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.

It is pretty easy to correct SRID mistance using the standard ALTER TABLE commands. Here are a couple of scenarios.

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

ALTER TABLE mytable
 ALTER COLUMN geom TYPE geometry(MultiPolygon, 4326)
  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:

ALTER TABLE mytable
  ALTER COLUMN geom
   TYPE geometry(MultiPolygon, 2163)
  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:

ALTER TABLE mytable
  ALTER COLUMN geom
  TYPE geometry(MultiPolygon, 2163)
  USING ST_Transform(ST_SetSRID(geom, 4326), 2163);