[postgis-users] accuracy of coverage

[Gold, Jack L (US SSA)]

I am trying to determine the accuracy of the ST_DWithin function.  I have a position at 32.52550166, -84.96989441 and I want to know if it is within a projected circle of 4km radius centered on 32.5, -85.

I am running the following query:

SELECT "id","location", ST_AsText(location) as "text_position" FROM "positions" WHERE (ST_DWithin(location, ST_geographyFromText("SRID=4326;POINT(-85.0000035762821 32.49999701976208)"), 4000.0))

"location" is defined as a geography


When I map this out, Google Maps shows the position report outside of the 4km ring as does my application, but the database is reporting that the position is within the 4km range of the ring.  I suspect the error is small, but I'm wondering where it lies.

Any ideas?

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