[PROJ] Alternate use model on EPSG v10+ datum ensembles

[Jack Riley - NOAA Federal]

Hi,

The discussion of datum ensembles in October ([PROJ] EPSG v10 update
status, [PROJ] Using latest realization of a datum ensemble?) talked about
the default use case and associated conundrum: Datum ensembles group
overlapping (1:1) realizations and proj is faced with sorting out the best
transform involving a CRS linked to an ensemble.

Under this default use case it makes sense that one should instead opt for
the specific datum from the ensemble if known (assuming it is also attached
to a CRS definition of its own), rather then the "generic" CRS-datum
ensemble construction that (may) result in less than optimal accuracy.

OK, so the alternate use model of datum ensembles I'd like to pursue is for
encapsulation of a set of realizations with extents that are mutually
exclusive.  For example, I'd like to register a vertical CRS for (say)
'NOAA Chart depth' with a datum height ensemble where the set of datum
members are realized within distinct extents. This vertical CRS/datum
ensemble would then be used to define a compound CRS (e.g., geog2D
NAD83(2011) + 'NOAA Chart depth'), and a coordinate operation would be
defined to transform to/from (reversible) between that and a geog3D CRS.
This transform involves one or more (if a multi-step operation) sets of
gridded transforms in a 1:1 relationship with the extents of the ensemble
members.  The idea being that proj would have to do the heavy lifting of
point-by-point spatial tests per the defined extents in the metadata.

Does this make sense and is it feasible?  The big savings here is to avoid
having to define distinct vertical CRSs plus the associated compound CRSs,
as well as the separate coordinate operations attached to each datum
member.  Not to mention alleviating the need for the user to identify and
partition data set conversions according to the individual extents. The
datum ensemble in question concerns NOAA VDatum which is comprised of
approximately 50 constituent datum members.  That's pretty much entirely a
consequence having to use regular gridding to define the transformations;
the resolution must be constrained in areas which involve complex
shoreline.  And we would would be looking at another set of defines for
'NOAA chart height' (more than a simple sign change; e.g., chart depth
typically on a MLLW datum, whereas chart height per a MHW datum).

There is one example in the v10 EPSG registry that demonstrates this
alternate use model definition (datum members with mutually-exclusive
extents), with vertical and compound CRSs, but no associated coordinate
operations defined as of yet:
Datum: British Isles height ensemble, EPSG Code 1288
https://epsg.org/datum_1288/British-Isles-height-ensemble.html
Vertical CRS: BI height, EPSG code 9451
https://epsg.org/crs_9451/BI-height.html
Compound CRS: ETRS89 + BI height, EPSG code 9452
https://epsg.org/crs_9452/ETRS89-BI-height.html
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