURCE CODE
.PP
Available at:
r.sunmask source code
(history)
.PP
Accessed: Sunday Dec 17 17:22:01 2023
.PP
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.PP
© 2003\-2023
GRASS Development Team,
GRASS GIS 8.3.1 Reference Manual