gis: Added the add_postgis_srs utility to ease the creation of spatial_ref_sys table entries.

git-svn-id: http://code.djangoproject.com/svn/django/branches/gis@7399 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/gis/utils/__init__.py

@@ -20,3 +20,4 @@ try:
 except:
     HAS_GEOIP = False
 
+from django.contrib.gis.utils.srs import add_postgis_srs

django/contrib/gis/utils/srs.py

@@ -0,0 +1,23 @@
+def add_postgis_srs(srs):
+    """
+    This function takes a GDAL SpatialReference system and adds its
+    information to the PostGIS `spatial_ref_sys` table -- enabling
+    spatial transformations with PostGIS.  This is handy for adding
+    spatial reference systems not included by default with PostGIS.  
+    For example, the following adds the so-called "Google Maps Mercator 
+    Projection" (available in GDAL 1.5):
+    
+    >>> add_postgis_srs(SpatialReference(900913))
+
+    Note: By default, the `auth_name` is set to 'EPSG' -- this should
+    probably be changed.
+    """
+    from django.contrib.gis.models import SpatialRefSys
+
+    if srs.srid is None:
+        raise Exception('Spatial reference requires an SRID to be compatible with PostGIS.')
+   
+    # Creating the spatial_ref_sys model.
+    sr, created = SpatialRefSys.objects.get_or_create(
+        srid=srs.srid, auth_name='EPSG', auth_srid=srs.srid, 
+        srtext=srs.wkt, proj4text=srs.proj4)