[Proj] old german topo

[Paul Kelly]

On Sat, 24 Jul 2004, Maciek Sieczka wrote:

[...]
> As to the datums I tried 3 different ones and I'm not sure which one is
> really suitable here. I believe I had a best result with the first of the
> mentioned below. Though the reprojection result was very similar in each
> case (max 1-2 m difference) I would like to know which one is right if
> somebody can tell me.

None of them are "right"; they are all only approximations to the shift 
between the two datums. Number 1 is for central West Germany, number 2 for 
southern west Germany, and number 3 seems to be general for the whole area 
the potsdam datum is used in. However there is also a more accurate 
7-parameter transformation for all of Germany on the CRS website (and also 
in GRASS).
So it all depends on which part of Germany your map covers, and e.g. if it 
is not in an area that any of the widely available transformations covers 
(e.g. former part of Germany that is now in Poland?) then for maximum 
accuracy you would have to derive your own transformation using locations 
of churches as was suggested. But I wouldn't know how to do that...

>
> 1. +towgs84=584.8,67.0,400.3,0.105,0.013,-2.378,10.29
> taken from the
> http://crs.bkg.bund.de/crseu/crs/descr/eu-countrysel.php?country=DE ->
> DE_DHDN / GK_3 -> Transf. -> DE_DHDN (Middle) to ETRS89
>
> 2. +towgs84: 597.1,71.4,412.1,0.894,0.068,-1.563,7.58
> GRASS 5.3, datum: potsdam
>
> 3. +towgs84=606.0,23.0,413.0
> GDAL 1.2.0, potsdam bessel, Potsdam Rauenberg 1950 DHDN
>
> Maciek
>
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