[Proj] HARN nadgrid files

[support at mnspoint.com]

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. 

Other questions with grid shift files are: 

How much their usage slows down the end product that uses Proj.4
library? 

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

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. 

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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