[Qgis-user] QGIS 3.22 breaks CRS Transform Files?

[John Layt]

Hi,

We're preparing to upgrade the company from 3.16 LTR to 3.22 LTR and in our
testing we're hitting issues apparently due to the Mac packages changing
from PROJ 6 to PROJ 8 which seems to have changed the transform file names
and formats. I've not seen any warnings or documentation on these breaking
changes and how to deal with them so was hoping the list could offer some
enlightenment.

Most of our work is in OSGB36, but with an occasional sprinkling of WGS84,
or at least projects created in WGS84 by default with OSGB36 data then
added. Where required we use the OSTN15 transform. Some of these project
files stretch back to QGIS2 days so may be quite crufty.

The preferred OSTN15 transform shipped with 3.16 and earlier versions was
OSTN15_NTv2_OSGBtoETRS.gsb. In 3.22 this appears to have changed name and
format to uk_os_OSTN15_NTv2_OSGBtoETRS.tif.

We're seeing 2 issues in 3.22 we don't have in 3.16.

1) When opening projects with the OSTN15 transform in them, we get a
warning dialog that the grid file OSTN15_NTv2_OSGBtoETRS.gsb can't be found
and must be manually installed. The dialog tries to be helpful by including
a download link, but it's to the uk_os_OSTN15_NTv2_OSGBtoETRS.tif file
instead, so you need to manually source the gsb file. The ideal solution
would be for QGIS to automatically convert to using the tif as it seems to
already know they are equivalent, or to ask the user for confirmation to
do so.

2) If you create a new Project and it defaults to WGS84, and then you
change the CRS to OSGB36, you get the "Ballpark Transform" warning even
though there is no data to be transformed, and may never be. Also, the
warning gets displayed as an inline alert on the canvas as soon as you
select the new CRS, so if you don't Apply and close the Properties you
still see the warning. This may be a regression of
https://github.com/qgis/QGIS/issues/34983, but the message probably
shouldn't be displayed in these circumstances.

In general, the "Ballpark transform" message leaves users a little
stranded. Our less-experienced users usually end up with it as they
panicked and hit Cancel on the Transform select dialog when opening an
existing project. Perhaps the warning could actually link back to the
Transform Select dialog again? Or at least list the affected layers?

As it stands, it appears I will need to somehow install the gsb file in
every user profile and set the new tif as the default for all new projects.
Any words of advice on how best to achieve this, or other solutions?

We do need to better educate our users on CRS and transforms (and using
better default settings!), but it's hard when many are only occasional
users. Heck, I struggle myself! Does anyone know of some clear and simple
explanations of CRS transforms we can use?

Cheers!

John.
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