Further tweaks and fixes for transformation selection dialog

[nyalldawson]

• Show area of use information, which gives users much more context and clues for selecting the right transform from the list
• Fix missing scope and remarks for single operations

Previously:

Now:

[nyalldawson]

@rduivenvoorde this one should also assist!

I think we're getting close now. It's definitely MUCH MUCH improved over the initial version. The next step I'd like to see would be to enable all entries, and highlight in red the ones which are unavailable due to missing grids (along with the explanatory text about downloading the missing grids).

@NathanW2 also pointed out that we could use the world map widget to show the area of use here, which would be a very nice touch if someone wants to take this on...

[andreasneumann]

These are all really nice improvements!

I guess one of the remaining questions for the not so skilled user is why this whole dialogue is popping up. Perhaps we should add one or two sentences why this datum transformation happens and why there is choice or different methods.

The other question: do we have to list so many alternatives?

[andreasneumann]

What is the difference between the light gray entries and the regular black ones?

[nyalldawson]

@andreasneumann
light grey ones are known operations, but unavailable on the system (e.g. due to missing grid files). With this pr you can click them and see the details (including a download link for some grid files), but you can't accept the dialog with an unavailable transform selected.

The other question: do we have to list so many alternatives?

Some of these are extreme situations. You'll usually only see a couple.

As to whether we could cull the list, it's 100% dependant on the data from the epsg registry. You'd need to get transforms deprecated and removed from EPSG's database in order to remove them from this list.... (I'm not sure that's a path you'd want to go down! 😉 )

[rduivenvoorde]

At #30501 I propose to write a more thorough introduction about projection/crs's/transformations/ etc etc here:

https://docs.qgis.org/testing/en/docs/user_manual/working_with_projections/working_with_projections.html

We can link to there (we already do) via the Help button. We could even add an extra sentence in the dialog, to read the Documenation (via Help button)

Then we 'only' need to write a good introduction in the docs hopefully by someone who understands this. Proj dev's pointed to EPSG documentation for reference (will look for the url later)

[andreasneumann]

@nyalldawson - thank you for the clarifications. Makes sense.

https://epsg.io/ is also a useful website that could be referenced in the documentation.

[nyalldawson]

I'll address a helper introductory text in a follow up pr

[rouault]

Now that you display the remarks, which sometimes refer to other transformations by their code (like your examples with GDA94->GDA2020), shouldn't the EPSG code (when available) of transformations be exposed too ?

[nyalldawson]

@rouault yes, good point. I'm guessing the same approach will be required for this (ie checking for the code for the operation, if it doesn't exist then checking step by step)?

[nyalldawson]

@andreasneumann

https://epsg.io/ is also a useful website that could be referenced in the documentation.

The problem with that site is that it only includes CRS information -- it doesn't have information on transform operations and the like.

And the official source (https://www.epsg-registry.org/) is... uh... a little underwhelming.

[nyalldawson]

Actually - I was wrong (had a typo!) - epsg.io DOES have operations.