PostGIS – Understanding ST_DWithin with Geometry or Geography Type

geography-data-typegeometry-data-typepostgisquery

assuming I use the following query to match the locations to a poi within 10km distance:

SELECT
poi.poi_id,
h.loc_id,
(ST_Distance(poi.geom,h.geom)*111.111) as distanceinkm
FROM iut_poi_transl AS poi, hrs_loc as h
WHERE poi.poigr_id != 30 AND ST_DWithin(poi.geom,h.geom,0.09)

Data is SRID 4326.

Does it use the geometry or the geography type?

If uses geometry how do I have to change the query to get use geography type?

Best Answer

As per documentation of function it can use both: http://postgis.net/docs/ST_DWithin.html

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

boolean ST_DWithin(geography gg1, geography gg2, double precision distance_meters);

boolean ST_DWithin(geography gg1, geography gg2, double precision distance_meters, boolean use_spheroid);

Watch out for this:

For geography units are in meters and measurement is defaulted to use_spheroid=true (measure around WGS 84 spheroid), for faster check, use_spheroid=false to measure along sphere.

But for 10km buffer radius it shouldn't be a big error.

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