[Proj] Time dependent datum transformations

[Nick Mein]

Hi all,

Scott mentioned "treating WGS84 as a 'static' or 'plate-fixed' datum in the
"+towgs84 approximation error" thread. I'll take that as my segue into the
topic of dynamic datums and time dependent datum transformations.

There is a good introduction/overview of the topic in
http://ceur-ws.org/Vol-1142/paper6.pdf. This was written from an Australian
and New Zealand perspective, but the general issues are global. There are
"International Case Studies" of USA and Great Britain.

Mike Craymer has written some nice accessible material on this topic as
well. See, for example, http://www.naref.org/transf/nad83_hydroscan2006.pdf.

As a community, we have managed to largely ignore the issue of dynamic
datums for a long time. As precise point positioning technology becomes
more and more available, and more and more users are able to access cm
level positions in the ITRF, we are not going to be able to continue to
ignore this issue.

Generally, there are two classes of challenges that we face with.

Firstly, we need to store the epoch of measurement with any position that
record - if that is not implicit in the definition of the reference frame
being used. That isn't a trivial challenge, if our database and/or our
software architecture assumes that a coordinate has only 3 dimensions.

Secondly, we need to use that epoch to perform a time dependent datum
transformation.

In general, transforming coordinates from ITRF to a plate-fixed datum will
require a 14 parameter datum transformation, and coordinate propagation to
the reference epoch for that datum.

The 14 parameter tansformation is straight-forward (as long as you know the
measurement epoch).

Coordinate propagation is not so straight-forward.

In GDA94 the coordinate propagation is combined with the 14 parameter datum
transformation. But Australia is the "lucky country", geodetically speaking.

An Euler pole approach works well if you are not close to a plate boundary.

For places like California or New Zealand you need a more sophisticated
velocity/displacement/distortion model.

So finally, a question for the Proj-4 community, and a plea to the wider
geodetic community:

To the Proj-4 community: Are there plans to add time-dependent datum
transformations to Proj-4? (Or is that support already there?)

To the wider geodetic community: Are we able to work together on
developing/defining standards for representing and sharing
velocity/displacement/distortion models, rather than having a "tower of
Babel" where every national authority develops and publishes a slightly
different model, and every software developer has to try to implement that
model.

Regards,
Nick.
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