Check that building height is in a sensible range.

Sometimes, data that doesn't make sense gets into OSM. For example, version 4 of [this building](https://www.openstreetmap.org/way/290535940) has `height=5e+70`, which is many orders of magnitude larger than the observable universe. Clearly this is a typo and causes problems for us when we try to write it out in tiles.

This patch drops values outside a sensible range, which is approximately that of the tallest current structure, the Burj Khalifa.

test/test_transform.py

@@ -1039,3 +1039,22 @@ def test_hole(self):
 
     def test_filled_hole(self):
         self._check(set([1, 2, 3, 4]), 5)
+
+
+class BuildingHeightCalculation(unittest.TestCase):
+
+    def test_nonsense_height(self):
+        # test that a nonsensically large value for a height input
+        # doesn't get returned in the output.
+        from vectordatasource.transform import _building_calc_height
+        from vectordatasource.transform import _building_calc_levels
+        height = _building_calc_height('1e6', None, _building_calc_levels)
+        self.assertIsNone(height)
+
+    def test_nonsense_levels(self):
+        # test that a nonsensically large value for the number of levels in
+        # a building doesn't get into the output.
+        from vectordatasource.transform import _building_calc_height
+        from vectordatasource.transform import _building_calc_levels
+        height = _building_calc_height(None, '1000', _building_calc_levels)
+        self.assertIsNone(height)

vectordatasource/transform.py

@@ -192,15 +192,24 @@ def _building_calc_min_levels(min_levels):
     return min_levels
 
 
+# slightly bigger than the tallest structure in the world. at the time
+# of writing, the Burj Khalifa at 829.8m. this is used as a check to make
+# sure that nonsense values (e.g: buildings a million meters tall) don't
+# make it into the data.
+TALLEST_STRUCTURE_METERS = 1000.0
+
+
 def _building_calc_height(height_val, levels_val, levels_calc_fn):
     height = _to_float_meters(height_val)
-    if height is not None:
+    if height is not None and 0 <= height <= TALLEST_STRUCTURE_METERS:
         return height
     levels = _to_float_meters(levels_val)
     if levels is None:
         return None
     levels = levels_calc_fn(levels)
-    return levels
+    if 0 <= levels <= TALLEST_STRUCTURE_METERS:
+        return levels
+    return None
 
 
 def add_id_to_properties(shape, properties, fid, zoom):