[Proj] Proj4 and Epoch

[Greg Troxel]

Chris Crook <ccrook at linz.govt.nz> writes:

First, let me say that your note was from my viewpoint an excellent
discussion of the issues.  In particular the point about the difference
between velocities of the frame and velocities of particular stations
adds a level of complexity that I wasn't thinking about.

But the biggest thing I realized is that there don't seem to be EPSG
codes for the different WGS84 realizations, so the first agenda item is
to resolve whether to push EPSG to do that or to model them outside of
EPSG or to decide that all WGS84 realizations are equivalent for proj
purposes.

> I think the following doesn't work ...
>
>> So, I suspect you have to transform to the standard epoch in some frame,
>> and then to another frame, and then to the epoch you want.
>
> The reason is that it is the conversion between frames that is time dependent.

Yes, I was trying to take that into account, and to let someone use proj
for part of the work, but do velocities of each frame separately.

What I meant as an example

  Given coordinates in NAD83(2011) epoch 2016.0, transform them
  (somehow, correctly) to NAD83(2011) epoch 2010.0.   This gets you
  coordinates that proj just calls NAD83(2011).

  Then use proj to transform to ITRF2008, which I think implies epoch
  2005.0 as the standard epoech.

  Then transform ITRF2008 epoch 2005.0 to ITRF2008 epoch 2016.0

This is probably not right because it blurs the frame motion vs ground
motion issues, and I think the conclusion is that this sort of geodetic
pickyness is beyond what proj is currently trying to or succeeding at
doing.



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