[PROJ] Calculating DTK25 from WGS84

[Dominik Vogt]

On Wed, Apr 08, 2020 at 12:58:39PM +0200, Even Rouault wrote:
> first, I'm not familiar at all with DTK25 but
> seeing
> http://www.adv-online.de/AdV-Produkte/Standards-und-Produktblaetter/Produktblaetter/binarywriterservlet?imgUid=3ab60d30-b338-9eff-de33-50376a112976&uBasVariant=11111111-1111-1111-1111-111111111111
> it seems that DTK25 products on Bessel ellipsoid use the Potsdam datum

Exactly.

> The corresponding geographic CRS is likely EPSG:4746 "PD/83"
>
> Assuming you have the BETA2007 grid installed,
>
> echo 49.12345 8.55555 | cs2cs -f "%.8f" EPSG:4326  EPSG:4746
>
> 49.12450370	8.55651912 0.00000000

Yes, great, that's what I was looking for.  After upgrading proj
to the latest version this works fine.

> However given the formula you give to compute DTK25 sheet numbers, the influence
> of the datum shift is completely neglectable, unless you would be
> just at the boundary between 2 sheets (but I assume they have some overlap ?)

That's the point of the coordinate transformation.  We do mapping
of plant species, and all findings are reported using quarters of
the DTK25 maps, e.g. "TK 6918/4".  When out in the field, we
usually have a printed map and make sure to stay inside the
official boundaries.  So, the +- 100 m difference actually does
matter.

Thanks a lot!

Ciao

Dominik ^_^  ^_^

--

Dominik Vogt