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Re: Problem with Albers and Spherical Earth

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  • Subject: Re: Problem with Albers and Spherical Earth
  • Date: Thu, 08 Oct 2009 16:45:17 -0600
Sorry, we only have the spherical case for Albers Equal Area. the closed-form elliptical looks pretty hairy. There are often good numerical solutions that are much faster, but i dont know of any java implementations. 

Roland Viger wrote:


Hi Rich,

Glad to see that something/anything comes up at this early point.
It would be nice to figure out the georeferencing issue. I suppose I could reproject all this stuff into something that is more readily handled (like geographic coordinates? not sure that ArcInfo will go to albers on a sphere...) and then convert again to netcdf. Probably good to think a few minutes before undertaking that. 

Roland


From:   Rich Signell <address@hidden>
To:     Roland Viger <address@hidden>
Cc:     John Caron <address@hidden>
Date:   10/08/2009 03:29 PM
Subject:        Problem with Albers and Spherical Earth
Sent by:        address@hidden


------------------------------------------------------------------------



Roland,

Attached is the NcML for your uniformly spaced Albers Equal Area
Delaware grid, and while NetCDF-Java figured out the geospatial
coordinates,  it didn't plot in exactly the right place (see
attached).

Do you think folks will mind?  ;-)

I figured that this was probably an ellipsoid issue, realizing that I
hadn't specified a datum or ellipsoid.

But then I discovered (remembered?) that it isn't possible to specify
anything -- the Unidata Common Data Model assumes a spherical earth
for Albers

http://www.unidata.ucar.edu/software/netcdf-java/reference/StandardCoordinateTransforms.html

and of course this data uses the GRS80 Spheroid.  ;-(

I tried adding an inverse_flattening, hopefully, but nothing happened,
so I guess the Albers Equal Area routine would have to be improved for
this to work correctly.

John, please correct me if I'm wrong.

-Rich
--
Dr. Richard P. Signell   (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
[attachment "2009-10-08_1650.png" deleted by Roland Viger/WRD/USGS/DOI] [attachment "de.ncml" deleted by Roland Viger/WRD/USGS/DOI]