Epochs associated with a deformation model

[ccrook]

@rstanaway comments:

Another point of discussion is the use of the terms "epoch", "reference epoch", and "observation epoch". With ITRF realisations, the coordinates of the frame and their uncertainties are defined at the reference epoch, with associated velocities and uncertainties. As ITRF evolves with different realisations, the uncertainties are to some extent reset for each realisation. In the case of crust-fixed reference frames such as NZGD2000 there's an interesting discussion about what exactly is a "reference epoch" in the context of a crust-fixed frame. From my understanding, NZGD2000 and its associated deformation model are monitored within ITRF and every now and then re-adjusted based on the current version of ITRF. The network adjustment is done in a strain free ITRF system close to the "observation epoch" at the time of the adjustment. In NZ's case, this is potentially 20+ years since the crust-fixed RF was realised. As patches are applied to NZGD2000 for seismic displacement updates, the usage of "reference epoch" can become misleading in the sense that the frame as realised in 2000.0 is not the same as the frame defined by the latest version. This of course impacts on how the velocity model is used for NZGD2000 to ITRFyy transformations at other (later) epochs and how uncertainties are estimated using standard error propagation. Since, 2000 the ITRF connection and velocity models have become more refined. So, the uncertainty should realistically be estimated from the observation epoch to another epoch. If 2000.0 is used in the time function of the DMFM, what should we call it (2000.0 that is)? Also, it seems unrealistic to propagate the uncertainty back to 2000.0 in the case of NZGD2000.

[ccrook]

This is covered in the functional model straw man

@rstanaway would you be ok with closing this issue in github. We may come back to this discussion when we look at the formulae associated with calculating deformation and uncertainty.