[fix] Make GeoJSON output valid by transforming to WGS84

Author: StefanBrand

README.rst

@@ -83,7 +83,7 @@ Provides a ``GeometryField``, which is a subclass of Django Rest Framework
 geometry fields, providing custom ``to_native`` and ``from_native``
 methods for GeoJSON input/output.
 
-This field takes three optional arguments:
+This field takes four optional arguments:
 
 - ``precision``: Passes coordinates through Python's builtin ``round()`` function (`docs
   <https://docs.python.org/3/library/functions.html#round>`_), rounding values to
@@ -97,6 +97,10 @@ This field takes three optional arguments:
 - ``auto_bbox``: If ``True``, the GeoJSON object will include
   a `bounding box <https://datatracker.ietf.org/doc/html/rfc7946#section-5>`_,
   which is the smallest possible rectangle enclosing the geometry.
+- ``transform`` (defaults to ``4326``): If ``None`` (or the input geometry does not have
+  a SRID), the GeoJSON's coordinates will not be transformed. If any other `spatial
+  reference <https://docs.djangoproject.com/en/5.0/ref/contrib/gis/geos/#django.contrib.gis.geos.GEOSGeometry.transform>`,
+   the GeoJSON's coordinates will be transformed correspondingly.
 
 **Note:** While ``precision`` and ``remove_duplicates`` are designed to reduce the
 byte size of the API response, they will also increase the processing time

rest_framework_gis/fields.py

@@ -18,14 +18,20 @@ class GeometryField(Field):
     type_name = 'GeometryField'
 
     def __init__(
-        self, precision=None, remove_duplicates=False, auto_bbox=False, **kwargs
+        self,
+        precision=None,
+        remove_duplicates=False,
+        auto_bbox=False,
+        transform=4326,
+        **kwargs,
     ):
         """
         :param auto_bbox: Whether the GeoJSON object should include a bounding box
         """
         self.precision = precision
         self.auto_bbox = auto_bbox
         self.remove_dupes = remove_duplicates
+        self.transform = transform
         super().__init__(**kwargs)
         self.style.setdefault('base_template', 'textarea.html')
 
@@ -34,6 +40,14 @@ def to_representation(self, value):
             return value
         # we expect value to be a GEOSGeometry instance
         if value.geojson:
+            # NOTE: For repeated transformations a gdal.CoordTransform is recommended
+            if (
+                self.transform is not None
+                and value.srid is not None
+                and value.srid != 4326
+            ):
+                value.transform(self.transform)
+
             geojson = GeoJsonDict(value.geojson)
         # in this case we're dealing with an empty point
         else:

tests/django_restframework_gis_tests/migrations/0005_othersridlocation.py

@@ -0,0 +1,37 @@
+# Generated by Django 5.1.5 on 2025-01-24 17:38
+
+import django.contrib.gis.db.models.fields
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("django_restframework_gis_tests", "0004_auto_20240228_2357"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="OtherSridLocation",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("name", models.CharField(max_length=32)),
+                ("slug", models.SlugField(blank=True, max_length=128, unique=True)),
+                ("timestamp", models.DateTimeField(blank=True, null=True)),
+                (
+                    "geometry",
+                    django.contrib.gis.db.models.fields.GeometryField(srid=31287),
+                ),
+            ],
+            options={
+                "abstract": False,
+            },
+        ),
+    ]

tests/django_restframework_gis_tests/models.py

@@ -54,6 +54,10 @@ class Location(BaseModelGeometry):
     pass
 
 
+class OtherSridLocation(BaseModelGeometry):
+    geometry = models.GeometryField(srid=31287)
+
+
 class LocatedFile(BaseModelGeometry):
     file = models.FileField(upload_to='located_files', blank=True, null=True)
 

tests/django_restframework_gis_tests/serializers.py

@@ -2,7 +2,7 @@
 from rest_framework import pagination, serializers
 
 from rest_framework_gis import serializers as gis_serializers
-from rest_framework_gis.fields import GeometrySerializerMethodField
+from rest_framework_gis.fields import GeometryField, GeometrySerializerMethodField
 
 from .models import (
     BoxedLocation,
@@ -13,6 +13,7 @@
     MultiPointModel,
     MultiPolygonModel,
     Nullable,
+    OtherSridLocation,
     PointModel,
     PolygonModel,
 )
@@ -40,6 +41,7 @@
     'GeometrySerializerMethodFieldSerializer',
     'GeometrySerializer',
     'BoxedLocationGeoFeatureWithBBoxGeoFieldSerializer',
+    'OtherSridLocationGeoSerializer',
 ]
 
 
@@ -53,6 +55,17 @@ class Meta:
         fields = '__all__'
 
 
+class OtherSridLocationGeoSerializer(gis_serializers.GeoFeatureModelSerializer):
+    """Other SRID location geo serializer"""
+
+    geometry = GeometryField(auto_bbox=True)
+
+    class Meta:
+        model = OtherSridLocation
+        geo_field = 'geometry'
+        fields = '__all__'
+
+
 class PaginatedLocationGeoSerializer(pagination.PageNumberPagination):
     page_size_query_param = 'limit'
     page_size = 40

tests/django_restframework_gis_tests/test_fields.py

@@ -7,6 +7,7 @@
 from rest_framework_gis import serializers as gis_serializers
 
 Point = {"type": "Point", "coordinates": [-105.0162, 39.5742]}
+Point31287 = {"type": "Point", "coordinates": [625826.2376404074, 483198.2074507246]}
 
 MultiPoint = {
     "type": "MultiPoint",
@@ -141,6 +142,102 @@ def normalize(self, data):
         return data
 
 
+class TestTransform(BaseTestCase):
+    def test_no_transform_4326_Point_no_srid(self):
+        model = self.get_instance(Point)
+        Serializer = self.create_serializer()
+        data = Serializer(model).data
+
+        expected_coords = (-105.0162, 39.5742)
+        for lat, lon in zip(
+            data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(lat, lon, places=5)
+
+    def test_no_transform_4326_Point_set_srid(self):
+        model = self.get_instance(Point)
+        model.geometry.srid = 4326
+        Serializer = self.create_serializer()
+        data = Serializer(model).data
+
+        expected_coords = (-105.0162, 39.5742)
+        for lat, lon in zip(
+            data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(lat, lon, places=5)
+
+    def test_transform_Point_no_transform(self):
+        model = self.get_instance(Point31287)
+        model.geometry.srid = 31287
+        Serializer = self.create_serializer(transform=None)
+        data = Serializer(model).data
+
+        expected_coords = (625826.2376404074, 483198.2074507246)
+        for lat, lon in zip(
+            data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(lat, lon, places=5)
+
+    def test_transform_Point_no_srid(self):
+        model = self.get_instance(Point31287)
+        Serializer = self.create_serializer()
+        data = Serializer(model).data
+
+        expected_coords = (625826.2376404074, 483198.2074507246)
+        for lat, lon in zip(
+            data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(lat, lon, places=5)
+
+    def test_transform_Point_to_4326(self):
+        model = self.get_instance(Point31287)
+        model.geometry.srid = 31287
+        Serializer = self.create_serializer()
+        data = Serializer(model).data
+
+        expected_coords = (16.372500007573713, 48.20833306345481)
+        for lat, lon in zip(
+            data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(lat, lon, places=5)
+
+    def test_transform_Point_to_3857(self):
+        model = self.get_instance(Point31287)
+        model.geometry.srid = 31287
+        Serializer = self.create_serializer(transform=3857)
+        data = Serializer(model).data
+
+        expected_coords = (1822578.363856016, 6141584.271938089)
+        for lat, lon in zip(
+            data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(lat, lon, places=1)
+
+    def test_transform_Point_bbox_to_4326(self):
+        model = self.get_instance(Point31287)
+        model.geometry.srid = 31287
+        Serializer = self.create_serializer(auto_bbox=True)
+        data = Serializer(model).data
+
+        expected_coords = (
+            16.372500007573713,
+            48.20833306345481,
+            16.372500007573713,
+            48.20833306345481,
+        )
+        for received, expected in zip(
+            data["geometry"]["bbox"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(received, expected, places=5)
+
+
 class TestPrecision(BaseTestCase):
     def test_precision_Point(self):
         model = self.get_instance(Point)

tests/django_restframework_gis_tests/tests.py

@@ -14,7 +14,7 @@
 from rest_framework_gis import serializers as gis_serializers
 from rest_framework_gis.fields import GeoJsonDict
 
-from .models import LocatedFile, Location, Nullable
+from .models import LocatedFile, Location, Nullable, OtherSridLocation
 from .serializers import LocationGeoSerializer
 
 
@@ -310,6 +310,33 @@ def test_geojson_false_id_attribute_slug(self):
         with self.assertRaises(KeyError):
             response.data['id']
 
+    def test_geojson_srid_transforms_to_wgs84(self):
+        location = OtherSridLocation.objects.create(
+            name="other SRID location",
+            geometry='POINT(625826.2376404074 483198.2074507246)',
+        )
+        url = reverse('api_other_srid_location_details', args=[location.id])
+        response = self.client.get(url)
+        expected_coords = (16.372500007573713, 48.20833306345481)
+        expected_coords_bbox = (
+            16.372500007573713,
+            48.20833306345481,
+            16.372500007573713,
+            48.20833306345481,
+        )
+        self.assertEqual(response.data['properties']['name'], 'other SRID location')
+        for received, expected in zip(
+            response.data["geometry"]["coordinates"],
+            expected_coords,
+        ):
+            self.assertAlmostEqual(received, expected, places=5)
+
+        for received, expected in zip(
+            response.data["geometry"]["bbox"],
+            expected_coords_bbox,
+        ):
+            self.assertAlmostEqual(received, expected, places=5)
+
     def test_post_geojson_id_attribute(self):
         self.assertEqual(Location.objects.count(), 0)
         data = {

tests/django_restframework_gis_tests/urls.py

@@ -17,6 +17,11 @@
         views.geojson_location_details,
         name='api_geojson_location_details',
     ),
+    path(
+        'geojson-other-srid/<int:pk>/',
+        views.other_srid_location_details,
+        name='api_other_srid_location_details',
+    ),
     path(
         'geojson-nullable/<int:pk>/',
         views.geojson_nullable_details,

tests/django_restframework_gis_tests/views.py

@@ -11,7 +11,14 @@
 )
 from rest_framework_gis.pagination import GeoJsonPagination
 
-from .models import BoxedLocation, LocatedFile, Location, Nullable, PolygonModel
+from .models import (
+    BoxedLocation,
+    LocatedFile,
+    Location,
+    Nullable,
+    OtherSridLocation,
+    PolygonModel,
+)
 from .serializers import (
     BoxedLocationGeoFeatureSerializer,
     LocatedFileGeoFeatureSerializer,
@@ -25,6 +32,7 @@
     LocationGeoSerializer,
     NoGeoFeatureMethodSerializer,
     NoneGeoFeatureMethodSerializer,
+    OtherSridLocationGeoSerializer,
     PaginatedLocationGeoSerializer,
     PolygonModelSerializer,
 )
@@ -49,6 +57,15 @@ class LocationDetails(generics.RetrieveUpdateDestroyAPIView):
 location_details = LocationDetails.as_view()
 
 
+class OtherSridLocationDetails(generics.RetrieveUpdateDestroyAPIView):
+    model = OtherSridLocation
+    serializer_class = OtherSridLocationGeoSerializer
+    queryset = OtherSridLocation.objects.all()
+
+
+other_srid_location_details = OtherSridLocationDetails.as_view()
+
+
 class GeojsonLocationList(generics.ListCreateAPIView):
     model = Location
     serializer_class = LocationGeoFeatureSerializer