Custom PROJ datum transformation (TIF) not applied when using external GNSS receiver via Bluetooth (regression in 2026.1.0)

[MTrim]

Since version 2026.1.0, a custom horizontal datum transformation grid file (at_bev_AT_GIS_GRID_2021_09_28.tif) is no longer applied when an external GNSS receiver is connected via Bluetooth. The GPS position is recorded with a significant horizontal offset, as if no datum transformation is performed.

This is a regression: the same project and setup worked correctly in version 2025.8.0.

Environment
∙ Mergin Maps mobile version: 2026.1.1 (regression introduced in 2026.1.0)
∙ Last working version: 2025.8.0
∙ Device / OS: Android
∙ External GNSS: Leica FLX100 plus and Leica GS05 connected directly via Bluetooth (also testet via mock location with the same results)
∙ Grid file: at_bev_AT_GIS_GRID_2021_09_28.tif (BEV Austria datum shift grid), placed in proj/ subfolder of the project.

It seems that the GNSS Enhancement Bundle introduced in #4234 (2026.1.0) somehow refactored the position processing for external providers. The horizontal datum transformation appears to be bypassed in the new code path we also tried mock lockation with the same results.

Note: this is a horizontal position issue, height info is not testet yet.

[Withalion]

Hello @MTrim, thanks for reporting this issue. We will try to find the cause and fix it ASAP.

[MTrim]

Thank you for the quick response.

This is quite urgent for us… many of our useres use external RTK GNSS receivers for precise surveying work and are directly affected. A horizontal offset of 30–50 cm is not acceptable in their workflows.

We have advised them to downgrade to 2025.8.0 for now. Looking forward to a fix.