Modification of Z and M coordinates when splitting a feature

[jmonticolo]

What is the bug or the crash?

When a feature of a vector layer with Z and/or M coordinates is splitted (split feature or split part), new Z coordinates are extrapolated and all M coordinates are set to 0 (or nan depending of the data provider).

• Why the Z and M coordinates have a different behavior?
• Can we choose the behavior for each type of coordinate ?
• Can have we the conservation of M values ?
• It may be not a bug and be a thoughtful decision, then is there any discussions (QGIS Dev user list, a ticket or PR) about that ?

Steps to reproduce the issue

1. Create a MULTIPOLYGON with Z and/or M vector layer in a projected coordinate system
   I created a MULTIPOLYGON ZM (200m x 100m rectangle) in EPSG:2154 - RGF93 v1 / Lambert-93:

wkt_geom	id
MultiPolygon ZM (((1000000 6800000 0 0, 1000000 6800100 1 1, 1000200 6800100 2 2, 1000200 6800000 0 0, 1000000 6800000 0 0)))	1

2. Edit this rectangle with the Split Features tool and cut it in the middle and create some new vertices inside.
3. Save and look the coordinates table of the 2 new features, you'll see:
   • Z coordinates are extrapolated
   • M coordinates are set to 0 (or nan) : for the new vertices, I can be ok with it but for the vertices like the second one (see illustrations, the second is the top left corner) passing from 1 to 0, I think it's not the attended result.

• Illustrations

  • Before

  
  In the bottom left corner, the labels are duplicated because it's the starting / ending point.

  • After

  

Versions

Version de QGIS	3.40.1-Bratislava
Révision du code	989853a7
Libraries
Version de Qt	5.15.13
Version de Python	3.12.7
Version de GDAL/OGR	3.9.3
Version de Proj	9.5.0
Version de la base de données du registre EPSG	v11.016 (2024-08-31)
Version de GEOS	3.13.0-CAPI-1.19.0
Version de SQLite	3.46.1
Version de PDAL	2.8.1
Version du client PostgreSQL	16.2
Version de SpatiaLite	5.1.0
Version de QWT	6.3.0
Version de QScintilla2	2.14.1
Version de l'OS	Windows 10 Version 2009

Supported QGIS version

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

New profile

• I tried with a new QGIS profile

Additional context

No response

[uclaros]

Is this a duplicate of #40796 ?

[jmonticolo]

Yes, for the M values.

[uclaros]

closing this in favor of #40796