converting geometry of SRID 0 to geography

Question

My geometry columns have SRID of 0. Can I simply convert them to geography as such:

routes.geom::geography <-> pc.geom::geography as distance

in order to get "distance" in metres?

P.S - "routes.geom" is a linestring column, and "pc.geom" is a point column.

here is a snippet of how it is used, (extracted from my main query):

    FROM point_center as pc
    
    CROSS JOIN LATERAL (
      SELECT routes.id_num, routes.geom::geography <-> pc.geom::geography AS dist 

Answer 1

A GEOGRAPHY type instantiation assumes EPSG:4326 by default, so you can simply cast your GEOMETRY types, and even get correct results if they actually are referenced in EPSG:4326.

---

However, it is highly advisable to add the correct SRID to the GEOMETRY column and values:

ALTER TABLE <table>
  ALTER COLUMN geom TYPE GEOMETRY(<TYPE>, <SRID>)
    USING ST_SetSRID(geom, <SRID>)
;

-- VACUUM ANALYZE <table>;

Note:

A CAST to GEOGRAPHY will render the respective statement unable to utilize a spatial index on the GEOMETRY column; in order to benefit from an index lookup where possible, add a functional index on the actual CAST:

CREATE INDEX ON <table> USING GIST ( (geom::GEOGRAPHY) );

-- VACUUM ANALYZE <table>;
;