[Proj] why doesn't the epsg file reference the hpgn grids?

Andre Joost andre+joost at nurfuerspam.de
Wed Mar 18 12:36:05 EST 2015

Am 18.03.2015 um 05:39 schrieb Hermann Peifer:

> I also did not mention that the meaning of e.g. +datum=potsdam changed
> over the few years I have been using PROJ. The potsdam datum seems to be
> hard-wired into pj_datums.c.
> $ grep potsdam pj_datums.c
> "potsdam",  "towgs84=598.1,73.7,418.2,0.202,0.045,-2.455,6.7", ...
> I assume that earlier changes to pj_datums.c were yet another downstream
> processing step. I can only assume, as this isn't mentioned in the
> indicated README.

The change of the potsdam datum was a follow-up of this openstreetmap 
forum topic: http://forum.openstreetmap.org/viewtopic.php?id=12723

Since high-resolution aerial image was georeferenced with a better 
transformation than the 3-parms used before, there was a need to adjust 
the proj.4 datum definition.

BTW: In the same file you find the only predefined usage of datum shift 
grids, NADCON/CONUS for the US and ntv2 for Canada.

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