Description

This PR fixes a bug in the geo-coordinate conversion system where latitude values changed incorrectly (inverted sign) when moving north/south in maps using Transverse Mercator (tmerc) or Universal Transverse Mercator (UTM) projections without an explicit <offset> tag in the OpenDRIVE file.

Root Cause:
Unreal Engine uses a left-handed coordinate system while geographic projections (Transverse Mercator, UTM) assume a right-handed system. The projection inverse transforms did not account for this handedness mismatch, causing the northing (Y) component to have an inverted sign relative to latitude changes.

Changes Made:

1. GeoProjection.cpp:
   • Modified TransformToGeoLocationTransverseMercator() to negate the Y-coordinate: double y = -((location.y - p.y_0) / p.k);
   • Modified TransformToGeoLocationUniversalTransverseMercator() to negate the Y-coordinate: double y = -((location.y - y_0) / k);
   • Added comments documenting the fix and the reasoning (Unreal left-handed coordinate system correction, applied 2026-02-13)

Behavior Change:

• Before: Moving an actor +Y in CARLA local coordinates produced decreasing latitude (incorrect)
• After: Moving an actor +Y in CARLA local coordinates produces increasing latitude (correct)
• Map origin (local 0,0) continues to map correctly to the expected geolocation

Fixes #9530 #9389

Where has this been tested?

• Platform(s): Ubuntu 22.04
• Python version(s): 3.10
• Unreal Engine version(s): 4.26 (CARLA ue4-dev branch)
• Test method: GNSS sensor output validation; confirmed latitude increases when actor moves north

Possible Drawbacks

• Maps that explicitly use an <offset> transform in OpenDRIVE may need validation to ensure the combined effect (offset negation + projection negation) is correct. However, the offset's OffsetTransform already applies a Y negation, so this change complements existing behavior.
• Any downstream code that relied on the incorrect (inverted) latitude behavior will need adjustment, but the fix aligns the system with correct geographic coordinate semantics.

Files Changed

1. LibCarla/source/carla/geom/GeoProjection.cpp

This change is