[Proj] Time dependent coordinate transformations

Greg Troxel gdt at ir.bbn.com
Tue Jan 6 19:13:27 EST 2015

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

I only read geodesy texts as a hobby, but let me say that I think your
outline of the issues is spot on.

> The question here is whether to have a separate API for transformations that require
> a time, or whether to use the same API and embed the time within the
> feature/coordinate being transformed.

I think there are actually three ways to do this:

  1) put time in the coordinates for each point

  2) construct an object which is a pair: "(datum, time)", and use that
  instead of using datum

  3) manage time out of band, and add time values in transformation

I find (2) to be the closest match to the world; people will still have
coordinates in a datum (be it crust-fixed or ITRF), but we'll be clear
about the epoch of the coordinates.  I would expect that (left coast
aside) the same values in NAD83 as of 2010 and NAD83 as of 2014 to
transform to almost zero difference in WGS84 or ITRF, but for the epoch
of the global frames to make a bigger difference.

It may be that 2 vs 3 is a distinction without a real difference in
changing a small number of procedures.

Another issue is horizontal datums vs 3D datums, which raises the issue
of time-dependent height transforms as well.  But presumably those fit
within the 15-parameter transforms you describe.   It does seem like an
extra source of complexity if a point is described by both a 2D and a
vertical datum, and one wants WGS84.  But, I've convinced myself that
there aren't vastly tricky additional time-dependent issues because of

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