[UK] Use database for co-ordinate transformation.

If the application is running on a server with GDAL 1.9 (and maybe 1.8),
its OSGB co-ordinate transformation can be up to a few metres out. As we
use the transformation by the database when saving new coordinates, also
use it when fetching so we can match and display accurate information.

https://trac.osgeo.org/gdal/ticket/4597 is the GDAL ticket that explains
that it is fixed in GDAL 1.10.

mapit/management/commands/mapit_import_postal_codes.py

@@ -131,17 +131,12 @@ def do_postcode(self, location=None, srid=None):
             pc = Postcode.objects.get(postcode=self.code)
             if location:
                 if pc.location:
-                    curr_location = (pc.location[0], pc.location[1])
+                    curr_location = pc.location
+                    # Postcode locations are stored as WGS84
                     if settings.MAPIT_COUNTRY == 'GB':
-                        if pc.postcode[0:2] == 'BT':
-                            curr_location = pc.as_irish_grid()
-                        else:
-                            pc.location.transform(27700)  # Postcode locations are stored as WGS84
-                            curr_location = (pc.location[0], pc.location[1])
-                        curr_location = tuple(map(round, curr_location))
+                        curr_location = pc.as_uk_grid()
                     elif srid != 4326:
-                        pc.location.transform(srid)  # Postcode locations are stored as WGS84
-                        curr_location = (pc.location[0], pc.location[1])
+                        curr_location = pc.location.transform(srid, clone=True)
                     if curr_location[0] != location[0] or curr_location[1] != location[1]:
                         pc.location = location
                         pc.save()

mapit/models.py

@@ -413,6 +413,10 @@ def filter_by_area(self, area):
         )
 
 
+def str2int(s):
+    return int(round(float(s)))
+
+
 @python_2_unicode_compatible
 class Postcode(models.Model):
     postcode = models.CharField(max_length=7, db_index=True, unique=True)
@@ -449,12 +453,13 @@ def as_dict(self):
             countries.augment_postcode(self, result)
         return result
 
-    # Doing this via self.location.transform(29902) gives incorrect results.
-    # The database has the right proj4 text, the proj file does not. I think.
-    def as_irish_grid(self):
+    # Doing this via self.location.transform(27700/29902) can give incorrect results
+    # with some versions of GDAL. Via the database produces a correct result.
+    def as_uk_grid(self):
         cursor = connection.cursor()
-        cursor.execute("SELECT ST_AsText(ST_Transform(ST_GeomFromText('POINT(%f %f)', 4326), 29902))" % (
-            self.location[0], self.location[1]))
+        srid = 29902 if self.postcode[0:2] == 'BT' else 27700
+        cursor.execute("SELECT ST_AsText(ST_Transform(ST_GeomFromText('POINT(%f %f)', 4326), %d))" % (
+            self.location[0], self.location[1], srid))
         row = cursor.fetchone()
         m = re.match('POINT\((.*?) (.*)\)', row[0])
-        return list(map(float, m.groups()))
+        return list(map(str2int, m.groups()))

mapit_gb/countries.py

@@ -81,15 +81,10 @@ def augment_postcode(postcode, result):
     pc = postcode.postcode
     if is_special_postcode(pc):
         return
-    if pc[0:2] == 'BT':
-        loc = postcode.as_irish_grid()
-        result['coordsyst'] = 'I'
-    else:
-        loc = postcode.location
-        loc.transform(27700)
-        result['coordsyst'] = 'G'
-    result['easting'] = int(round(loc[0]))
-    result['northing'] = int(round(loc[1]))
+    loc = postcode.as_uk_grid()
+    result['coordsyst'] = 'I' if pc[0:2] == 'BT' else 'G'
+    result['easting'] = loc[0]
+    result['northing'] = loc[1]
 
 
 # Hacky function to restrict certain geographical links in the HTML pages to

mapit_gb/tests.py

@@ -67,21 +67,27 @@ def setUp(self):
             code='E05000025',
         )
 
+    def test_ni_postcode(self):
+        postcode = models.Postcode(postcode='BT170XD', location=Point(-6.037555, 54.556533))
+        grid = postcode.as_uk_grid()
+        self.assertEqual(grid, [327011, 369351])
+
     def test_postcode_json(self):
         pc = self.postcode.postcode
         url = '/postcode/%s' % urllib.parse.quote(pc)
         response = self.client.get(url)
         content = get_content(response)
 
-        in_gb_coords = self.postcode.location.transform(27700, clone=True)
+        in_gb_coords = self.postcode.as_uk_grid()
+        self.assertEqual(in_gb_coords, [529090, 179645])
         pc = countries.get_postcode_display(self.postcode.postcode)
         self.assertDictEqual(content, {
             'postcode': pc,
             'wgs84_lat': self.postcode.location.y,
             'wgs84_lon': self.postcode.location.x,
             'coordsyst': 'G',
-            'easting': round(in_gb_coords.x),
-            'northing': round(in_gb_coords.y),
+            'easting': round(in_gb_coords[0]),
+            'northing': round(in_gb_coords[1]),
             'areas': {
                 str(self.area.id): {
                     'id': self.area.id,
@@ -113,14 +119,14 @@ def test_partial_json(self):
         response = self.client.get(url)
         content = get_content(response)
         countries.get_postcode_display(self.postcode.postcode)
-        in_gb_coords = self.postcode.location.transform(27700, clone=True)
+        in_gb_coords = self.postcode.as_uk_grid()
         self.assertDictEqual(content, {
             'wgs84_lat': self.postcode.location.y,
             'wgs84_lon': self.postcode.location.x,
             'coordsyst': 'G',
             'postcode': 'SW1A',
-            'easting': round(in_gb_coords.x),
-            'northing': round(in_gb_coords.y)
+            'easting': round(in_gb_coords[0]),
+            'northing': round(in_gb_coords[1])
         })
 
     def test_partial_json_link(self):
@@ -133,16 +139,16 @@ def test_nearest_json(self):
         response = self.client.get(url)
         content = get_content(response)
         pc = countries.get_postcode_display(self.postcode.postcode)
-        in_gb_coords = self.postcode.location.transform(27700, clone=True)
+        in_gb_coords = self.postcode.as_uk_grid()
         self.assertDictEqual(content, {
             'postcode': {
                 'distance': 0,
                 'wgs84_lat': self.postcode.location.y,
                 'wgs84_lon': self.postcode.location.x,
                 'coordsyst': 'G',
                 'postcode': pc,
-                'easting': round(in_gb_coords.x),
-                'northing': round(in_gb_coords.y)
+                'easting': round(in_gb_coords[0]),
+                'northing': round(in_gb_coords[1])
             }
         })