[Proj] HARN nadgrid files

[Even Rouault]

On jeudi 22 décembre 2016 13:40:23 CET Richard Greenwood wrote:
> I've been following a few of the recent threads on this list regarding
> enhancements to Proj's datum transformations. Pretty exciting stuff!
> 
> One group of transformations that seems to have been ignored for a number
> of years is applying the HARN hpgn grids in the Proj epsg file. For example
> the Proj epsg entry for epsg:3758 uses +towgs84=0,0,0,0,0,0,0 but I believe
> that is should use +nadgrids=wyhpgn.gsb, or at least that's what I change
> it to, which produces results that match NADCON.
> 
> Is anyone aware of a reason that nadgrids are not referenced in the epsg
> file? Is it maybe just an oversight, something that nobody ever got around
> to? Or because distributing nadgrids would become burdensome?

Richard,

I don't think we would want to force people using grids to be able to use EPSG codes 
(because of the required space or licensing terms), but we could generate +nadgrids=@bla 
syntax to make them optional. What could be useful is to have both +nadgrids= and 
+towgs84 with the priority given to the grids when they are present but this isn't 
implemented right now in proj.4 (if you have both +nadgrids and +towgs84, I believe 
+nadgrids is always used, even if the grids are missing on the system)

As far as adding the grid info, in the current workflow, this would be a matter of doing that in 
the https://svn.osgeo.org/metacrs/geotiff/trunk/libgeotiff/csv/build_pcs.py script, 
presumably adding a new column in the gcs.csv file, and some logic/dictionary to match the 
datum name with a grid name, and then updating GDAL to use that new information when 
reading the .csv files.

Even

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