[PROJ] Static/Dynamic Webmapping Problem version 2.0

Duncan Agnew dagnew at ucsd.edu
Wed Jul 17 11:35:46 PDT 2019

Just to add to this discussion, I note that the NGS HTDP program assumes
the following:

WGS84(G730) is equivalent to  ITRF91
WGS84(G873) is equivalent to  ITRF94
WGS84(G1150) is equivalent to  ITRF2000
WGS84(G1674) is equivalent to  ITRF2008

and the transformations between these ITRF's are at the 1-2 cm level at
most, and are a few mm for the latest frames. I'd note that the later ITRF
frames now all have an epoch and velocities for at least some things:
again, not important at the 0.1 m level (and probably the 0.01m level as

So the changes in WGS84 are, for just about any application, irrelevant.

The problem is *entirely* that WGS/ITRF is tied to a global everage of
plate velocities, not fixed to any plate, so repeated measurements over the
same ground point in those systems will show changing coordinates because
of motion of whatever plate it is on.

My understanding is that both GDA2020 and GDA94 are "snapshots" of the
ITRF, with different transformation parameters to allow for plate motion.
So if you start out in 1994 and keep measuring the same point in GDA94, its
coordinates will change and get increasingly far from their initial value.
In GDA2020 they will also change with time, but instead will approach the
GDA94 numbers measured in 1994, until in 2020 they will be the same--but
then start diverging again. The point is that the coordinate values
measured at a place in 1994, expressed in GDA94, will match the values
measured at the same place in 2020 expressed in GDA2020. Neither is
plate-fixed.  There will be a time-dependent reference frame known as the
Australian Terrestrial Reference Frame (ATRF) that is plate-fixed: in 2020
it will match GDA2020.

I continue to prefer "plate-fixed" to "static" because it is immediately
obvious what is meant; "static" clearly means that something doesn't
change, but what, the frame or the coordinates of a ground point?

Duncan Agnew
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