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Coordinate Discussions:
The Coordinates
above represent an updated survey from June 26th, 2006. 10
separate dual frequency GPS data files were post-processed using the NGS
OPUS site. Each file was a 12 hour AM or PM logging session
spanning May 1st through May 10th, 2006. The results were
geometrically averaged using Trimble Pathfinder Office. The coordinates encoded as the reference
location in the base files themselves use the NAD83 (CORS96 -
Ep:2002.0000) coordinate system.
The ITRF (2006 Epoch) coordinates
are useful if you're interested in true global geodetics. The
International Terrestrial Reference Frame is updated continuously from
GPS sites all over the world. As plate tectonic action moves the
continents around by a few centimeters per year, the ITRF folks update
the whole coordinate system.
It is often asked what the relationship
between NAD83, ITRF, and WGS84 really is. The NGS WEB Site has a
good explanation: http://www.ngs.noaa.gov/faq.shtml
In short,
if one were to take a GPS receiver,
place it on a point, and let it average
sufficiently long, it would resolve
to a coordinate that we would call WGS84.
However, it doesn't really make sense
to talk about coordinates without specifying
the time epoch involved. The surface
of the earth moves due to plate tectonics
- several centimeters per year in all
different directions. WGS84, as
defined by the broadcast ephemeris of
the GPS, is from a 2002 Epoch
which translates to WGS84 (G1150).
To move coordinates forward and backward
in time (should you wish to get really
technical about your geodetics), use
the NGS Horizontal Time Dependant Positioning
(HTDP) at http://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml
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