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