[Proj] old german topo

[Maciek Sieczka]

"Paul Kelly" <paul-grass at stjohnspoint.co.uk> wrote:

> 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).

First I'd like to explain myself. Before I got to understand that there are
different datum transformation parameters available for Potsdam datum I have
had propably accidently selected the "2." mentioned below when setting up
the mapset in Grass. After that I got to the
http://crs.bkg.bund.de/crseu/crs/descr/eu-countrysel.php?country=DE,
understood a bit more and came to conclussion that "1." would be better (not
"right" :), ok) in my case - my maps are within 50°20'N - 52°20'N. And after
your message I finally noticed different Potsdam datum paramteres available
in Grass. Briefly that's why I was, in error, reffering to the "2." as to
the "GRASS 5.3, datum: potsdam". Sorry for 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

> 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?)

Yes, former part of Germany, now in Poland, circa 50°45'N, 15°30' - 15°50'E.

> 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...

Then me neither for sure.

I've noticed that all the German/Bessel 1841 systems on the CRS site
mention slightly different ellipsoid inverse flattening than Grass:

Grass 5.3: 299.1528128
CRS:        299.15281285

Propably not a big difference but how big anyway?

One thing more about the projection I've remembered that the guy I got the
map from mentioned the name "von Müffling". He seemed to know about
the cartography even less than me (would you believe it?) but according to
him the projection was "multilateral projection elaborated by general major
von Müffling" (my own translation from Polish). I treid a brief search about
this general but nothing. Is it telling anything to anybody?

Maciek