"Ballpark Transform" warning when viewport (but not data!) is outside of Transformation area

[floledermann]

What is the bug or the crash?

I have a layer that I want to be reprojected on the fly to my project CRS and whose features are wholly within the boundaries of the "area of use" of the selected coordinate transformation.

The reprojection works as expected as long as I keep the map view zoomed in on the reprojected layer's extent. However, when I zoom out the map viewport, I get a "Used a ballpark transform" warning in the orange bar on the top of the viewport. This happens even though there is no geometry in any layer outside the "area of use" of the on-the-fly transformation!

IMO this is a bug and can be very confusing to newcomers (I am using QGIS in teaching). If no geometry is reprojected outside of the area of use the ballpark transform should not kick in (which it doesn't) and the warning should not appear even when the map view is zoomed to outside the area of use.

Screenshot 1 - no warning upon opening the project:

Screenshot 2 - ballpark transform warning when zooming out:

Zip File with test project & data:
testcase_qgis.zip

Steps to reproduce the issue

Use the attached test project to reproduce.

Upon opening the project -> no warning
Upon zooming out -> ballpark transform warning

Versions

QGIS version
3.20.3-Odense
QGIS code revision
495fbae
Qt version
5.15.2
Python version
3.9.5
GDAL/OGR version
3.3.1
PROJ version
8.1.1
EPSG Registry database version
v10.028 (2021-07-07)
GEOS version
3.9.1-CAPI-1.14.2
SQLite version
3.35.2
PDAL version
2.3.0
PostgreSQL client version
13.0
SpatiaLite version
5.0.1
QWT version
6.1.3
QScintilla2 version
2.11.5
OS version
Windows 10 Version 1607

Active Python plugins
contour
openlayers_plugin
QuickOSM
db_manager
MetaSearch
processing

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

Additional context

No response

[gioman]

even though there is no geometry

@floledermann I guess this happens because to trigger the warning are used the bounding boxes of the CRS of the layers and not the extent of the features.

[floledermann]

@gioman This is what I inferred from the behaviour, too. Although I want to point out that in the test case none of the layers has geometry outside the "area of use" of the CRS transformation. It seems to be entirely triggered by the bounds of the map viewport.

As a short-term measure, the warning could be rephrased along the lines "Map viewport extends beyond the area for which the transformation from EPSG:X to EPSG:Y is valid. A ballpark transform would be used outside of this area". Such warning would need to be hidden again when zooming in, which is unfortunately not currently happening. The warning stays visible until dismissed by the user.

The warning as it stands "Used a ballpark transform ..." is definitely wrong, as the ballpark transform is never applied to even a single coordinate pair in the described scenario.

[github-actions]

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[floledermann]

If you would like to continue with this issue, please provide any missing information or answer any open questions.

I am not sure what further information I should provide on the issue. I have provided a description, screenshot and test project above, and tried to clarify remaining questions in my answer to @gioman above.

The issue is very much still present. Of course it is not a critical bug, but still the warning is misleading and confusing.