[Proj] Difference between Proj4 datums and EPSG database

[Frank Warmerdam]

Erich Sadlowski wrote:
> Hello all,
> 
> using the same epsg number one can get different results for proj4 (e.g. cs2cs) 
> and other programms.
> For example: When I use 31466 (DHDN, Gauss-Kruger zone 2) a datum "potsdam" ist 
> used. This points to "Potsdatum Rauenberg 1950 DHDN". But in the EPSG database 
> Potsdam 1995 ist used. Inspecting the "towgs84" parameters you can find great 
> differences.
> 
> My question is: Why is a different and obsolete definition used in proj4? Why 
> aren't the lines in the epsg file corrected to the "offical" EPSG towgs84 
> parameters - although in the release notes I can find the statement "Upgraded 
> to EPSG 6.11.1 database". 

Erich,

I believe a PROJ.4 user submitted a request that the towgs84 parameter
606,23,413 be used for the "Potsdam Rauenberg 1950 DHDN" datum and indicated
that this was equivelent to EPSG datum 6314 (or GCS 4314).  This relationship
is now hardcoded into the code that converts from EPSG to PROJ.4 format.

An expansion of EPSG:31466 into WKT shows:

PROJCS["DHDN / Gauss-Kruger zone 2",
     GEOGCS["DHDN",
         DATUM["Deutsches_Hauptdreiecksnetz",
             SPHEROID["Bessel 1841",6377397.155,299.1528128,
                 AUTHORITY["EPSG","7004"]],
             AUTHORITY["EPSG","6314"]],
         PRIMEM["Greenwich",0,
             AUTHORITY["EPSG","8901"]],
         UNIT["degree",0.01745329251994328,
             AUTHORITY["EPSG","9122"]],
         AUTHORITY["EPSG","4314"]],
     PROJECTION["Transverse_Mercator"],
     PARAMETER["latitude_of_origin",0],
     PARAMETER["central_meridian",6],
     PARAMETER["scale_factor",1],
     PARAMETER["false_easting",2500000],
     PARAMETER["false_northing",0],
     UNIT["metre",1,
         AUTHORITY["EPSG","9001"]],
     AUTHORITY["EPSG","31466"]]

This does not have any TOWGS84 parameters associated with the datum
Deutsches_Hauptdreiecksnetz, presumably because EPSG publishes more than
one possible solution though I'd have to do some digging to verify that.

If you believe that DHDN / Deutsches_Hauptdreiecksnetz is clearly different
than "Potsdam Rauenberg 1950 DHDN" then please let me know though a review
of the GCS list shows only this one entry with the name DHDN.

Things to keep in mind:
  o when translating EPSG datums to PROJ (or WKT) I ignore towgs84 values
    if the epsg database includes more than one possibility for the datum.
  o a variety of overrides are in place based on user provided input for
    datum towgs84 values - I have no way to evaluate their correctness so
    I just take folks on faith.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org