[Proj] HARN nadgrid files

Richard Greenwood richard.greenwood at gmail.com
Tue Dec 27 16:08:55 EST 2016

On Mon, Dec 26, 2016 at 11:47 PM, <support at mnspoint.com> wrote:

> Hello,
> It looks like the reasons why grid shift files are not much used or
> referenced is that usually there are other ways available to fix small map
> errors ... and using grid shift files would require too much knowledge in
> the subject ... and also give too much unrequired accuracy since the source
> material is already so much off the line. So most users and applications
> are happy with the towgs84 mechanism.
I don't think most users have any idea what datum transformation is being
used. But they do expect results that match other geospatial software.

> Other questions with grid shift files are:
> How much their usage slows down the end product that uses Proj.4 library?
 I'd venture to guess that grid transformations are computationally less
expensive than 7 parameter transformations.

> How difficult their usage is for end users to be realistic that they ever
> used them?

I'm proposing changes only to the "epsg" file that is a part of the Proj.4
distribution.  However this would require that the hpgn grids here
https://trac.osgeo.org/proj/wiki/HarnGrids be included in the Proj.4
distribution. They are 354kB zipped. There are at least two grid files in
the nad/ directory that are already part of the distribution. But as time
goes by we see more, and larger, grid shift files. So I can see some
resistance to adding more.

> How could the above questions (or problems) be fixed the most easy way
> (for the end user)? Meaning that adding more good code to Proj.4 and/or
> choosing better data formats instead of writing more complex manuals would
> be preferred.
Even Rouault suggested adding code that would use the +nadgrids definition
if the required grid shift file was present, otherwise the +towgs84
definition would be used.

> Janne.
> ----------------------------------------------------------------------
> Richard Greenwood kirjoitti 2016-12-22 22:40:
> 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.
> Rich
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Richard W. Greenwood, PLS
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