[Proj] DHDN / Gauss Krueger to WGS84

[Frank Warmerdam]

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).

Best regards,
-- 
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