On Sun, 30 Mar 2008, Nikos Alexandris wrote:
[...]
( http://wald.intevation.org/tracker/?func=detail&aid=603&group_id=21&atid=204 )
I am struggling to understand why g.proj picks the wrong ellipsoid when
using epsg:2100 to create a new location in GRASS.
Hi Nikos,
Thanks for reporting this. It looks to be a bug as a result of my change
to lib/proj/convert.c on 2006-11-05 to add the ellipsoid name into the
PROJ_INFO files generated from WKT sources. Previously the ellipsoid name
was missing when a location was generated with g.proj.
The code tries to match the numerical ellipsoid parameters against those
in GRASS's ellipse.table. As you may know, GRS80 and WGS84 are almost
identical, close enough for them to make no difference to datum transform
calculations, for example. Because they are almost the same it is
obviously very difficult to tell them apart. I thought I'd tested it
reasonably well, but as you've found it fails in some circumstances. It
shouldn't affect the accuracy of your calculations though, but is
obviously less than ideal.
When I originally wrote the ellipsoid comparing code I made a compromise
between handling the case of a sphere correctly and keeping the best
precision. I've changed it now so it handles the sphere as a special case
and it now seems also to work for your example. I'd appreciate some more
testing though.
This bug will have been present in 6.2.3 but not earlier versions.
Best regards,
Paul
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