[Proj] HARN nadgrid files

Richard Greenwood richard.greenwood at gmail.com
Thu Dec 22 15:40:23 EST 2016

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?

It seems like pretty low hanging fruit. HARN has been around for about 20
years. The nadgrid files are already there. Differences between
+towgs84=0,0,0,0,0,0,0 and +nadgrids=XXhpgn.gsb are on the order of a meter
in the few places I have looked, so it's a fairly significant correction
(or error, depending on your point of view).  "grep -c HARN epsg" returned
264 and "grep -c 'hpgn' epsg" returned 0, so effects a lot of definitions.

Maybe I'm missing something. I'd be interested in any comments.


Richard W. Greenwood, PLS
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