[PROJ] Finnish KKJ with triangle-wise affine transformation

[Even Rouault]

Oskari,

> In many places we still need to support the old KKJ (EPSG:2393) system. The
> National Land Survey of Finland (Maanmittauslaitos) provides a TIN network
> and affine transformation for each triangle for converting between
> ETRS-TM35FIN and KKJ. This is the preferred and most accurate way to
> convert to and from KKJ. More information and the plain text data files can
> be found from NLS website though only in Finnish but you should be able to
> get the idea
> https://www.maanmittauslaitos.fi/kartat-ja-paikkatieto/asiantuntevalle-kayt
> tajalle/koordinaattimuunnokset
> 
> My question is, is this kind of transformation something that could be
> implemented inside PROJ at some point in the future? I don't know if this
> kind of thing is used anywhere else so it might be that this has very
> little use outside of Finland.

I'm currently working with National Land Survey of Finland to bring those transformations 
into PROJ. So stay tuned. Should come in coming weeks / months. Triangulation-based 
transformations are likely of interest in other cases too as a number of grid-based 
transformations come initially from triangulations.

> If this isn't viable, are there any plans about plugin support for PROJ? It
> would be cool to be able to implement custom steps and represent them as
> part of the normal PROJ pipeline to implement special needs :-)

I can see pros & cons to a plugin approach. From a community point of view, plugins tend to 
"isolate" people. Sometimes what one thinks to be a custom need can be useful for others.

Even

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