[PROJ] Transforming from epsg:4326 to epsg:31468 produces a 2 meter offset

[Greg Troxel]

Javier Jimenez Shaw <j1 at jimenezshaw.com> writes:

> That relates to a question I have:
>
> The conversion from ITRF2014 to NAD83(2011) has velocities. What should I
> set in the 4th dimension converting with proj?

The traditional view is that ITRF2014 is a dynamic datum, that stations
have velocities and you need to specify the epoch of data but that NAD83
is a static plate-fixed datum.  However, NAD83 really has velocities
too, just about 10x smaller than the same station's ITRF velocities (on
the NA plate; as always YMMY in CA).  People don't worry about this much
in practice because they are small, but also because NAD83 coordinates
are ~always expressed as "NAD83(2011) epoch 2010"; it requires extra
effort to get current epoch NAD83.

> - 2010 (that is the EPOCH of ITRF2014 I guess. BTW, strange name then)

2010 is the reference epoch, yes, but that means that the published
coordinates are given for 2010.0 and then you use another date - 2010.0
* velocity to get current coordinates.

> - 2021.3 (the date of the measurement I did this morning)

probably this

> - something else

If you did a navigation solution that is somehow accurate enough or
you're being careful for fun, then the right answer is hard.  A guess is
the epoch at which the station coordinates used for broadcast orbits
were updated (seems to be end of January every year), combined with some
plate motion model, and this is I suspect impossible to distinguish from
2021.3.

> - it does not matter (it does, because it returns different values in
> projinfo)

The big question is how you did your measurement.  Surely you are
talking carrier phase differential, because otherwise this wouldn't
matter.  Therefore your output coordinates are in the frame of the
reference stations.  I have the impression that for NGS OPUS (PAGES) you
get NAD83(2011) epoch 2010 or you get ITRF2014 epoch of data (because
they reckon the CORS coordinates to that epoch).  And similar for NRCAN
PPP.

In the US, you may also want to look at HTDP, which attempts to do more
careful modeling of velocities than a fixed value.  This was used as
part of the NAD83(2011) adjustment process.

  https://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml
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