Hi Martin:
On 2/3/2011 11:13 AM, Martin Desruisseaux wrote:
Hello John
Le 03/02/11 18:30, John Caron a écrit :
i guess whats missing is a method to convert lat/lon on one ellipsoid
to lat/lon on another?
Close. A method to transform lat/long on one *datum* to lat/lon on
another.
Many different datum may exist for the same ellipsoid. The ellipsoid
axis length alone are not suffisient.
ok, so if i understand, a datum might just use the ellipsoid as the
datum, but others may specify a different surface, but possibly using
some ellipsoid as a reference. Is that accurate?
if so, does WSG84 use the ellipsoid as the datum for calculating
lat/lon? Or does it also need other parameters ? If other, how big might
the deviation be?
(minor note: in ISO 19111 terminology, this is a "transformation", not
a "conversion". A Map projection is a "conversion" but a datum shift
is a "transformation". The difference is that conversions are
determined by mathematical formulas with infinite (in theory)
precision while transformations are determined by empirical parameters
with statistical errors).
However there is no generic method for doing the above transformation.
Datum shift require empirical parameters (typically the "Bursa-Wolf
parameters", but other methods are possible) which need to be stored
in some kind of database (a real database like the http://www.epsg.org
one, or a text file, etc.).
On a related note, the GeoAPI 3.0 interfaces are now going through the
final voting process (after much delay for reasons that we don't
control) in the Open Geospatial Consortium. ESRI, CNES (a French space
agency) and other are among the submitters. I would suggest to set the
NetCDF referencing classes as implementation of GeoAPI interfaces (I
can provide help for that if you wish), so NetCDF users can start with
a relatively "lightweight" referencing implementation, while other
implementations are possible. For example the Geotoolkit.org
referencing module alone (not including dependencies like ISO 19115
metadata) is a 900 kb JAR file, which suggest that referencing stuff
may complex enough to allow the possibility to use exernal libraries.
that sounds promising, ill put that on my to-do list to investigate.
regards,
John
Regards,
Martin
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