diff --git a/src/c3nav/mapdata/cache.py b/src/c3nav/mapdata/cache.py
index 24eaa974..1cd0a059 100644
--- a/src/c3nav/mapdata/cache.py
+++ b/src/c3nav/mapdata/cache.py
@@ -256,13 +256,23 @@ class GeometryChangeTracker:
         self._geometries_by_level = {}
         self._deleted_levels = set()
 
-    def save(self, last_update, new_update):
+    def finalize(self):
         for level_id in self._deleted_levels:
             try:
                 os.remove(MapHistory.level_filename(level_id, mode='base'))
             except FileNotFoundError:
                 pass
             self._geometries_by_level.pop(level_id, None)
+        self._deleted_levels = set()
+
+    def combine(self, other):
+        self.finalize()
+        other.finalize()
+        for level_id, geometries in other._geometries_by_level.items():
+            self._geometries_by_level.setdefault(level_id, []).extend(geometries)
+
+    def save(self, last_update, new_update):
+        self.finalize()
 
         for level_id, geometries in self._geometries_by_level.items():
             geometries = unary_union(geometries)
diff --git a/src/c3nav/mapdata/models/geometry/base.py b/src/c3nav/mapdata/models/geometry/base.py
index d57519dc..55018cc3 100644
--- a/src/c3nav/mapdata/models/geometry/base.py
+++ b/src/c3nav/mapdata/models/geometry/base.py
@@ -5,9 +5,10 @@ from django.db import models
 from django.utils.functional import cached_property
 from django.utils.translation import ugettext_lazy as _
 from shapely.geometry import LineString, Point, mapping
+from shapely.ops import unary_union
 
 from c3nav.mapdata.models.base import SerializableMixin
-from c3nav.mapdata.utils.geometry import assert_multilinestring
+from c3nav.mapdata.utils.geometry import assert_multilinestring, assert_multipolygon
 from c3nav.mapdata.utils.json import format_geojson
 
 
@@ -117,7 +118,12 @@ class GeometryMixin(SerializableMixin):
         return False
 
     def get_changed_geometry(self):
-        return self.geometry if self.orig_geometry is None else self.geometry.symmetric_difference(self.orig_geometry)
+        if self.orig_geometry is None:
+            return self.geometry
+        difference = self.geometry.symmetric_difference(self.orig_geometry)
+        if self._meta.get_field('geomety').geomtype in ('polygon', 'multipolygon'):
+            difference = unary_union(assert_multipolygon(difference))
+        return difference
 
     def save(self, *args, **kwargs):
         self.recalculate_bounds()
diff --git a/src/c3nav/mapdata/models/update.py b/src/c3nav/mapdata/models/update.py
index 1c19eaa6..048fce7a 100644
--- a/src/c3nav/mapdata/models/update.py
+++ b/src/c3nav/mapdata/models/update.py
@@ -9,6 +9,7 @@ from django.utils.http import int_to_base36
 from django.utils.timezone import make_naive
 from django.utils.translation import ugettext_lazy as _
 
+from c3nav.mapdata.cache import changed_geometries
 from c3nav.mapdata.tasks import process_map_updates
 
 
@@ -89,17 +90,29 @@ class MapUpdate(models.Model):
             if not new_updates:
                 return ()
 
-            last_processed_update = cls.objects.filter(processed=True).latest().to_tuple
-            for new_update in new_updates:
-                with suppress(FileNotFoundError):
-                    changed_geometries = pickle.load(open(new_update._changed_geometries_filename(), 'rb'))
-                    changed_geometries.save(last_processed_update, new_update.to_tuple)
-                new_update.processed = True
-                new_update.save()
+            changed_geometries.reset()
 
             from c3nav.mapdata.models import AltitudeArea
             AltitudeArea.recalculate()
 
+            for new_update in new_updates:
+                print(new_update)
+                try:
+                    changed_geometries.combine(
+                        pickle.load(open(new_update._changed_geometries_filename(), 'rb'))
+                    )
+                except FileNotFoundError:
+                    print('Changed geometries file not found. File not found!')
+                with suppress(FileNotFoundError):
+
+                    print('done')
+                new_update.processed = True
+                new_update.save()
+
+            last_processed_update = cls.objects.filter(processed=True).latest().to_tuple
+
+            changed_geometries.save(last_processed_update, new_updates[-1].to_tuple)
+
             from c3nav.mapdata.render.data import LevelRenderData
             LevelRenderData.rebuild()