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