NAV 44/7
ANNEX 1
THE INTERNATIONAL TBRRESTRIAL REFERENCE
FRAME
In 1994 the United States
Defense Mapping Agency (DMA) slightly changed the original
WGS 84 reference datum to agree with the widely accepted
International Earth Rotation Service Terrestrial Reference
Frame (ITRF). Despite changes to the precise ephemeris and
co-ordinates set for the global GPS monitor stations the
effect on the maritime navigation community was
insignificant.
But the movement of the Earth's
crust (over 10 cm/year in some places) must be allowed for
and can be detected within the ITRF datum. The Intemational
Earth Rotation Service computes annual ITRF positions and
velocities for many stations covering the world's surface.
WGS 84 co-ordinates should
strictly therefore be qualified as LAT ... LONG...(Framework:ITRF
93, at epoch 1996.3650).
Consideration might therefore
be given to replacing the term WGS 84 in IMO documentation
with WGS 84 (ITRP 93). Similarly the Russian Federation might
like to qualify PZ 90.
Actual figures for Hong Kong,
China provided by Nick Emerson of the Hong Kong Hydrographic
Office are:
| WGS 84 Position in
1991 |
|
|
WGS 84 Position in 1996 |
| |
|
|
|
| LAT XN |
|
|
LAT X+.002158 N |
| LONG Y |
|
|
E LONG Y + .012007" E |
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