[Proj] cs2cs: Gauss-Krüger to WGS84

[Martin Trautmann]

On 12-12-10 0:41, Jose Gonçalves wrote:
> Hello
> 
>      cs2cs +init=/usr/local/share/proj/world:gk4-d +towgs84
>     the result is
>      56d30'44.77"E  39d38'19.023"N 0.000
> 
> 
> This use of cs2cs is not correct. You must run something like:
> 
> cs2cs /string_for_system_1/ +to/string_for_system2/

That's not what I did? I thought that
	+init=/usr/local/share/proj/world:gk4-d
would be system1 and that
	+towgs84
would include the system2 part.

> The +towgs84=... is the part of a string that you use to give datum
> parameters of that system

Ah, :-(

>     That's far away. Another online example names:
>      cs2cs +init=epsg:31466 +to +init=epsg:4326
> 
>     which returns even worse:
>      32d6'20.91"E   46d39'54.638"N 16.676
> 
> 
> The EPSG code 31466 is for GK zone 2. Your X coordinate is very large,
> so it is more likely that the point is in a different zone, probably 4,
> which has a false easting of 4500 km. In that case you get:

I had missed 314666 vs. 31468, although I used gk4 above.

> echo 4491957.175 5511210.407 | cs2cs +init=epsg:31468 +to +init=epsg:4326
> 11d53'13.028"E  49d44'14.871"N 48.659 

That's close enough to
 11.88695229	49.73746425 (here)
 11.88695	49.737475   (from a known opposite conversion)

But what's the third part of the output line? I did not mind as long as
it was 0.000, but I do not see it documented.

> At least this point is in Germany... The datum transformation parameters
> used are the following:
> +towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7

That's done automagically? Or do I have to apply this anywhere?

I'm just lost in the docu, where to find the epsg ids, how to apply
towgs84 and and how to read the output.

Thanks!
Martin