[QGIS-Developer] QGIS 3.8 Transformations dialog

[Nyall Dawson]

On Thu, 27 Jun 2019 at 17:40, Richard Duivenvoorde <rdmailings at duif.net> wrote:
>
> Hi,
>
> Starting up a fresh 3.8 and loading a EPSG:28992 (our national crs)
> layer I got a dialog questioning me which Transformation to use.

Looks like something/someone has changed a setting on your QGIS.
(Options - CRS - "Ask for datum transformation if several are
available"). The default is off.

> I wrote something down here:
>
> http://www.qgis.nl/2019/06/27/english-proj-select-datum-transformations-for-epsg28992/?lang=en
>
> Question to devs: what is the meaning of the 'green' item in this
> dialog?

It's just the first, non-deprecated one. I.e. - more or less random.
On proj < 6.1 builds treat any transformations with great suspicion.

> 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).

> Question 2: IF somebody makes a mistake, how do you get this dialog back
> to choose another one?

The setting is saved in the project properties (or in the options, CRS
tab if you clicked "make default").

> Cannot find something in docs or changelog about it (besides:
> http://changelog.qgis.org/en/qgis/version/3.8/#much-improved-coordinate-transform-handling)

On proj < 6.1 builds none of this is new. It's been cleaned up a bit,
and some bugs fixed, but it's been relatively unchanged since 3.0.

Nyall

>
> Hoping that this question and writing this down in a blog will help
> others too.
>
> Regards,
>
> Richard Duivenvoorde
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