[Proj] Accuracy Cassini Solder -> lat lon 200m?!

[Sebastian Schubert]

Dear all,

* On Friday 03 July 2009, 11:39:31, Jean-Claude Repetto wrote:
> Sebastian Schubert a écrit :
> > I want to convert from Solder system used in the city of Berlin
> > (EPSG:3068) to the standard lat lon system.
>
> What do you mean "standard lat lon system" ? If you mean WGS84, you
> should try :
> cs2cs -v +init=epsg:3068 +to +init=epsg:4326

I would like to understand this a bit more. I know this is probably not 
the perfect place to ask but I do not know where to ask. I would be 
also very happy to be given a hint where to find the solution myself.

As far as I understand, the standard settings, I applied before:

schubert at linux-8ubn:~> cs2cs -v +init=epsg:3068
...
# ---- To Coordinate System ----
#Lat/long (Geodetic alias)
#
# +proj=latlong +datum=potsdam +ellps=bessel +towgs84=606.0,23.0,413.0

refer to the EPSG::6314 Deutsches Hauptdreiecksnetz. The not vanishing 
values of towgs84 show that this is not wgs84. This gives the same 
results as the formulas found in the epsg guide
http://www.epsg.org/guides/G7-2.html
when using the parameters for the ellipsoid given in the epsg database 
for EPSG::6314 Deutsches Hauptdreiecksnetz.

However, I thought that when I replace these parameters with the ones of 
wgs84, I should actually get wgs84 coordinates. This does not seem to 
be true. Does this mean that eg the ellipsiod has another centre? So 
the formulas given in the guide are only applicable for exactly one 
Base Geodetic CRS (in this case DHDN)? So proj does a two step 
conversion when I want to convert to wgs84: convert to Base Geodatic 
system, then convert to wgs84? Are the formulas for the latter given 
somewhere?

Thanks a lot!
Sebastian
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