Why do the Centroid function and centroid method return (slightly) different results

I have a GeoDjango (v5.2) model ("Location") with a MultiPolygonField ("coordinates") and use PostGIS (v17-3.5) as database backend. When I annotate a queryset with the multi-polygon's centroid, I get slightly different results compared to the centroid method of a single instance. They only seem to be rounding errors, but given the names of function and method, I was expecting the same results.

from django.contrib.gis.db import models
from django.contrib.gis.db.models.functions import Centroid

class Location(models.Model):
    coordinates = models.MultiPolygonField(
        unique=True,
        srid=DATABASE_SRID,
        dim=2,
        geography=True,
    )
    ...

# Get an arbitrary location that is centered at (20°N, 20°E).
foo = Location.objects.get(pk=1)
bar = Location.objects.filter(pk=1).annotate(center=Centroid("coordinates"))

print(foo.coordinates.centroid.coords)
# > (21.99999999999877, 22.0000000000065)
print(bar.first().center.centroid.coords)
# > (22.000000000008804, 21.999999999992266)

Вернуться на верх