ST_Distance (Numeric Distance)

Use this skill when you need the numeric distance value between geometries (for reporting, ranking display, metadata).

If you need a boolean “within X distance” filter, prefer ST_DWithin (faster). A common pattern is:

• use ST_DWithin to filter candidates

• use ST_Distance to compute the returned distance value

Core rules

• Units depend on type:

• geometry → SRID units (degrees/meters/feet depending on SRID)

• geography → meters

• Geometries must be in the same SRID for geometry math

• Do not compute meaningful distances in EPSG:4326 geometry (degrees) unless you intend degrees

Canonical patterns

Distance in meters (recommended): geography

Works well when data is stored in EPSG:4326 and you want meters.

SELECT ST_Distance(a.geom::geography, b.geom::geography) AS distance_m FROM a JOIN b ON ...;

Distance in projected SRID units: geometry

Use a projected/client SRID where units are meaningful (feet/meters).

SELECT ST_Distance( ST_Transform(a.geom, client_srid), ST_Transform(b.geom, client_srid) ) AS distance_units FROM a JOIN b ON ...;

Fast filter + distance output (recommended pattern)

Filter candidates with ST_DWithin , then compute distance.

WITH p AS ( SELECT ST_SetSRID(ST_MakePoint($1, $2), 4326) AS geom ) SELECT f.*, ST_Distance(f.geom::geography, p.geom::geography) AS distance_m FROM features f, p WHERE ST_DWithin(f.geom::geography, p.geom::geography, 5000) -- 5km ORDER BY f.geom <-> p.geom LIMIT 50;

This is a common “map / nearby” endpoint pattern:

• ST_DWithin constrains work

• <-> ranks quickly

• ST_Distance provides the numeric value

Common mistakes

• Using ST_Distance(geom, geom) on EPSG:4326 geometry and assuming meters

• Using ST_Distance(...) < x in WHERE instead of ST_DWithin(...)

• Mixing SRIDs without transforming

• Computing distances on huge tables without bounding the search

Summary

• Use geography for meters

• Use projected SRIDs for planar geometry distances

• Use ST_DWithin for filtering, ST_Distance for reporting