Reproject Geometry with ST_Transform

Use this skill when you need to reproject geometry between coordinate systems (SRIDs) in PostGIS.

This documents the canonical, safe patterns for using ST_Transform.

When to use

• You need planar distance, buffering, or area calculations
• You are comparing geometries from different SRIDs
• You are preparing geometry for web maps (EPSG:4326)

Core rules

• Geometry must have a valid SRID before transforming
• Use projected SRIDs for distance/buffer math
• Serve web maps in EPSG:4326
• Avoid transforming large tables inside WHERE clauses

Basic transform

SELECT
  ST_Transform(geom, 4326)
FROM my_table;

Assign SRID before transforming (required if SRID = 0)

SELECT
  ST_Transform(
    ST_SetSRID(geom, 3857),
    4326
  )
FROM my_table;

Never call ST_Transform on geometry with SRID = 0.

Transform for distance or buffer calculations

Store geometry in 4326, transform only for math:

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

Do not compute distance or buffers in EPSG:4326.

Transform only one side (performance)

When joining datasets, transform the smaller side:

SELECT a.*
FROM large_table a
JOIN small_table b
  ON ST_Intersects(
    a.geom,
    ST_Transform(b.geom, ST_SRID(a.geom))
  );

Transforming both sides disables spatial index usage.

Serving geometry to web clients

Always serve GeoJSON in EPSG:4326:

ST_AsGeoJSON(ST_Transform(geom, 4326))::jsonb

Common mistakes

• Transforming geometry without checking SRID
• Transforming both sides of a spatial join
• Buffering or measuring distance in EPSG:4326
• Serving projected coordinates directly to browsers

Summary

• Store geometry consistently
• Transform explicitly
• Use projected SRIDs for math
• Use EPSG:4326 for maps