SpatialReference.srid incorrectly assumes first AUTHORITY value to be projection SRID

[Stefan Brand]

Problem

When reading data into our PostGIS database, we noticed strange locations for our geometries. The data was supposed to be EPSG:31287, but investigation showed that Django was assuming EPSG:6312. This led to wrong EWKTs when saving geometries to PostGIS.

It turns out that the dataset did not have any SRID stored for the projection, just for the DATUM. Django assumed the DATUM's AUTHORITY value to be the SRID of the dataset's projection. This contradicts the ​documentation for SpatialReference.srid:

Returns the SRID of top-level authority, or None if undefined.

Since the layer's srid cannot be set to a different value, the only workaround is to set the srid for each OGRGeometry individually.

Current Result

​`SpatialReference.srid` calls int(self.attr_value("AUTHORITY", 1)), which returns 6312 (wrong SRID).

Expected Result

SpatialReference.srid should call auth_code(target=None) to get the top-level SRID.

Django should not assume a wrong SRID if the projection does not have a SRID. For example, QGIS reads the whole spatial reference information, correctly projects the data, but displays "Unknown CRS" because it does not assume any AUTHORITY value to be the dataset SRID.

Steps to reproduce

from django.contrib.gis.gdal import SpatialReference

WKT = """PROJCRS["MGI / Austria Lambert",
    BASEGEOGCRS["MGI",
        DATUM["Militar-Geographische Institut",
            ELLIPSOID["Bessel 1841",6377397.155,299.1528128,
                LENGTHUNIT["metre",1]],
            ID["EPSG",6312]],
        PRIMEM["Greenwich",0,
            ANGLEUNIT["Degree",0.0174532925199433]]],
    CONVERSION["unnamed",
        METHOD["Lambert Conic Conformal (2SP)",
            ID["EPSG",9802]],
        PARAMETER["Latitude of false origin",47.5,
            ANGLEUNIT["Degree",0.0174532925199433],
            ID["EPSG",8821]],
        PARAMETER["Longitude of false origin",13.3333333333333,
            ANGLEUNIT["Degree",0.0174532925199433],
            ID["EPSG",8822]],
        PARAMETER["Latitude of 1st standard parallel",49,
            ANGLEUNIT["Degree",0.0174532925199433],
            ID["EPSG",8823]],
        PARAMETER["Latitude of 2nd standard parallel",46,
            ANGLEUNIT["Degree",0.0174532925199433],
            ID["EPSG",8824]],
        PARAMETER["Easting at false origin",400000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8826]],
        PARAMETER["Northing at false origin",400000,
            LENGTHUNIT["metre",1],
            ID["EPSG",8827]]],
    CS[Cartesian,2],
        AXIS["easting",east,
            ORDER[1],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]],
        AXIS["northing",north,
            ORDER[2],
            LENGTHUNIT["metre",1,
                ID["EPSG",9001]]]]"""



geodjango_srs = SpatialReference(WKT)
geodjango_srs.validate()  # raises for invalid SRS
print(geodjango_srs.attr_value("AUTHORITY", 1))
print(geodjango_srs.auth_code(target=None))

6312
None

[Claude Paroz]

Thanks for the report. Would you like to prepare a pull request for Django?

[Stefan Brand]

Thanks for asking, yes, I would like that. I hope I can tackle it this week. If I don't find time, someone else feel free to create a PR.

[Stefan Brand]

While applying the change, I got a failing test. It turns out that GeoDjango has a different behaviour than the GDAL Python bindings.

Compare:

from django.contrib.gis.gdal import SpatialReference
from osgeo import ogr

WEB_MERCATOR = """PROJCS["WGS 84 / Pseudo-Mercator",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Mercator_1SP"],
    PARAMETER["central_meridian",0],
    PARAMETER["scale_factor",1],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["Easting",EAST],
    AXIS["Northing",NORTH],
    EXTENSION["PROJ4","+proj=merc +a=6378137 +b=6378137 +lat_ts=0 +lon_0=0 +x_0=0 +y_0=0 +k=1 +units=m +nadgrids=@null +wktext +no_defs"],
    AUTHORITY["EPSG","3857"]]"""


geodjango_srs = SpatialReference(WEB_MERCATOR)
geodjango_srs.validate()  # raises for invalid SRS
print(geodjango_srs.attr_value("AUTHORITY", 1))
print(geodjango_srs.auth_code(target=None))

ogr_srs = ogr.osr.SpatialReference(WEB_MERCATOR)
if ogr_srs.Validate(): raise
print(ogr_srs.GetAttrValue("AUTHORITY", 1))
print(ogr_srs.GetAuthorityCode(None))

3857
None
3857
3857

The reason is that ​`django/utils/force_bytes` turns None into the byte string b'None'. Since ctypes.c_char_p also accepts None, we can bypass force_bytes if target is None.

I will do some more testing and push a patch tomorrow.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In d77762d:

Refs #34302 -- Fixed SpatialReference.auth_name()/auth_code() when target is None.

force_bytes() turns None into the byte string b"None". Since
ctypes.c_char_p() also accepts None, we can bypass force_bytes() if
target is None.

[Mariusz Felisiak <felisiak.mariusz@…>]

In eacf6b73:

Fixed #34302 -- Fixed SpatialReference.srid for objects without top-level authority.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 341f33ed:

[4.2.x] Refs #34302 -- Fixed SpatialReference.auth_name()/auth_code() when target is None.

force_bytes() turns None into the byte string b"None". Since
ctypes.c_char_p() also accepts None, we can bypass force_bytes() if
target is None.

Backport of d77762de038d1ab46cdcda2b7202d36c80956e25 from main

[Mariusz Felisiak <felisiak.mariusz@…>]

In efcc0f2:

[4.2.x] Fixed #34302 -- Fixed SpatialReference.srid for objects without top-level authority.

Backport of eacf6b73d8eace004f840bd9b80c8c671caab9da from main