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

Hermann Peifer peifer at gmx.eu
Tue Mar 17 23:39:29 EST 2015

On 2015-03-18 4:53, Even Rouault wrote:
> Selon Hermann Peifer <peifer at gmx.eu>:
>> Here a few observations, from a simple user perspective: (...)
> As an open source code, there is no real "mystery" involved, although it is
> indeed non trivial. The process is decently documented here :
> http://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/README

In my first line, I mentioned the "simple user" on purpose. The one who 
might not know what SVN or trunk is, or who might not speak English in 
the first place (there is machine translation, I know..)

Don't understand me wrong: local/share/epsg is more than fine for 
beginners, but somewhat advanced users might want to have more control 
and stability when it comes to transformation parameters, at least I do.

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.

In summary: local/share/epsg is probably as fine as it can be, I do 
however feel safer when using local/share/hermann.


More information about the Proj mailing list