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

[Hermann Peifer]

On 2015-03-18 2:59, Richard Greenwood wrote:
>
> Does the proj epsg file get created from data at www.epsg.org
> <http://www.epsg.org>? And is that data lacking the reference to the
> grid shift files, or is it "lost in translation"? It seems like there is
> the wonderful resource in the harn grids that isn't getting used, but
> maybe I'm overlooking something.
>

Here a few observations, from a simple user perspective:
There is a somewhat complex (mysterious?) epsg-to-csv conversion process 
with "upstream" elements around libgeotiff and "downstream" locations 
near PROJ.4, which results in ever surprising changes in the generated 
lookup files.

The changes are partly based on EPSG database updates (which is fine, of 
course) but also on changing conversion logic: about which parameter set 
to select in case of several available, which numeric precision to 
choose when converting radians from EPSG into arc seconds used in 
towgs84 values, and so on.

There are plenty of helpful grid shift files listed at 
http://trac.osgeo.org/proj/wiki, but PROJ.4's epsg lookup table lists 
none of them, only the null grid appears there:

$ grep nadgrids local/share/proj/epsg
<3785> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 
+y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs <>
<3857> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 
+y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext  +no_defs <>

There haven't been many releases of PROJ lately, which brought some 
stability into coordinate transformations, as the positive side-effect 
of an otherwise sad story. However, this is expected to change and what 
I did anyway some time ago was to add my own lookup table as 
local/share/proj/hermann in order to gain control over my own 
transformations.

Now I can use: +init=hermann:4314, which resolves to my transformation 
parameters (+proj=longlat +ellps=bessel +nadgrids=BETA2007.gsb), whereas 
epsg:4314 translates to: +proj=longlat +datum=potsdam +no_defs

Hope this helps, Hermann