[MetaCRS] Fwd: Re: [Proj] Time dependent coordinate transformations

[Chris Crook]

On the subject of HTDP transformations below:

> > > > It seems fairly straightforward to expand the definitions of coordinate
> > > > systems in PROJ and the EPSG registry to include additional parameters.
> > > > The to_wgs84 parameters could readily be extended to include the 14
> > > > parameter plus reference epoch time dependent Bursa Wolf
> >>
> > > transformation.
> > >
> > > > Some datum transformations already include nested grids, so extending
> >>
> > > this
> > >
> > > > to include grids with associated time functions also seems feasible.
> >>
> > > Agreed although in the case of HTDP, the production of grids result from
> > > the result of a (apparently complex) computation process where you have
> > > to provide
> > > src_crs, src_crs_date, dst_crs and dst_crs_date. Regarding proj.4 one
> > > could have a fixed (dst_crs, dst_crs_date) by convention, but one would
> > > still have varying src_crs_date for a given src_crs. Perhaps
> > > pre-generate grids for each year and linearly interpolate between them ?
> >
> > I believe that there is just one one date for these transformations.
>
> I meant that for HTDP, my impression with my limited testing is that you can
> actually specify one date for each of the both CRS and it will use each one,
> presumably using some pivot as a reference, to generate the final grid.
>
> Transforming from NAD83(2011) at T=2014.0 into WGS84(G730) at T=1991.0 gives a
> different result from NAD83(2011) at T=2011.0 into WGS84(G730) at T=1991.0, and
> different from NAD83(2011) at T=2014.0 into WGS84(G730) at T=1986.0

While this may give different results, it is not at all clear to me why it should give different
results.  The question here is what is it supposed to be doing I guess.

Perhaps this difference between HTDP and the deformation in New Zealand is that in NZ
the deformation model is defined to be part of the datum.  In effect it is a function which
defines the transformation from ITRF96 at any epoch to NZGD2000.  It isn't considered or
used as a function for computing NZGD2000 at arbitrary epochs - any such coordinates
would not be NZGD2000 at all.

By contrast HTDP transforms coordinates between different epochs (or more accurately,
predicts the moment of objects fixed to the crust over time).  But there is not a
well defined commonly used reference epoch that is used to represent coordinates, and
this deformation is not part of the datum.  NAD83 is in this sense more like a global
datum.   And features that are fixed to the crust in the western US do not have fixed
coordinates.

I wonder how this is managed by GIS users?

> > This
> > is not about predicting where a feature might be at another time, it is
> > about transforming between coordinate systems which are related by a time
> > dependent function.  Of course if the feature is considered to be fixed in
> > one of those coordinate systems then the effect of transforming at
> > different dates is to track it's trajectory in the other.
> >

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