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<p>Hello all</p>
<p>Below is just adding a little bit more historical context to
what Thomas said.</p>
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<p>Le 02/06/2022 à 12:39, Thomas Knudsen a écrit :</p>
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<p>The conceptual problem here is the dubious historical
ISO19100/OGC notion that a CRS has some kind of internal
state, which makes it possible to derive its relation to
other CRS.</p>
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<p align="justify">This notion came from OGC 01-009, which was prior
to ISO 19111. The (now superseded) OGC 01-009 standard had a
notion of CRS state in the form of "<font face="monospace">TOWGS84</font>"
information, and this design has been used by PROJ4 as well. But
ISO 19111 never had this notion as far as I know. The "bound CRS"
notion introduced in WKT 2 is somewhat similar, but the
specification said that this is a compromise for existing
practices, not something that they recommend. The EPSG
documentation discusses the problem of "statefull CRS" in their
discussion about "early binding" (stateful CRS) versus "late
binding" (stateless CRS) implementations of map projection
libraries. PROJ 4 was an "early binding" implementation, PROJ 6
and later are now "late binding" implementations (but it seems to
me that habits inherited from PROJ 4 are still well entrenched).<br>
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<p>The meridian convergence (and any other relevant
characteristic) is a property of the transformation (i.e.
the mathematical prescription), not of the CRS per se,
because *there is no such thing as a CRS per se*: It is just
a label, and you cannot perform any kind of mathematical
analysis on a label.</p>
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<p align="justify">OGC 01-009 specified a way to not only transform
coordinates, but also to obtain the Jacobian matrix at a given
point for a given transformation. That was (I think) a very good
feature from OGC 01-009 which has not been kept by ISO 19111.
Maybe because considered too complex, I do not know.<br>
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<p>The practically available implementations of the ISO/OGC
standards for "referencing by coordinates" has an
unfortunate focus on systems, rather than transformations.</p>
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<p>It may be a matter of popularity. The Java world has some
implementations of ISO/OGC standards done in the "right" way
(late-binding implementation + support of Jacobian matrices) for
10~20 years. But they are not well-known like PROJ, which may give
the impression that they do not exist.<br>
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<p> Martin</p>
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