[Proj] HARN/HPGN

[Frank Warmerdam]

Folks,

At the request of a client, I have developed some tools to translate
between the .los/.las format used for datum grid shift files distributed
by NOAA for NADCON to NTv2 for convenient use with PROJ.4.  This was
done primarily to make use of the NOAA HARN datum shift files practical.
This has gone ok.

However, now that I look at utilizing the data I find that there are
distinct HARN grid shift files for each state.  On examination I find
that they overlap, and that the overlapping areas do not agree.  I read a
bit of material from NOAA/NGS which seems to suggest each state does it's
own thing.

When I look in the EPSG database, there seems to be only one GCS mentioning
HARN which is EPSG:4152 NAD83(HARN).

The online HARN conversion tool on the NGS site requires you to pick a
state grid file.

So my question is basically - is there really a distinct HARN datum for
each state?  Does it make sense to treat HARN as a datum, or should it be
treated as ~52 datums?

I'd appreciate a bit of educating on this...

PS. If you want the NGS HPGN files in NTv2 let me know.  I'm also hoping
the NTv2 writing capability I have integrated into GDAL will be useful
to convert other ideosyncratic datum grids into a common format.  It also
allows easy examination of NTv2 files in GDAL based applications.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent