[PROJ] Meaning of (x) numbers in proj transformations in QGIS

[Nyall Dawson]

On Fri, 28 Jun 2019 at 19:03, Even Rouault <even.rouault at spatialys.com> wrote:
>
> Another thing is that currently the import process of the EPSG dataset to the
> PROJ dataset doesn't import remarks, so they are lost to the end user. I
> didn't want to do that for now to limit the size of the database, but that
> might be discussed.
>

I'd be a big +1 to having access to these remarks through proj. It's
really the only loss we had when porting QGIS to proj 6 and dropping
our own bodgy db. I'd love some way to expose more information to end
users to help inform them of the suitability of different pipelines,
and having access to the EPSG "scope" and "remarks" seems ideal for
this purpose.

E.g. I'd want to expose this from the EPSG registry when giving users
a choice between an operation involving EPSG::8447 and EPSG::8446:

EPSG:8447
Scope: Transformation of GDA94 coordinates when localised distortion
needs to be taken into account, e.g. if GDA94 coordinates were derived
survey control monuments.
Remarks: See GDA94 to GDA2020 (1) or (3) (codes 8048 and 8446) for
alternative conformal-only transformation without local distortion
modelling. GDA2020 Technical Manual and fact sheet T1 give guidance on
which to use.

EPSG:8446:
Scope: Conformal transformation of GDA94 coordinates that have been
derived through GNSS CORS.
Remarks: Gives identical results to Helmert transformation GDA94 to
GDA2020 (1) (code 8048). See GDA94 to GDA2020 (2) (code 8447) for
alternative with local distortion modelling included. GDA2020
Technical Manual and fact sheet T1 give guidance on which to use.

Both the fields are needed to fully inform users on the applicability
of either pipeline.

Nyall



> > But my main question: is "which proper information an average QGIS user
> > can use to determine what transformation to pick.." if presented with
> > one of those dialogs (as I had....)... and not being a projection guru :-)
>
> Ah ah, anyone knowning the answer would be multi-millionaire ;-)
>
> But to come back to your particular transformation, the thing is that which
> one of "Amersfoort to WGS 84 (3)" or "Amersfoort to WGS 84 (4)" should be
> picked up depends probably more on which one was used by other data producers
> to do similar transformations. Given the accuracy of definition of both
> Amersfoort and WGS 84, both are OK. This is mostly an issue of using
> consistently the same one to avoid alignment problems.
>
> Even
>
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