PostGIS Distance lookups use ST_Distance_Sphere() and not ST_Distance()

[Bibhas C Debnath]

Currently ​the doc says that PostGIS uses ST_Distance() for distance lookups like distance_gte etc. But actually ST_Distance_Sphere() is used. We need to fix that on the doc.

The doc also says in the beginning -

... an optional third element, 'spheroid', may be included to tell GeoDjango to use the more accurate spheroid distance calculation functions on fields with a geodetic coordinate system (e.g., ST_Distance_Spheroid would be used instead of ST_Distance_Sphere).

But then ST_Distance_Sphere is not mentioned anywhere else.

Here is what currently is happening -

current_localities = Locality.objects.filter(centroid__distance_lte=(pnt, 1000))

results in -

(0.005) SELECT "map_locality"."id", "map_locality"."name", "map_locality"."slug", "map_locality"."centroid", "map_locality"."radius" FROM "map_locality" WHERE ST_Distance_Sphere("map_locality"."centroid", ST_GeomFromEWKB('\x0101000020e6100000865ad3bce3685340e9b7af03e7ec2940'::bytea)) <= 1000 LIMIT 21; args=(<django.contrib.gis.db.backends.postgis.adapter.PostGISAdapter object at 0x7fdcc9d320d0>, 1000)

Attaching a patch with the issue. I can send a PR.

[Bibhas C Debnath]

patch to rename st_distance to st_distance_sphere