Layers in a custom projection are not refreshed when the projection definition is changed

[fuzzysolutions]

Describe the bug
When I have a layer in a custom projection and I change the projection definition, e.g. edit the false easting parameter, there is no repaint triggered for the existing layer. I need to close and reload the project to get the layer displayed according to the new projection definition.

How to Reproduce

1. Create a custom projection, e.g. by taking thedefinition of a UTM zone and adding a custom false_easting and false_northing
2. Create a layer with this projection and add some features
3. Edit the false_easting and false_northing parameter for this custom projection to some other value
4. The features stay where they have been on the map before the redefinition until the project is closed and reloaded

QGIS and OS versions

QGIS-Version	3.16.1-Hannover	QGIS-Codeversion	b381a90
Kompiliert gegen Qt	5.11.2	Laufendes Qt	5.11.2
Kompiliert mit GDAL/OGR	3.1.4	Läuft mit GDAL/OGR	3.1.4
Kompiliert mit GEOS	3.8.1-CAPI-1.13.3	Läuft mit GEOS	3.8.1-CAPI-1.13.3
Kompiliert mit SQLite	3.29.0	Läuft mit SQLite	3.29.0
PostgreSQL-Client-Version	11.5	SpatiaLite-Version	4.3.0
QWT-Version	6.1.3	QScintilla2-Version	2.10.8
Kompiliert mit PROJ	6.3.2	Läuft mit PROJ	Rel. 6.3.2, May 1st, 2020
BS-Version	Windows 10 (10.0)
Aktive Python-Erweiterungen	db_manager; MetaSearch; processing

[gioman]

@fuzzysolutions can you attach a sample layer, thanks.

[roya0045]

I'm not sure this is a bug as people are not expected to change crs definition. This might be more of a feature request. I'd guess that one definition change you'd have to go through and the layer using this crs and replace it with the new definition. Otherwise the layers still use their internal 'outdated' definition.

[fuzzysolutions]

@gioman
Here is a test layer with some railway points in a UTM 32 coordinate system with a local offset. That is my use case, such local coordinate systems are common for engineering projects in order to keep the number coordinate digits low (that is not for data size reasons, but in order to have more human-reading-friendly coordinates, so I easily see whether to points are 1m or 100m away from each other).

Load that Layer, create a user defined CRS according to the definition below and redefine the layer on that CRS (the layer first carries its definition in its own). Then change the FALSE EASTING / FALSE NORTHING parameters in the user-defined CRS and see that nothing happens.

test layer user-defined CRS.zip

PROJCRS["ETRS89 / UTM zone 32N",
    BASEGEOGCRS["ETRS89",
        DATUM["European Terrestrial Reference System 1989",
            ELLIPSOID["GRS 1980",6378137,298.257222101,
                LENGTHUNIT["metre",1]]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["degree",0.0174532925199433]],
        ID["EPSG",4258]],
    CONVERSION["UTM zone 32N",
        METHOD["Transverse Mercator",
            ID["EPSG",9807]],
        PARAMETER["Latitude of natural origin",0,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8801]],
        PARAMETER["Longitude of natural origin",9,
            ANGLEUNIT["degree",0.0174532925199433],
            ID["EPSG",8802]],
        PARAMETER["Scale factor at natural origin",0.9996,
            SCALEUNIT["unity",1],
            ID["EPSG",8805]],
        PARAMETER["False easting",-5123501,
            LENGTHUNIT["metre",1],
            ID["EPSG",8806]],
        PARAMETER["False northing",-1630240,
            LENGTHUNIT["metre",1],
            ID["EPSG",8807]]],
    CS[Cartesian,2],
        AXIS["(E)",east,
            ORDER[1],
            LENGTHUNIT["metre",1]],
        AXIS["(N)",north,
            ORDER[2],
            LENGTHUNIT["metre",1]],
    USAGE[
        SCOPE["unknown"],
        AREA["Europe - 6?E to 12?E and ETRS89 by country"],
        BBOX[38.76,6,83.92,12]]]

[fuzzysolutions]

@roya0045: Yes, while creating the example layer, I better understood to use the "Test" widget below the custom CRS definition. So the importance is not high. Still, I think not updating the layer is a kind of unexpected behavior, so to me it is a bug.
But I'd leave that decision to the community. Priority can be set to low, anyway.

I think it's already valuable for others that the behavior is now in some way documented here.

[roya0045]

@fuzzysolutions 100% agreed

[drf5n]

My workaround after a CRS edit is to set the layer's CRS to something else, then back to the edited CRS. You do not have to close and reload the whole project in that case.

It might be nice if you could just click on the greyed-out current CRS under Layer CRS and have it refresh/reapply without the doubly re-assigning it.