Extract X / Y from Geometry (ST_X, ST_Y)

Use this skill when you need to extract numeric longitude/latitude values from a PostGIS geometry.

This is common when:

• returning tabular data alongside geometry
• exporting points to CSV
• debugging coordinate issues
• integrating with non-GIS systems

Core rules

• ST_X and ST_Y only work on POINT geometries
• Units and meaning depend on the geometry’s SRID
• For web use, extract coordinates after transforming to EPSG:4326

Canonical patterns

Extract from point geometry (already in EPSG:4326)

SELECT
  ST_X(geom) AS lng,
  ST_Y(geom) AS lat
FROM points
WHERE geom IS NOT NULL;

• ST_X → longitude
• ST_Y → latitude

Transform before extracting (recommended)

If geometry is stored in a projected or client SRID:

SELECT
  ST_X(ST_Transform(geom, 4326)) AS lng,
  ST_Y(ST_Transform(geom, 4326)) AS lat
FROM points;

Always extract in 4326 when values are meant for humans or maps.

Non-point geometry (use centroid)

For lines or polygons, extract from the centroid:

SELECT
  ST_X(ST_Centroid(geom)) AS lng,
  ST_Y(ST_Centroid(geom)) AS lat
FROM features;

Be aware:

• Centroid may fall outside the polygon for concave shapes
• For guaranteed interior points, use ST_PointOnSurface

Guaranteed interior point (polygons)

SELECT
  ST_X(ST_PointOnSurface(geom)) AS lng,
  ST_Y(ST_PointOnSurface(geom)) AS lat
FROM polygons;

JSON-friendly extraction

Useful when building API responses:

SELECT
  jsonb_build_object(
    'lng', ST_X(ST_Transform(geom, 4326)),
    'lat', ST_Y(ST_Transform(geom, 4326))
  )
FROM points;

Common mistakes

• Calling ST_X / ST_Y on non-point geometries
• Forgetting to transform before extraction
• Swapping latitude and longitude
• Assuming projected coordinates are degrees

Summary

• Use ST_X for longitude, ST_Y for latitude
• Transform to EPSG:4326 before extracting for web use
• Use centroids or point-on-surface for non-point geometry