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

[Andre Joost]

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.