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<p class=MsoSalutation><font size=3 face=Arial><span lang=EN-US
style='font-size:12.0pt;font-family:Arial'>Hello, (I hope this is approximately
the right forum).<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>I have had some trouble with meteorological GeoTIFF data,
stored in stereographic<br>
based on a GEOGCS called “NinJo sphere”, without any EPSG code, using
a sphere<br>
with radius 6378137 m.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>The main problem wasn’t the lack of an ordinary
geodetic datum, since the most detailed<br>
file had the resolution 4.6 km, much more than the typical difference between
datums. <o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>What bothers me more, with spherical data, is that it
seems impossible to know if the<br>
spherical latitude should be interpreted as identical to WGS84 latitude
(geodetic latitude),<br>
or if it should be interpreted as geocentric latitude that needs conversion to
WGS84.<br>
The difference between the two interpretations can be up to 21 km, which can be<br>
significant even when the datum choice isn’t. <o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>I was hoping to find that one of the alternatives was
a de facto standard, but<br>
instead I found evidence that both of them are in use.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Spherical latitude = geodetic latitude is used by:<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'> Data in Web Mercator, like Open Street Map,<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'> Meteorological data in any of the models WRF,
MM5 and NinJo.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Spherical latitude = geocentric latitude is used by:<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'> MISR Regional UAE2 Imagery (from NASA Earth
Data).<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>In an old post, Melita Kennedy has written that it is
“probably more correct” to use the<br>
second convention; maybe she has experience with data like MISR. <o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>The Proj.4 FAQ discusses Web Mercator and mentions “many
other datasets … such as<br>
many NASA datasets” as using the same convention, so maybe NASA is not consistent<br>
in its choice of convention. <o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Anyway, what I would like is some standard way to tag
the metadata, in GeoTIFF in<br>
my case but more generally in WKT, to specify which convention is used. <br>
Of course, the problem is already solved in Proj.4, as the FAQ says, since we
can use<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'> +nadgrids=@null<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>for the first convention, and<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'> +towgs84=0,0,0<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>for the second one.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>In GeoTiff, specifying TOWGS84[0,0,0] in the same way
will be possible as soon<br>
as the proposal <a href="http://trac.osgeo.org/geotiff/wiki/TOWGS84GeoKey">http://trac.osgeo.org/geotiff/wiki/TOWGS84GeoKey</a>
is supported,<br>
but there will still be no way to specify the first convention.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>In WKT, too, only the second convention can be
specified.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>So I wonder, is there already some discussion going
on about how the first convention<br>
could be specified? (Perhaps within OGC or EPSG.) If not, can we start it? <o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Best regards<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Mikael Rittri<br>
Carmenta<br>
<st1:country-region w:st="on"><st1:place w:st="on">Sweden</st1:place></st1:country-region><br>
<a href="http://www.carmenta.com/">http://www.carmenta.com</a><o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>REFERENCES:<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>The way to interpret WRF and MM5 data: <a
href="http://www.ie.unc.edu/cempd/projects/mims/spatial/grids_ellipsoids_map_proj.html">http://www.ie.unc.edu/cempd/projects/mims/spatial/grids_ellipsoids_map_proj.html</a><o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>The way to interpret NinJo data: personal
trial-and-error.<o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>The way to interpret MISR UAE2 data: <a
href="http://eosweb.larc.nasa.gov/PRODOCS/misr/uae/imagery/regional/map_projection.html">http://eosweb.larc.nasa.gov/PRODOCS/misr/uae/imagery/regional/map_projection.html</a><o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span lang=EN-US style='font-size:
12.0pt;font-family:Arial'>Melita Kennedy’s post: <a
href="http://lists.osgeo.org/pipermail/metacrs/2008-August/000144.html">http://lists.osgeo.org/pipermail/metacrs/2008-August/000144.html</a><o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'>Proj.4 FAQ: </span></font><font face=Arial><span lang=EN-US
style='font-family:Arial'><a
href="http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator"><span
lang=SV>http://trac.osgeo.org/proj/wiki/FAQ#ChangingEllipsoidWhycantIconvertfromWGS84toGoogleEarthVirtualGlobeMercator</span></a></span></font><font
face=Arial><span style='font-family:Arial'><o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'> <o:p></o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoBodyText><font size=3 face=Arial><span style='font-size:12.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
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