[GIS] Django Postgis Distance from point

geodjangopointpostgis

I have a point where the objcet is and another point for mylocation.

I want to calculate the distance

>> object_location
<Point object at 0x7f9ceaac7be0>

>> mylocation
<Point object at 0x7f9cf9311b50>

>> mylocation.distance(location)
0.13980702957351368

But the result is wrong is there some different way with geodjango to calculate the distance between two points

This way in ipdb is correct

ipdb> Item.objects.filter(id=self.object.id).distance(mylocation, field_name="address__location")[0].distance
Distance(m=15368.0088756)

Best Answer

This might just be a problem of projections/units, it looks like the projection of mylocation.srid and object_location.srid might be WGS84 (the default projection in GeoDjango).

If you want to have distance in meters without making another database lookup, you could transform your points with something like this:

mylocation.transform(3857)
object_location.transform(3857)

and then calculate the distance, which will be returned in meters. The distance would be linear distance between the two points. The exact results will still depend on the projection you choose. Also the accuracy will be limited because it does not calculate spherical distances according to the GeoDjango documentation. So depending on your use case, using a DB query is still the better solution.

Related Solutions

[GIS] Simple PostGIS ST_Distance query for two points from the same geometry table

You need to reference your table twice, giving it different aliases:

SELECT ST_Distance(a.geom, b.geom)
FROM points_table a, points_table b
WHERE a.id='x' AND b.id='y';

PostGIS – Calculate Minimum Distance Between Points

It's not a simple problem, it involves some kind of forced iteration over the set of candidate points. This chapter from the workshop shows a similar problem, but not exact (your problem is slightly easier)

http://postgis.net/workshops/postgis-intro/advanced_geometry_construction.html

The nearest neighbor searching chapter from the workshop shows the tools you might use to do an index-assisted approach with some external loop driving the query

http://postgis.net/workshops/postgis-intro/knn.html

If your points have a distinct id and you know a distance tolerance (9999) they will all fall within, a self-join and use of the "DISTINCT ON" filter will get you the answer in one go.

WITH unfiltered AS
(
  SELECT t1.id AS id1, t2.id AS id2, ST_Distance(t1.geom, t2.geom) as dist
  FROM t t1, t t2 WHERE ST_DWithin(t1.geom, t2.geom, 9999) AND t1.id <> t2.id
  ORDER BY t1.id, ST_Distance(t1.geom, t2.geom) ASC
)
SELECT DISTINCT ON (id1) id1, id2, dist FROM unfiltered;

It first gathers the candidates combinations of points, and sorts them by distance. Then the "distinct on" filter strips out just the first member of each candidate group, which conveniently is the closest, thanks to the pre-sorting.

Best Answer

Related Solutions

[GIS] Simple PostGIS ST_Distance query for two points from the same geometry table

PostGIS – Calculate Minimum Distance Between Points

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