Michael Bedward ha scritto:
> Hi Andrea,
>
>> http://jira.codehaus.org/browse/GEOT-2744
>>
>
> I don't see any difference between the two snippets not matter how
> hard I stare at them :)
SPHEROID["Clarke 1866", 6378206.4, 294.9786982138982,
AUTHORITY["EPSG","7008"]],
vs
DATUM["D_NAD_1927_CGQ77",
SPHEROID["Clarke_1866", 6378206.4, 294.9786982]],
that is: 294.9786982138982 vs
294.9786982
This is enough for the current referencing subsystem to state
the two projections are different (whilst by all intents and purposes
they are the same one, but using a different accuracy level for their
parameters).
>> Using an absolute difference in accuracy would not work,
>> other parameters have different scales.
>
> Could you explain this... why wouldn't a per-parameter absolute
> difference work with the example that you give ?
If we could have a per parameter accuracy it could be an absolute
difference, but then we'd end up with soo many absolute
differences.
Consider for example:
294.9786982138982
304800.0
Using an absolute difference of 10e-6 would make sense for the
first, but would not make much sense for the latter, which is
big. If we use a tolerance based on the size of the value we
get something that adapts to the order of magnitude of the
value itself. If we apply 10-6 as a percentage for the first value
we'd tolerate differences of 2.94 * 10e-4 and for the second
of 0.3 instead. Makes sense?
Cheers
Andrea
--
Andrea Aime
OpenGeo - http://opengeo.org
Expert service straight from the developers.
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