I have a number of points in Gauss-Kruger, Zone 3 that I want to convert to display them within Google-Earth. Assuming WGS84 for Google-Earth the transformed coordinated differ from the original ones about 140m to the north-east.
I am doing the coordinate transformation using the "PROJ.4 – Cartographic Projections Library" (http://trac.osgeo.org/proj/) with the following proj4 strings:
Gauß Krüger, Zone 3:
+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
WGS84:
+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs - "
Could somebody check if my proj4 strings are correct (especially WGS84 for Google Earth) or suggest other causes of the error?
Best Answer
Your Gauss-Krueger projection uses
+datum=potsdam
. Up to 2012, this was hard coded in proj4 to a very unprecise value using a 3-parameter-transformation.You find more exact values for 7-parameter transformations in this topic:
http://forum.openstreetmap.org/viewtopic.php?id=12723
There is an even better ntv2-grid transformation available here (take the binary), that has to be in the same folder as your application and data, unlsss you specify full pathnames.
To compare the different possible values, I made this test batch file:
with any sample Gauss-Krüger lon-lat coordinate pair in 31467.txt