[OSRS-PROJ] Datum Shifts

[Norman Vine]

Frank Warmerdam writes:
>
> 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?


See

http://www.ngs.noaa.gov/faq.shtml#WhatHARN
http://www.ngs.noaa.gov/TOOLS/Nadcon/Nadcon.html
ftp://ftp.ngs.noaa.gov/pub/pcsoft/nadcon/Readme.txt
http://www.ngs.noaa.gov/PUBS_LIB/NGS50.pdf

HTH

Norman



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