[gdal-dev] Re: EASE-grid projection

[Frank Warmerdam]

Bruce Raup wrote:
> All,
> 
> We at NSIDC had the EASE-Grid projections added to the EPSG code list
> last year or so, and I recently noticed that in the "share/proj/epsg"
> file in FWTools, for the entry for 3410, it says:
> 
> # NSIDC EASE-Grid Global
> # Unable to translate coordinate system EPSG:3410 into PROJ.4 format.
> 
> I don't understand why it says this, especially since the Proj.4
> parameters are defined at

Bruce,

It says this because EPSG projection method 9834 (Lambert Cylindrical
Equal Area) has no translation support in gdal/ogr/ogr_fromepsg.cpp.

If you file a ticket on this issue I can add it.

> http://nsidc.org/data/atlas/epsg_3410.html
> 
> as pointed out by Jose.  Can anyone say why the epsg file says that?
> 
> Proj.4 doesn't deal with datums, so it can handle EASE-Grid just fine,
> but it should be noted that EASE-Grid maps lon/lat, usually defined on
> the WGS-84 datum, to a projection that uses a non-standard spheroid.
> In other words, the datum ellipsoid and the projection ellipsoid are
> not the same.  This is common for satellite-era data.  Unfortunately,
> lots of software and data formats do not properly make this
> distinction, or allow for this distinction, and so one can run into
> problems.  For example, display software can display the wrong lon/lat
> under the cursor by erroneously doing a datum shift after
> unprojection.

This is most annoying since there seems to be no standard way of representing
such a coordinate system in WKT (for instance).

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent