[PROJ] [gdal-dev] Static/Dynamic datum problems

[Duncan Agnew]

All:

        As a geophysicist who studies crustal deformation, I'll weigh
in and say that the issue is not (I think) the dynamic nature of the
datum, but plate tectonics.

        Taking WGS84 (of various dates) to be matched to the various
releases of the ITRF, I'd say that the latter is as fixed as it is
possible to make it: origin at the Earth's center of mass, Z axis
to the Conventional International Origin, X axis defined so the X-Z
plane is parallel to the local vertical through Greenwich (to maintain
continuity of Universal Time). The ITRF deals with plate tectonics
by including a model of what the plate motions are, and then chooses
a variation of orientation that makes the these motions,
averaged over the Earth, zero: as close as we can get to "average
Earth motion". As the data series get longer and the coverage better,
there are new releases, but the differences between them are small.

        What isn't small is the plate motion relative to this system:
in it, a location on the Australian plate moves northeast at the rate
of of 1.8 m from 1994 to 2020. This isn't because of updates to the
datum, it is because the ITRF coordinate system is designed to be fixed
relative to the average Earth. That is, the datum, and the coordinate
system, is fixed, but the location of a point on the ground in that
system varies with time.

        The only way around this, so far as I know, is to include dates
with coordinates and have some code that will allow you to convert
measured coordinates between epochs. For Australia, Europe, and much
of North America this is just a time-dependent shift that is nearly the
same everywhere: on a plate boundary it is a lot more complicated, as
is shown by the NGS HTDP program, which has lots of grids and allowances
for earthquakes.

        In summary: the different releases of the WGS/ITRF are different,
but this doesn't make them "dynamic datums": they are fixed, to the
the Earth on average, but relative to that different parts of the Earth's
surface are moving. Not a new problem, but one that complicates things.

        Hope this clarifies rather than muddies.

Thanks
Duncan Agnew
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