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

[Jack Riley - NOAA Federal]

>
> Just got a very comprehensive answer from Roger Lott, from IOGP:
> ---snip---
> As far as Proj issue 9980 [note from E. Rouault: issue 9980 is this
> discussion
> thread about NOAA VDatum] is concerned, I would be very wary about using
> the
> ensemble construct to include a mixture of onshore geodetic vertical datums
> and offshore tidal datums - these have different conceptual reference
> systems. One models the geoid, the other a tidal surface. Although the
> geoid
> is a surface which best fits MSL, they are not the same or even parallel
> with each other.
>

Quick point of clarification: In the NOAA VDatum we junction the onshore
geodetic vertical datum and offshore tidal datum through a *local* mean sea
surface (LMSL) construction.  This LMSL tidal surface represents mean
oceanographic topography computed over some epoch, and is modeled as a grid
of offsets relative to the geoid (height) realization.  Heights at the
tidal datums of interest (e.g., Mean High Water, Mean Lower Low Water,
etc.) are modeled in turn as grids relative to the LMSL.
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