[gdal-dev] Re: EASE-grid projection

[Bruce Raup]

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

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.

Best regards,
Bruce

On Fri, Oct 10, 2008 at 12:09 PM, Dan Stahlke <dan at gina.alaska.edu> wrote:
> Jose,
>
> The north pole version works for me, perhaps because of the '+init=' part:
> (although that probably shouldn't matter...)
>   gdalwarp -t_srs '+init=epsg:3408' ...
> Maybe the equatorial version of EASE is different.
>
> If you are giving the full proj4 definition "+proj=cea ..." you might try
> adding "+no_defs" at the end.  Otherwise, you usually end up getting some
> sort of default flatting value or something that tends to interfere whenever
> an explicit radius is specified.  Probably the "+ellps=sphere" ends up
> overriding the faulty default value.
>
> - Dan
>
> Jose Gomez-Dans wrote:
>>
>> Hi,
>> I know it's unpolite to reply to self, but I think I might have cracked
>> this one. Comments welcomed...
>>
>>
>> 2008/10/10 Jose Gomez-Dans <jgomezdans at gmail.com
>> <mailto:jgomezdans at gmail.com>>
>>
>>
>>    $ gdalwarp --debug on -of GTiff -s_srs "+proj=cea +lat_0=0
>>    +lon_0=0 +lat_ts=30 +a=6371228.0 +units=m" -t_srs 'EPSG:4326'
>>    INPUT OUTPUT.tif
>>    OGR_PROJ4: Can't find ellipse definition in:
>>    +proj=cea +lat_0=0 +lon_0=0 +lat_ts=30 +a=6371228.0 +units=m
>>    ERROR 1: Translating source or target SRS failed:
>>    +proj=cea +lat_0=0 +lon_0=0 +lat_ts=30 +a=6371228.0 +units=m
>>
>>
>> If I add the +ellps=sphere (hint is here:
>>  <http://nsidc.org/data/ease/ease_grid.html#whySpherical>. What's wrong with
>> having a flat Earth!!! :D), I am able to reproject the data.
>>
>> Is this a sound approach?
>>
>> Thanks,
>> Jose
>>
>>
>> --
>> Centre for Terrestrial Carbon Dynamics
>> Department of Geography, University College London
>> Gower Street, London WC1E 6BT, UK
>> ------------------------------------------------------------------------
>>
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-- 
Bruce Raup
http://cires.colorado.edu/~braup/