[OSRS-PROJ] Datum Shifts

[Ed McNierney]

Frank -

Yes, the conus, Alaska, Hawaii, prvi, stgeorge, stlrnc, and stpaul files
make up the NAD27 grid shift set; as far as I'm aware they are the only
files used for NAD27 definitions in PROJ.  The HARN files are used in
bidirectional conversions in NAD83.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
Phone: (978) 251-4242  Fax: (978) 251-1396
ed at topozone.com


-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Thursday, March 13, 2003 4:54 PM
To: PROJ.4 Discussion
Subject: [OSRS-PROJ] Datum Shifts 

Folks,

I am working on implementation of NTv2 support, and as part of that
effort
I am trying to add a small test script for the existing datum support
... try
to make sure I don't break anything while improving things.

In doing so, I have started looking at the files for datum shifting and
realized I am confused about them.

The files conus.lla, alaska.lla, hawaii.lla, prvi.lla, stgeorge.lla,
stlrnc.lla, and stpaul.lla are all apparently files with grid shifts
from
NAD27 to NAD83, right?

The files FL.lla, MD.lla, TN.lla, WI.lla and WD.lla are listed in the
README as being for "high precision conversion of geographic coordinates
from NAD83 to NAD83".  What does that mean?  I presume this relates to
HPGN, which I gather is a higher precision form of NAD83, is that right?
Do these files shift from "old NAD83" to "HPGN NAD83" or the other way?

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


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