determine whether a circle intersects a county boundary in postGIS

Question

I'm using PostGIS 2.0.1 and need to determine whether a circle (center + radius) intersects a county boundary (polygon).

The query I'm using is this:

SELECT DISTINCT(u.id) FROM siteareas_poly sa, userlocations ul, 
            clienttypes ct INNER JOIN users_clienttypes uct on ct.id = uct.clienttype_id 
            INNER JOIN users u on uct.user_id = u.id 
            WHERE sa.id = 'concord' and ct.id = 2 
            AND ul.geog_lnglat is not null
            AND  ST_Intersects(st_exteriorring(sa.boundary), ST_BUFFER(ul.geog_lnglat,ul.radius_km * 1000)::geometry)

where the ul.geog_lnglat is a

geography(Point,4326)

and the sa.boundary is

geometry

I've tried a few variations on this and always gotten no results, when I expect to get at least one, as I have a row in userlocations that lies in that county (by zip lookup).

Am I making any glaring errors in my approach?

Answer 1

Use ST_DWithin. You will need to convert both to either planer geometry or geography types.

For example, to use ST_DWithin on geography inputs:

SELECT ...
FROM ...
WHERE ...
  AND ST_DWithin(ST_Transform(sa.boundary, 4326)::geography,
                 ul.geog_lnglat,
                 ul.radius_km * 1000);