[PROJ] Creating a dynamic system

[Greg Troxel]

Even Rouault via PROJ <proj at lists.osgeo.org> writes:

> for all practical purposes, it seems to be one. I don't know why it
> hasn't been formally tagged as such in EPSG. I see that back in times
> I added in CoordinateMetadata::create() an exception to check if a CRS
> had a point motion operation attached to it to allow it to be used to
> create a CoordinateMetadata object

NAD83 is a funny case.  Depending on one's view of accuracy, it can be
static or dynamic.  Originally, it was a crust-fixed frame and formally
static.  But then two things happened:

  - greater ability to measure, so the plate not being rigid and part of
    NA being on the pacific plate starts to matter more.
  - redefinition in terms of ITRF (NAD83(2011)), with euler pole
    parameters that in hindsight were not exactly right

For the US, NAD83(2011) is technically dynamic, with station velocities.
The velocities are very small, a few mm/y, vs the much larger 2cm/year
that NA plate stations have in ITRF.  So neglectable in more situations.

However, nobody uses that datum.   When people say NAD83(2011), they
mean "NAD83(2011) epoch 2010.0" coordinates, which is effectively a
static datum.   (Or they don't understand.)

The next question is what EPSG:6319 means.   It doesn't mention the
epoch:
  https://spatialreference.org/ref/epsg/6319/

but i think everybody who uses it (who is aware of this issue at all)
firmly knows that data tagged in EPSG:6319 is epoch 2010.0.

The RTK networks run by the various StateDOT agencies are essentially
all in NAD83(2011) epoch 2010.0.  I know of one that also offers some
ITRF and draft NATRF2022 frames.


I am unclear on precise Canadian practice but my impression is that the
most recent CSRS is more or less the same concept but adjusted to
Canadian data and a few cm different then NAD83(2011).


I suggest that datums that are technically dynamic but not really be
avoided in examples like this.



A further issue is to defuzz NAD83(2011) in EPSG in terms of if it is a
dynamic datum with a reference epoch with stations having velocities, or
if it means epoch 2010.0.  I vote for epoch 2010.0 to match US practice.