[QGIS-Developer] QGIS 3.8 Transformations dialog

[Nyall Dawson]

On Thu, 27 Jun 2019 at 19:04, Richard Duivenvoorde <rdmailings at duif.net> wrote:
>
> On 27/06/2019 10.04, Nyall Dawson wrote:
> >> Is it the old one? Or a default one?
> >> Because reading the texts there, it appears that the last item is to be
> >> used?
> > Well - (don't read this as snark, I'm just trying to accurately convey
> > the situation): if you want to make the correct choice, fire up proj
> > 6.1's projinfo tool and see what it suggest for transforming between
> > the two systems. On QGIS < 3.8 or proj <  6.1, this dialog is using a
> > custom database of "transformations", which was ad hoc, unreliable,
> > and basically untrustworthy.
> >
> > The good news is that the solution is already done -- you just need a
> > QGIS 3.8 build based on Proj 6.1. And then you can trust QGIS'
> > transformation capabilities as being the best there are available
> > anywhere (thanks entirely to the proj team).
>
> Ok, by accident I have a QGIS with proj6 here :-)
>
> That one shows 'just' 2 options (added to the qgis.nl post)
> IF the green line is just the first one, but we are not sure if it is
> the right one... should we make it green anyway?

Ah, on proj 6 builds, we ARE sure that the first/green one is the most
recommended one :) That's because proj gives us an ordered list of
possible operations in order of preference (calculated using all
proj's internal smarts and knowledge about the accuracy of different
transform operations).

>
> Other remark (again, see qgis.nl post): in the new transformation
> dialog, I do not see the information that I saw in the other (proj5)
> dialog, information like:
>
> EPSG Transformation Code: 1672
> Source CRS: Amersfoort
> Destination CRS: WGS 84
> Remarks: Parameter values from Amersfoort to ETRS89(1) (code 1751)
> assuming that ETRS89 is equivalent to WGS84 within the accuracy of the
> transformation.
> Replaces Amersfoort to WGS84 (1) (code 1112).
> Replaced by Amersfoort to WGS84 (3) (code 15934).
>
> Is that info not available (any more?),

Unfortunately not -- proj doesn't expose this information. I'm not
actually sure where it even came from in the first place. It was
present in the QGIS custom db, but with no provenance or source.
I would love to see this properly handled and exposed, but it's a
discussion you'll need to bring up with the proj team to resolve.

> Plz do not see this as critiques:

Not at all! This is all relatively new territory for QGIS, so your
feedback is invaluable.

Nyall



> I understand that actually QGIS and
> proj are getting better and better, and we just are getting more
> precise! Thanks all for this work!
>
> Regards,
>
> Richard Duivenvoorde
>
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