GeoDjango + PostGIS: pure geometry SRID - django

Question

I am thinking about using django (2.0.6) with Postgres/postGIS as a backend (10/2.4?).
However, the model geometries I intend to use will not be stored "against" a spheroid (whether earth mars, etc) or any other kind of SRID. Rather a 100% pure geometry in 3D cartesian coordinates, where 1 unit = 1 meter.
How am I to declare model fields and insure they are pure geometry both:
at database level
at application level
Would this work?
geometry = models.MultiPolygonField(_('Geometry'), spatial_index=True, dim=3)
Or better to set the SRID?:
geometry = models.MultiPolygonField(_('Geometry'), spatial_index=True, dim=3, srid=0)
Many thanks

Answer 1

You can use SRID 3857 for 1m Cartesian grid:
models.py:
class Place(models.Model):
geom = models.PointField(srid=3857, dim=3)
def __str__(self):
return str(self.geom.coords)
However, GeoDjango does not support 3d distance calculations out of the box. For example:
from django.contrib.gis.geos import Point
from django.contrib.gis.measure import Distance
from myapp.models import Place
p = Point(100, 100, 0)
Place.objects.create(geom=p)
Place.objects.create(geom=Point(150, 100, 0))
Place.objects.create(geom=Point(100, 100, 22))
Place.objects.create(geom=Point(100, 100, 500))
Place.objects.create(geom=Point(150, 150, 0))
Place.objects.create(geom=Point(250, 250, 0))
for o in Place.objects.filter(geom__distance_lte=(p, Distance(m=100))):
print(o.geom.coords, p.distance(o.geom))
Output:
(100.0, 100.0, 0.0) 0.0
(150.0, 100.0, 0.0) 50.0
(100.0, 100.0, 22.0) 0.0
(100.0, 100.0, 500.0) 0.0
(150.0, 150.0, 0.0) 70.71067811865476
In pure PostGIS, compare ST_Distance and ST_3DDistance :
SELECT *
FROM (
SELECT
"myapp_place"."id",
ST_Distance("myapp_place"."geom",
ST_GeomFromEWKT('SRID=3857;POINT(100 100 0)')) d,
ST_AsEWKT("myapp_place"."geom")
FROM "myapp_place"
) t
WHERE d <= 100.0
vs.
SELECT *
FROM (
SELECT
"myapp_place"."id",
ST_3DDistance("myapp_place"."geom",
ST_GeomFromEWKT('SRID=3857;POINT(100 100 0)')) d,
ST_AsEWKT("myapp_place"."geom")
FROM "myapp_place"
) t
WHERE d <= 100.0