[Proj] Accuracy of results

[Bjoern Ruff]

Hi,

I tried a projection into Gauss-Krueger

proj +proj=tmerc +lat_0=0 +lon_0=9 +k=1.000000 +x_0=3500000 +y_0=0
+ellps=bessel +datum=potsdam +units=m +no_defs
9.27593e 47.69597n
3520709.54      5284121.62

this result differs about 80m in x and in y from the conversion from the
official site:

https://upd.geodatenzentrum.de/cts/server?REQUEST=GetCoordinates&FROMSRS=GEO
84&TOSRS=GK3&COORDS=9.27592 47.69597 
- <CTS_Response version="1.0">
  <SRS name="GK3" /> 
  <COORDS values="3520788.1716 5284220.4243" /> 
  </CTS_Response>


Can someone tell me the reason?

Thanks,
Bjoern

===================================
Bjoern Ruff
http:/www.forstware.com
br at forstware.de