Issue converting projected coordinates to wgs1984 in Postgis

Question

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.

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My .prj is this:

PROJCS["Argentina_GKBsAs",
GEOGCS["GCS_Campo_Inchauspe",
DATUM["D_Campo_Inchauspe",
SPHEROID["International_1924",6378388.0,297.0]],
PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
PROJECTION["Transverse_Mercator"],
PARAMETER["False_Easting",100000.0],
PARAMETER["False_Northing",100000.0],
PARAMETER["Central_Meridian",-58.4627],
PARAMETER["Scale_Factor",0.999998],
PARAMETER["Latitude_Of_Origin",-34.6297166],
UNIT["Meter",1.0]]

The projection number in spatialreference.org is 8333, and here you can see it in wkt as html for comparison:

PROJCS["Gauss Krugger BA", GEOGCS["GCS_Campo_Inchauspe", DATUM["D_Campo_Inchauspe", SPHEROID["International_1924",6378388.0,297.0]], PRIMEM["Greenwich",0.0], UNIT["Degree",0.0174532925199433]], PROJECTION["Transverse_Mercator"], PARAMETER["False_Easting",100000.0], PARAMETER["False_Northing",100000.0], PARAMETER["Central_Meridian",-58.4627], PARAMETER["Scale_Factor",1.0], PARAMETER["Latitude_Of_Origin",-34.6297166], UNIT["Meter",1.0]]

Here is the Postgis insert statement:

INSERT into spatial_ref_sys (srid, auth_name, auth_srid, proj4text, srtext) values ( 98333, 'sr-org', 8333, '+proj=tmerc +lat_0=-34.6297166 +lon_0=-58.4627 +k=1 +x_0=100000 +y_0=100000 +ellps=intl +units=m +no_defs ', 'PROJCS["Gauss Krugger BA",GEOGCS["GCS_Campo_Inchauspe",DATUM["D_Campo_Inchauspe",SPHEROID["International_1924",6378388.0,297.0]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",100000.0],PARAMETER["False_Northing",100000.0],PARAMETER["Central_Meridian",-58.4627],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",-34.6297166],UNIT["Meter",1.0]]');

So, I have one point in wgs:

-58.47532313, -34.63847446

And I make the geometry (Point, 4326) in one table

Then I take the same point in the PCS (8333):

POINT(98897.6171875 98978.84375)

And I make the geometry (Point, 4326) out of the following transformation:

update test_t 
set geom = ST_Transform(ST_SetSRID(ST_GeomFromText(coord), 98333), 4326);

I get this when I pick the point in Qgis:

Being the green point the correct one (the one originally in wgs), and the red the misplaced one (the transformed).

Answer 1

The spatial_ref_sys entry that spatialreference.org gave you was incomplete. It did not include the data to do the transformation from the Campo Inchauspe GCS to WGS84. Here's an entry that includes the transform:

INSERT into spatial_ref_sys 
(srid, auth_name, auth_srid, proj4text, srtext) 
values ( 98334, 'sr-org', 8333, 
'+proj=tmerc +lat_0=-34.6297166 +lon_0=-58.4627 +k=1 +x_0=100000 +y_0=100000 +ellps=intl +units=m +towgs84=-148,136,90,0,0,0,0 +no_defs ', 
'PROJCS["Gauss Krugger BA",GEOGCS["GCS_Campo_Inchauspe",DATUM["D_Campo_Inchauspe",SPHEROID["International_1924",6378388.0,297.0]],TOWGS84[-148,136,90,0,0,0,0], PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",100000.0],PARAMETER["False_Northing",100000.0],PARAMETER["Central_Meridian",-58.4627],PARAMETER["Scale_Factor",1.0],PARAMETER["Latitude_Of_Origin",-34.6297166],UNIT["Meter",1.0]]');

Using the entry above, I can reproduce your good geographic coordinates from your PCS point.

select st_astext(st_transform('SRID=98334;POINT(98897.6171875 98978.84375)'::geometry,4326));

POINT(-58.475323109322 -34.6384744452138)

I found the TOWGS84 transform parameters by looking for entries in spatial_ref_sys that had "Inchauspe" in the srtext until I found one that had the same datum as your PRJ file had.

In general if you're seeing a transform that is almost-but-not-quite right (like your 50m drift), looking for problems in the datum transform is your most likely solution.