Normalize area geometries to two dimensions

backend/app/utils/geometry.py

@@ -3,13 +3,14 @@ from __future__ import annotations
 from typing import Any
 
 from pyproj import Transformer
+from shapely import force_2d
 from shapely.geometry import GeometryCollection, MultiPolygon, Polygon, box, shape
 from shapely.ops import transform
 from shapely.validation import make_valid
 
 
 def normalize_to_multipolygon(raw_geometry: dict[str, Any]) -> MultiPolygon:
-    geom = shape(raw_geometry)
+    geom = force_2d(shape(raw_geometry))
     if geom.is_empty:
         raise ValueError("Geometry is empty")
 

backend/tests/test_rc4_national_scope_operator.py

@@ -15,6 +15,7 @@ if str(SCRIPTS) not in sys.path:
     sys.path.insert(0, str(SCRIPTS))
 
 import provision_belgium_north_sea_scope as operator  # noqa: E402
+from app.utils.geometry import normalize_to_multipolygon  # noqa: E402
 
 
 def marine_feature(identifier: str, geometry):
@@ -77,6 +78,26 @@ def test_marine_scope_derivation_fails_when_a_required_unit_is_missing() -> None
         )
 
 
+def test_area_geometry_normalization_drops_source_z_dimension() -> None:
+    geometry = normalize_to_multipolygon(
+        {
+            "type": "Polygon",
+            "coordinates": [
+                [
+                    [2.5, 49.5, 0],
+                    [6.4, 49.5, 0],
+                    [6.4, 51.5, 0],
+                    [2.5, 51.5, 0],
+                    [2.5, 49.5, 0],
+                ]
+            ],
+        }
+    )
+
+    assert geometry.has_z is False
+    assert geometry.geom_type == "MultiPolygon"
+
+
 def test_archive_extraction_accepts_one_safe_geopackage_and_rejects_traversal(tmp_path: Path) -> None:
     archive = tmp_path / "adminvector.zip"
     with zipfile.ZipFile(archive, "w") as handle: