[Proj] Google Earth Accuracy

[Duncan Agnew]

Since I am in the crustal-motion business I want to correct the previous
posting about ITRF. This is "space-based" only in the sense that it uses
data from outside the Earth (GPS and other satellites, along with 
distant
radio sources).  It was not designed with space use in mind. Since ITRF 
and
WGS84 are both space-based in this sense, they align closely: after all,
the center of the Earth and the rotation pole are both non-arbitrary.

ITRF was designed to be an ultraprecise reference frame for measuring 
motions
of the Earth as a whole, and of different parts relative to each other. 
Since
the frame is designed to be fixed relative to the velocities of all the
tectonic plates, averaged over the whole Earth, any particular point 
will
have time-varying coordinates in this frame--as is is in fact true for 
any
frame not tied to a particular plate (WGS-84 included), though many 
softwares
do not take account of this. The people who develop and update ITRF are
working at the millimeter level.

Two "plate-fixed" frames are NAD (at least east of the Rockies), and 
EUREF
(for Northern Europe). Rather weirdly, Australia has chosen not to 
define
a local frame, even though it has a plate all to itself.

For more on ITRF, see http://itrf.ensg.ign.fr/

Duncan Agnew