Improve data

pipeline/transform/postcode_boundaries/test_postcode_boundaries.py

@@ -906,6 +906,37 @@ class TestToWgs84Geojson:
         assert result is not None
         assert result["type"] == "Polygon"
 
+    def test_pointlike_input_gets_building_scale_footprint(self):
+        """A tower-block postcode (all UPRNs at one point) must not collapse to a
+        sub-metre dot; it gets a building-scale footprint instead."""
+        import pyproj
+        from shapely.geometry import Point, shape
+        from shapely.ops import transform as transform_geometry
+
+        to_bng = pyproj.Transformer.from_crs(
+            "EPSG:4326", "EPSG:27700", always_xy=True
+        )
+        result = to_wgs84_geojson(Point(360000, 170000))
+        assert result is not None
+        area_m2 = transform_geometry(to_bng.transform, shape(result)).area
+        assert area_m2 > 100, f"point footprint only {area_m2:.1f} m^2"
+
+    def test_thin_sliver_keeps_minimal_buffer(self):
+        """A genuine elongated sliver still carries length, so it is NOT inflated
+        to building scale — only truly pointlike inputs are."""
+        import pyproj
+        from shapely.geometry import LineString, shape
+        from shapely.ops import transform as transform_geometry
+
+        to_bng = pyproj.Transformer.from_crs(
+            "EPSG:4326", "EPSG:27700", always_xy=True
+        )
+        sliver = LineString([(360000, 170000), (360040, 170000)]).buffer(0.05)
+        result = to_wgs84_geojson(sliver)
+        assert result is not None
+        area_m2 = transform_geometry(to_bng.transform, shape(result)).area
+        assert area_m2 < 100, f"sliver inflated to {area_m2:.1f} m^2"
+
     def test_coordinates_have_limited_precision(self):
         """GeoJSON coordinates should be rounded to 6 decimal places."""
         import json