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manual: r.sunmask and i.topo.corr improvements #3159

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Oct 3, 2023
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manual: r.sunmask and i.topo.corr improvements #3159

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6e89bbf
manual: r.sunmask and i.topo.corr improvements
neteler Sep 14, 2023
1658d04
fix grammar/typos
neteler Sep 26, 2023
9b97ae9
correct hemisphere and azimuth angle definition
neteler Oct 3, 2023
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24 changes: 14 additions & 10 deletions imagery/i.topo.corr/i.topo.corr.html
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Expand Up @@ -3,12 +3,11 @@ <h2>DESCRIPTION</h2>
<em>i.topo.corr</em> is used to topographically correct reflectance
from imagery files, e.g. obtained with <em>i.landsat.toar</em>, using a
sun illumination terrain model. This illumination model represents the
cosine of the incident angle <em>i</em>, i.e. the angle between the normal to the
ground and the sun rays.
cosine of the incident angle <em>i</em>, i.e. the angle between the normal
to the ground and the sun rays.
<p>
Note: If needed, the sun position can be calculated for a given date with
<em>r.sunmask</em>.

Note: If needed, the sun position can be calculated for a given date and
time with <em>r.sunmask</em>.

<center>
<img src="i_topo_corr_angles.png" alt="Figure showing terrain and solar angles"><br>
Expand All @@ -18,15 +17,20 @@ <h2>DESCRIPTION</h2>
<p>
Using the <b>-i</b> flag and given an elevation basemap (metric),
<em>i.topo.corr</em> creates a simple illumination model using the formula:

<ul>
<li> cos_i = cos(s) * cos(z) + sin(s) * sin(z) * cos(a - o) </li>
</ul>

where,
<em>i</em> is the incident angle to be calculated,
<em>s</em> is the terrain slope angle,
<em>z</em> is the solar zenith angle,
<em>a</em> the solar azimuth angle,
<em>o</em> the terrain aspect angle.

<ul>
<li> <em>i</em> is the incident angle to be calculated,</li>
<li> <em>s</em> is the terrain slope angle (from <em>r.slope.aspect</em>),</li>
<li> <em>z</em> is the solar zenith angle (i.e., 90&deg; - solar horizon angle from <em>r.sunmask</em>),</li>
<li> <em>a</em> the solar azimuth angle (from <em>r.sunmask</em>),</li>
<li> <em>o</em> the terrain aspect angle (from <em>r.slope.aspect</em>).</li>
</ul>

<p>
For each band file, the corrected reflectance (ref_c) is calculate from
Expand Down
73 changes: 48 additions & 25 deletions raster/r.sunmask/r.sunmask.html
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@@ -1,34 +1,57 @@
<h2>DESCRIPTION</h2>

<em>r.sunmask</em> 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
<a href="http://rredc.nrel.gov/">NREL</a> 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 <em>r.sunmask</em> itself
have to be used.

<p>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.
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
<a href="https://www.nrel.gov/">National Renewable Energy Laboratory</a>
(NREL). SOLPOS operates in two modes, either

<ul>
<li>(A) parameters to specify the exact known position of the sun, or</li>
<li>(B) parameters to specify the date/time for the sun position calculation
by <em>r.sunmask</em> itself</li>
</ul>
must be used.

<p>
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.

<p>
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&deg;, when the sun is directly overhead, to 0&deg;,
when the sun is on the horizon. Values lower than 0&deg; indicate that the
sun is below the horizon.

<p>
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&deg; and 360&deg;. Specifically, an azimuth of 0&deg; means
the sun is in the north, 90&deg; in the east, 180&deg; in the south and
270&deg; in the west.

<h2>NOTES</h2>

<em>r.sunmask</em> and daylight savings: Rather than converting the
local time to GMT, the SOLPOS algorithm uses what is called Local
Standard Time, which is generally politically defined as an offset
from GMT. So the key is the offset from GMT, which the solpos Time
<em>r.sunmask</em> 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 seasonally in <em>r.sunmask</em> (timezone parameter). See also
<a href="http://en.wikipedia.org/wiki/Daylight_saving_time_by_country">Daylight saving time by region and country</a>.
winter and summer), s/he would have to change the Time Zone parameter in
<em>r.sunmask</em> (<b>timezone</b> parameter) seasonally. See also
<a href="https://en.wikipedia.org/wiki/Daylight_saving_time_by_country">Daylight saving time by region and country</a>.
<p>

<p>
Note: In latitude/longitude locations the position coordinates pair
(east/west) has to be specified in decimal degree (not D:M:S). If
(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 <em>g.region -l</em> displays the map center's coordinates in
latitude/longitude (or <em>g.region -c</em> in the actual coordinate
Expand Down Expand Up @@ -98,7 +121,7 @@ <h2>EXAMPLE</H2>
<h2>Acknowledgements</h2>

Acknowledgements: National Renewable Energy Laboratory for their
<a href="http://rredc.nrel.gov/solar/codesandalgorithms/solpos/">SOLPOS 2.0</a> sun position
<a href="https://www.nrel.gov/grid/solar-resource/solpos.html">SOLPOS 2.0</a> sun position
algorithm.

<h2>SEE ALSO</h2>
Expand All @@ -112,6 +135,6 @@ <h2>SEE ALSO</h2>

<h2>AUTHORS</h2>

Janne Soimasuo, Finland 1994<br>
update to FP by Huidae Cho 2001<br>
SOLPOS algorithm feature added by Markus Neteler 2001
Janne Soimasuo, Finland, 1994<br>
update to FP by Huidae Cho, 2001<br>
SOLPOS algorithm feature added by Markus Neteler, 2001
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