[OSRS-PROJ] Datum Shifts

[Clifford J Mugnier]

Frank,

You are correct regarding the HARN interpolation files.  They may be used
to go in either direction with the original NAD83.  Note that NTv2 has a
variable mesh grid because of the great expanses of Canada that had zero
control points.  NADCON on the other hand, has a constant mesh grid.  I
have not compared the bi-linear interpolation code between NADCON and NTv2,
so I don't know if "one size fits all."

I would guess that one can convert in either direction for Canada also.

Cliff Mugnier
LOUISIANA STATE UNIVERSITY
------------------------------------
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,

--
---------------------------------------+--------------------------------------
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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