The value you get there is most likely in decimal degrees...and thus quite useless; a degree doesn't represent the same ground distance in different latitudes and getting from there to metric values is tricky and unnecessary.<br><br>
First off, you should always define a SRID for your geometries. You can, for example, run *ogrinfo* on the shapefile, the CRS is usually defined in the metadata.<br>
Now, since *ST_Distance* returned decimal degrees, it's definetely referenced in a geographical CRS. Possibly the direct and most precise way then is to cast your *geometries* to *geography* type, i.e.:

    ... max(ST_Distance(dump.geom::geography, ST_Centroid(t.geom)::geography)) AS distance ...

This should return values in meter, based on an unknown datum, but calculated on a spheroid and possibly still quite precise, despite the missing CRS. Which is no excuse for not having tried harder to find one!