GeoDjango dwithin errors when using django.contrib.gis.measure.D

Question

First off: Python 2.7.6, Django 1.6.5, Postgres 9.3.4, PostGIS 2.1.3, psycopg2 2.5.3 on RHEL 6.5

Here\'s the relevant model:

class Location(models.Model)         


        

Answer 1

I got a response to the ticket I submitted (https://code.djangoproject.com/ticket/22830). Apparently, I found a seemingly undocumented (or at least not clearly documented) problem that dwithin queries have with Distance objects. A dev says this:

As your objects are in geographic coordinates (geometry fields default to WGS84), you have to provide the distance as degree units. This is for example matching the PostGIS definition:

boolean ST_DWithin(geometry g1, geometry g2, double precision distance_of_srid);

distance_of_srid being degrees for WGS84. So the 5 that works in your example means 5 degrees, not 5 km!

It looks like they're going to clarify the documentation to make this clearer (great!).

Since what I want is 5km, I need to convert 5km to degrees. 1 degree is approximately 111.325km. Therefore, 1km = 1/111.325 degrees. 5km is therefore approximately 0.0449 or about 0.05 degrees. So I just need to change my call to this:

touching_locations = Location.objects.filter(geometry__dwithin=(location.geometry, 0.05))