GeoDjango & SpatiaLite - filtering nearby objects

Question

My models.py has User and Business models that have this field:

location = PointField(geography=True)

I'm getting Google Maps coordinates (in EPSG 4326) via Geocode service from an address which the user specifies.
Then I'm saving it in the above field (also EPSG 4326).

Now, what I want is to get all Business objects within a specified radius (of 1km for example), based on the user location:

Business.gis.filter(location__distance_lt=(request.user.location, D(km=1)))

But it doesn't work, it gives me this error:

ValueError: SpatiaLite does not support distance queries on geometry fields with a geodetic coordinate system. Distance objects; use a numeric value of your distance in degrees instead.

So, does anyone know how to properly solve this? Thanks in advance!

Answer 1

Theory:

This is a pretty obscure error related to those two:

• GeoDjango dwithin errors when using django.contrib.gis.measure.D
• Geodjango distance query not retrieving correct results

Since you are using geodetic coordinate systems (like the EPSG 4326) you need to provide the distance in degrees.

Here is a very good explanation on how to transform km to degrees accurately enough: How do I convert kilometres to degrees in Geodjango/GEOS?

Finally I would suggest to use the dwithin.

---

Praxis:

• 1km ~= 0.008 deg

• The query now should look:

  Business.gis.filter(location__dwithin=(request.user.location, 0.008))