# [Proj] Trouble georeferencing data stored in a spherical projection.

Noel Zinn (cc) ndzinn at comcast.net
Tue Mar 6 09:42:50 EST 2012

Mikael,

Here's another way of looking at the problem.  Our Web Mercator discussion a few years ago turned out to be about a (new, though some have commented that it was around before Google) non-conformal map projection (Web Mercator) and NOT about the differences between an ellipsoidal and a spherical earth.  Your situation may be the same.

You say your data are stored in stereographic, but stereographic using spherical or ellipsoidal equations?  Do you know?  Probably spherical (and I'd guess there's no difference between Snyder and Double Stereographic for the sphere) because if ellipsoidal then you'd have to be using an eccentricity and your reference surface would be an ellipsoid (probably WGS84) and not the NinJo sphere.

If so, then we have just defined the Web Stereographic map projection, spherical stereographic equations applied to ellipsoidal WGS84 coordinates, probably non-conformal.  You can work out the distortions (scale and convergence) and you're done.

Noel

Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

From: Mikael Rittri
Sent: Tuesday, March 06, 2012 7:38 AM
To: proj at lists.maptools.org
Subject: [Proj] Trouble georeferencing data stored in a spherical projection.

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

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

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

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