I'm trying to convert a local PCS (city of Buenos Aires) to wgs1984. Since the PCS is not included in Postgis, I added it to spatial_ref_sys using spatialreference.org (I compared the arguments with a .prj I know is right and they match). My coordinates for the PCS are in WKT, so to make my geometry column in wgs I used:

update <my_table> 
set geom = ST_Transform(ST_SetSRID(ST_GeomFromText(<wkt_column>), <PCS_number>), 4326);

Which works at first glance, but the points are off by approx. 50 meters in relation with my other wgs tables.

When I make the transformation using the Project tool on Arcgis, it works perfect. The difference I see is that Arcgis offers me to use different 'Geographic transformation methods' to make the transformation. The one I choose, and it works fine, is 'Campo_Inchauspe_To_Posgar_2007_1 + Posgar_2007_To_Wgs_1984_1'. I suspect that this method is the key to solve this, but in that case I don't have a clue on how to include that method in the Postgis transformation.

I would appreciate a lot any insight on this issue. Thanks in advance.