Merge pull request #4091 from alphagov/fix-combining-polygons-from-multiple-areas

Don’t combine polygons with different projections

app/models/broadcast_message.py

@@ -218,12 +218,33 @@ class BroadcastMessage(JSONModel):
         )
 
     def get_polygons_from_areas(self, area_attribute):
-        polygons = Polygons(
-            list(itertools.chain(*(
-                getattr(area, area_attribute) for area in self.areas
-            ))),
-            utm_crs=self.areas[0].simple_polygons.utm_polygons.utm_crs,
-        )
+        areas_polygons = [
+            getattr(area, area_attribute) for area in self.areas
+        ]
+        coordinate_reference_systems = {
+            polygons.utm_crs for polygons in areas_polygons
+        }
+
+        if len(coordinate_reference_systems) == 1:
+            # All our polygons are defined in the same coordinate
+            # reference system so we just have to flatten the list and
+            # say which coordinate reference system we are using
+            polygons = Polygons(
+                list(itertools.chain(*areas_polygons)),
+                utm_crs=next(iter(coordinate_reference_systems)),
+            )
+        else:
+            # Our polygons are in different coordinate reference systems
+            # We need to convert them back to degrees and make a new
+            # instance of `Polygons` which will determine a common
+            # coordinate reference system
+            polygons = Polygons(
+                list(itertools.chain(*(
+                    area_polygon.as_wgs84_coordinates
+                    for area_polygon in areas_polygons
+                )))
+            )
+
         if area_attribute != 'polygons' and len(self.areas) > 1:
             # We’re combining simplified polygons from multiple areas so we
             # need to re-simplify the combined polygons to keep the point

tests/app/models/test_broadcast_message.py

@@ -91,3 +91,43 @@ def test_areas_treats_missing_ids_as_custom_broadcast(notify_admin):
 
     assert len(list(broadcast_message.areas)) == 2
     assert type(broadcast_message.areas) == CustomBroadcastAreas
+
+
+@pytest.mark.parametrize('area_ids, approx_bounds', (
+    ([
+        'ctry19-N92000002',  # Northern Ireland (UTM zone 29N)
+        'ctry19-W92000004',  # Wales (UTM zone 30N)
+    ], [
+        -8.2, 51.5, -2.1, 55.1
+    ]),
+    ([
+        'lad20-E06000031',  # Peterborough (UTM zone 30N)
+        'lad20-E07000146',  # Kings Lyn and West Norfolk (UTM zone 31N)
+    ], [
+        -0.5, 52.5, 0.8, 53.0
+    ]),
+    ([
+        'wd20-E05009372',  # Hackney Central (UTM zone 30N)
+        'wd20-E05009374',  # Hackney Wick (UTM zone 30N)
+    ], [
+        -0.1, 51.5, -0.0, 51.6
+    ]),
+    ([
+        'wd20-E05009372',  # Hackney Central (UTM zone 30N)
+        'test-santa-claus-village-rovaniemi-a',  # (UTM zone 35N)
+    ], [
+        -0.1, 51.5, 25.9, 66.6
+    ]),
+))
+def test_combining_multiple_areas_keeps_same_bounds(area_ids, approx_bounds):
+    broadcast_message = BroadcastMessage(broadcast_message_json(
+        areas={'ids': area_ids}
+    ))
+
+    assert [
+        round(coordinate, 1) for coordinate in broadcast_message.polygons.bounds
+    ] == [
+        round(coordinate, 1) for coordinate in broadcast_message.simple_polygons.bounds
+    ] == (
+        approx_bounds
+    )