[Proj] DHDN / Gauss Krueger to WGS84

[Roger Oberholtzer]

On Fri, 2012-03-30 at 12:32 +0000, Mikael Rittri wrote:
> Hermann, 
> you are right, of course.

After further investigation, I have found that the epsg:31468 in proj is
incorrect.

The proper (meaning it works for coordinates used by BASt, which is my
requirement) definition should be (Zone 4):

+proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel +towgs84=582,105,414,1.04,0.35,-3.08,8.3 +units=m +no_defs

The definition of +datum=potsdam as

	+ellps=bessel +towgs84=606.0,23.0,413.0

seems to be the core problem. When I changed that to what is shown
above, I get reasonable values for my test data.

Ignore the fact that my original post had +x_0=3500000 and +lon_0=9.0
instead of +x_0=4500000 and +lon_0=12. I had been trying many
combinations and only posted the most recent. It was only the change to
+towgs94 that made any difference.


Here is another mystery:

I updated my epsg file to my preferred definition. Then, when I use
+init=epgs:13468 in the C API, I get locations that are 10 meters or so
away from when I specify the same values 'by hand' in the C API. My
entry in the file is:

<31468> +proj=tmerc +lat_0=0 +lon_0=12 +k=1 +x_0=4500000 +y_0=0 +ellps=bessel +towgs84=582,105,414,1.04,0.35,-3.08,8.3 +units=m +no_defs <>

Curious, no?


Yours sincerely,

Roger Oberholtzer

OPQ Systems / Ramböll RST

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer at ramboll.se
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