document scaling and ellipsoidal heights

src/imagery/i.sentinel1.gdargeocode/i.sentinel1.gdargeocode.html

@@ -5,14 +5,33 @@ <h2>DESCRIPTION</h2>
 reference systems (CRS) different from the current LOCATION will be projected to the CRS of the LOCATION
 and results will be aligned with and resampled to the pixels in the input DEM.
 
+<p>
+<em>i.sentinel1.gdargeocode</em> requires a digital elevation model with ellipsoidal reference height as
+input in the <b>elevation</b> option. Usualy, elevation models represent geoidal heights. Users can apply an
+Earth Gravitational Model (EGM) to compute ellipsoidal heights from their geoidal elevation models. EGMs can
+be downloaded as GeoTiffs here: https://cdn.proj.org/ and added to the heights in the geoidal elevation model:
+<div class="code"><pre>
+r.mapcalc expression="DTM_ellipsoidal=DTM+EGM"
+</pre></div>
+
+<p>
+Values in the output raster maps can be scaled in four different ways using the <b>scale</b> option:
+<ul>
+  <li><em>abs</em>: absolute values</li>
+  <li><em>intensity</em>: absolute squared values</li>
+  <li><em>dba</em>: dB absoulute values</li>
+  <li><em>dbi</em>: dB intensity values</li>
+</ul>
+
+<p>
 If the <b>a-flag</b> is set, a precise orbit file is fetched from ESAs web service if available (recommended).
 Note that there is usually a several days time lag after sensing time before these files are made available. If
 precise orbit files for the given scene are not available, the process will proceeed without and give a warning.
 
 <p>
-<b>input</b> can be a single Sentinel-1 SAFE file, a comma separated list of Sentinel-1 files or a text
-file with a list of Sentinel-1 files to geocode. Geocoded images are stored in GeoTIFF format in the
-<b>output_directory</b> and linked to the GRASS GIS database using <em>r.external</em>.
+<b>input</b> can be a single Sentinel-1 SAFE file, a directory containing Sentinel-1 files, a comma separated
+list of Sentinel-1 files or a text file with a list of Sentinel-1 files to geocode. Geocoded images are stored
+in GeoTIFF format in the <b>output_directory</b> and linked to the GRASS GIS database using <em>r.external</em>.
 
 <p>
 If only a specifc extent of the Sentinel-1 images is supposed to be geocoded, the user can define
@@ -26,7 +45,7 @@ <h2>DESCRIPTION</h2>
 supported.
 
 <h2>EXAMPLES</h2>
-  
+
 <div class="code"><pre>
 i.sentinel1.gdargeocode -a input=S1A_IW_GRDH_1SDV_20200304T161544_20200304T161609_031528_03A1CB_4A98.SAFE output_directory=./ elevation=elevation
 </pre></div>