[PROJ] EPSG v10 update status

[Even Rouault]

Greg,

> > for dynamic datum & CRS (like "WGS 84 (Gxxxx)", "ITRFxxxx"), ingest the
> > frame reference epoch from the database and emit it in WKT. No/little
> > backward compatibility issue foreseen.
> 
> Sorry if this is dragging you off topic, but I continue to have a hard
> time following exactly what people mean by "dynamic datum".  Sometimes
> it seems to mean "defined relative to ITRFxxxx via a velocity model, so
> that coordinates of crust-fixed stations are stable".  And sometimes it
> seems to mean "a datum that we ackknowledge is not crust-fixed".
> 
> Here, I wonder if you are referring the to progression of realizations,
> or one of the above, or something else?

Sorry to just copy&paste
https://docs.opengeospatial.org/as/18-005r4/18-005r4.html , but this is the inspiring 
document for WKT:2019, all PROJ "metadata" handling and EPSG database content

"""
3.1.20
dynamic reference frame
dynamic datum
reference frame in which the defining parameters include time evolution
Note 1 to entry: The defining parameters that have time evolution are usually a coordinate 
set.
"""

Here's the extensive list of what is considered in EPSG as a dynamic datum:

$ sqlite3 data/proj.db  "select 'EPSG:' || code || ', ' || name from geodetic_datum where 
frame_reference_epoch is not null order by name" 

EPSG:1291, Australian Terrestrial Reference Frame 2014
EPSG:1245, IGS00
EPSG:1247, IGS05
EPSG:1141, IGS08
EPSG:1191, IGS14
EPSG:1244, IGS97
EPSG:1246, IGb00
EPSG:1248, IGb08
EPSG:1272, IGb14
EPSG:6647, International Terrestrial Reference Frame 1988
EPSG:6648, International Terrestrial Reference Frame 1989
EPSG:6649, International Terrestrial Reference Frame 1990
EPSG:6650, International Terrestrial Reference Frame 1991
EPSG:6651, International Terrestrial Reference Frame 1992
EPSG:6652, International Terrestrial Reference Frame 1993
EPSG:6653, International Terrestrial Reference Frame 1994
EPSG:6654, International Terrestrial Reference Frame 1996
EPSG:6655, International Terrestrial Reference Frame 1997
EPSG:6656, International Terrestrial Reference Frame 2000
EPSG:6896, International Terrestrial Reference Frame 2005
EPSG:1061, International Terrestrial Reference Frame 2008
EPSG:1165, International Terrestrial Reference Frame 2014
EPSG:6740, Parametry Zemli 1990
EPSG:1157, Parametry Zemli 1990.02
EPSG:1158, Parametry Zemli 1990.11
EPSG:1227, SIRGAS Continuously Operating Network DGF00P01
EPSG:1228, SIRGAS Continuously Operating Network DGF01P01
EPSG:1229, SIRGAS Continuously Operating Network DGF01P02
EPSG:1230, SIRGAS Continuously Operating Network DGF02P01
EPSG:1231, SIRGAS Continuously Operating Network DGF04P01
EPSG:1232, SIRGAS Continuously Operating Network DGF05P01
EPSG:1233, SIRGAS Continuously Operating Network DGF06P01
EPSG:1234, SIRGAS Continuously Operating Network DGF07P01
EPSG:1235, SIRGAS Continuously Operating Network DGF08P01
EPSG:1236, SIRGAS Continuously Operating Network SIR09P01
EPSG:1237, SIRGAS Continuously Operating Network SIR10P01
EPSG:1238, SIRGAS Continuously Operating Network SIR11P01
EPSG:1239, SIRGAS Continuously Operating Network SIR13P01
EPSG:1240, SIRGAS Continuously Operating Network SIR14P01
EPSG:1241, SIRGAS Continuously Operating Network SIR15P01
EPSG:1242, SIRGAS Continuously Operating Network SIR17P01
EPSG:1293, Sistem Referensi Geospasial Indonesia 2013
EPSG:6324, WGS 72 Transit Broadcast Ephemeris
EPSG:6760, World Geodetic System 1966
EPSG:6322, World Geodetic System 1972
EPSG:1154, World Geodetic System 1984 (G1150)
EPSG:1155, World Geodetic System 1984 (G1674)
EPSG:1156, World Geodetic System 1984 (G1762)
EPSG:1152, World Geodetic System 1984 (G730)
EPSG:1153, World Geodetic System 1984 (G873)
EPSG:1166, World Geodetic System 1984 (Transit)

>From a quick look, at least for the datums with which I've some familiarity, it looks like they 
are datums which are not crust-fixed.

For example, Australia's GDA2020 which is crust-fixed is not there, altough it has a time-
dependent Helmert transformation to ATRF2014 and ITRF2014 (both listed above)

> datum ensembles... This is the most annoying part. The "World Geodetic
> System 1984" and "European Terrestrial Reference System 1989" datum
> have now in EPSG 10 a " ensemble" suffix in their names to reflect the
> new nature of those objects.
> I don't follow why NAD83 isn't listed here, and I wonder if you think
> there's a good reason, or just 'it ought to be but nobody has done the
> work yet'.

I don't know why the NAD83 family isn't there. Probably depends on NOAA deciding on how 
it wants to deal with that.

> Also, I wonder if ITRFxxxx is going to be treated as datum ensemble.  As
> I understand it, ITRFxxxx are successive realizations for ITRS (which is
> itself not a datum).

I don't know. But given the below definition taken again from 18-005r4, I'm wondering if 
someone who bothers enough to express coordinates in a ITRFxxxx datum would be really 
keen in seeing them degraded in a coarser ITRS ensemble. You could just use the WGS 84 
ensemble for the same purpose I guess.

"""
A Datum Ensemble is a construct to facilitate the merging of realizations of the same 
Terrestrial Reference System or Vertical Reference System for lower accuracy spatial 
manipulation. In this document, datum ensemble is a collection of two or more reference 
frames that are realizations of one Terrestrial or Vertical Reference System and which for all 
but the highest accuracy requirements may be considered to be insignificantly different from 
each other. Datasets referenced to the various realizations may be merged without change 
of coordinates.
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