Coordinate reference systems¶

Features are served in CRS84, longitude and latitude on WGS 84, unless a request asks for another coordinate reference system (CRS) that the collection offers, as defined by OGC API - Features - Part 2.

The CRS of a collection¶

The description of a collection lists the CRS it offers in crs, and gives the one its geometries are stored in, the SRID of its geometry field, as storageCrs:

{
  "id": "my_app.building",
  "crs": [
    "http://www.opengis.net/def/crs/OGC/1.3/CRS84",
    "http://www.opengis.net/def/crs/EPSG/0/2056"
  ],
  "storageCrs": "http://www.opengis.net/def/crs/EPSG/0/2056",
  ...
}

By default, a collection offers CRS84 and its storage CRS, or CRS84 alone when its geometries are stored in EPSG:4326. A collection without geometry offers none, and ignores the CRS parameters. The spatial extent of a collection is always given in CRS84.

Reading in a CRS¶

The crs parameter of /collections/{collectionId}/items and /collections/{collectionId}/items/{featureId} asks for the features in another CRS, and bbox-crs gives the CRS of the bbox parameter:

curl "https://example.com/oapif/collections/my_app.building/items?crs=http://www.opengis.net/def/crs/EPSG/0/2056"

Both default to CRS84. The response gives the CRS of its coordinates in its Content-Crs header, with the URI in angle brackets:

Content-Crs: <http://www.opengis.net/def/crs/EPSG/0/2056>

Writing in a CRS¶

The Content-Crs header of a POST, PUT or PATCH gives the CRS of the coordinates of the feature sent, with or without angle brackets, and defaults to CRS84 too. The feature returned is in CRS84.

CRS URIs¶

A CRS is given by its URI, http://www.opengis.net/def/crs/OGC/1.3/CRS84 or http://www.opengis.net/def/crs/EPSG/0/{code}, which is also accepted with https://.

A CRS that the collection does not offer is rejected with a 400 Bad Request, whether in crs, bbox-crs or Content-Crs. In particular, EPSG:4326 is not CRS84: its axis order is latitude first, which the API does not follow, and a client that sent coordinates latitude first would have them stored swapped.

Offering more CRS¶

Override supported_crs() to offer other CRS:

from django_oapif import OapifCollection
from django_oapif.crs import CRS

@oapif.register(Building)
class BuildingCollection(OapifCollection):
    def supported_crs(self):
        return (*super().supported_crs(), CRS("EPSG", 3857))

The features are reprojected by PostGIS, which writes the coordinates x first, whatever the axis order of the CRS: offer only CRS whose first axis is the easting or the longitude.

Reprojecting the features of a request takes a prepared transformation, which PostGIS keeps for the lifetime of the database connection: see deployment to keep connections open.