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