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