ST_Transform and ST_SetSRID: To project or not to project?

People often get confused between the ST_Transform and ST_SetSRID functions.

1. ST_SetSRID doesn't change the coordinates but adds meta data to state what spatial reference system the coordinate actually are. If you stamped your WGS 84 long lat data as a meter based projection. Guess what? Its still long lat. A spade by any other name is still a spade so don't use ST_SetSRID and expect to magically get meter coordinates.

2. ST_Transform is used to change the underlying coordinates from a known spatial reference system to another known spatial reference system.

In PostGIS 2+ it's pretty easy to correct mistakes you've made with standard ALTER TABLE commands. We'll demonstrate a couple of scenarios

Examples

You forgot to specify the spatial reference system of your data or specified it wrong, but you know its WGS 84 long lat

1ALTER TABLE mytable
2 ALTER COLUMN geom TYPE geometry(MultiPolygon,4326)
3  USING ST_SetSRID(geom,4326);

Your data is WGS 84 long lat, and you tagged it correctly but you want it in US National Atlas meters

1ALTER TABLE mytable
2 ALTER COLUMN geom TYPE geometry(MultiPolygon,2163)
3  USING ST_Transform(geom,2163);

You brought your data in as unknown, you know its wgs 84 lon lat, but you want to convert it to US National Atlas meters

1ALTER TABLE mytable
2 ALTER COLUMN geom TYPE geometry(MultiPolygon,2163)
3  USING ST_Transform(
4     ST_SetSRID( geom,4326 )
5       , 2163
6       );