Altitude mismatch and loss of precision when using External (Mock) position provider on iOS

Start with the iOS External (Mock) position provider and the handoff of altitude into the MM GPS info panel. Compare that path with the direct Trimble connection, then reproduce using the listed DA2 and iOS versions. Done means the displayed altitude matches TMM's orthometric height and preserves at least centimetre-level precision.

Description

When connecting a Trimble GNSS receiver via the External (Mock) position provider on iOS, the altitude displayed in the MM GPS info panel differs from the value shown in Trimble Mobile Manager by approximately 0.4–0.5 m. Additionally, altitude values in the MM app appear quantised to 0.1 m increments (e.g. 129.00, 129.10, 129.30), indicating sub-decimetre precision is lost in transit through the iOS mock location layer.

The issue does not occur when using a direct Trimble connection — in that case altitudes match between TMM and MM.

Environment
[x] Staging [ ] PROD

Application (+ app version, build, operating system)
Mergin Maps iOS app 2026.4.0 / iOS 26.0
Trimble DA2, Serial: 6238100128, Firmware: 1.75 (Up to date)

Steps to reproduce

1. Connect a Trimble DA2 GNSS receiver in the MM app using the External (Mock) provider on iOS
2. Open the GPS info panel in MM — note the Altitude value
3. Open Trimble Mobile Manager → Status → note the Orthometric height (MSL) value
4. Compare the two values

Actual results

• MM GPS info panel shows 129.00 m (External Mock)
• TMM shows Orthometric height (MSL): 128.59 m — a difference of ~0.41 m
• MM altitude values are rounded to 0.1 m steps; sub-decimetre precision from the receiver is lost

Expected results

Altitude in the MM GPS info panel should match the MSL value reported by TMM within the receiver's stated vertical precision (0.02 m RMS). Precision should be preserved to at least centimetre level, consistent with behaviour when using the direct Trimble connection.

Screenshots or logs
(attached — MM GPS info panel + TMM Status screen, both captured at 11:30 CEST)

Conclusion
Created by Claude and supervised by a human test analyst.