[PROJ] Calculating meridian convergence

[Martin Desruisseaux]

Hello all

Below is just adding a little bit more historical context to what Thomas 
said.


Le 02/06/2022 à 12:39, Thomas Knudsen a écrit :

> 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.
>
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superseded) OGC 01-009 standard had a notion of CRS state in the form of 
"TOWGS84" 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).


> 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.
>
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.


> The practically available implementations of the ISO/OGC standards for 
> "referencing by coordinates" has an unfortunate focus on systems, 
> rather than transformations.
>
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.

     Martin

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