AW: [Proj] DHDN / Gauss Krueger to WGS84

[uwe.schmitz at lverma.nrw.de]

Oliver,
> 
> Oliver Eichler wrote:
> >> This may or may not be the best datum shift parameters for 
> your area of
> >> work.  You may need to research a better local shift.
> >>
> > Does that mean that the shift parameters depend on the 
> location within
> > the map? Or that the whole map might have some kind of bad offset
> > produced by a bad projection?
> > 
> > And how would I get better shift parameters? Is there a 
> tutorial somewhere?
> 
> Oliver,
> 
> The 3/7 parameter +towgs84 parameters are just used for an approximate
> transformation between datums. Sometimes, especially for old 
> datums, the
> differences are quite irregular over the area of the datum 
> and are not well
> represented by a simple transformation.
> 
> So, you will often see datums having various towgs84 
> parameters, some which
> might be a decent general transformation over a large area 
> and others which
> might be a more accurate transformation in a fairly local area of the
> datum.
> 
> Ugly, isn't it?
> 
> No, there is no obvious way to get better parameters other 
> than researching
> various source (ie. national mapping agencies, other 
> publications, ask other
> users in your region).

please take a look at 
http://crs.bkg.bund.de/crseu/crs/descrtrans/BeTA/de_dhdn2etrs_beta.php

There you will find information about a "standard" solution 
for your problem. The documentation gives a brief example
for using PROJ.4. (I think, the german language isn't a problem
for you :-)).

If you have any problems, please let me know.

Best wishes
Uwe
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