[PROJ] Ellipsoidal height VCRS in WKT/EPSG

[Greg Troxel]

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

> Does exist any EPSG or WKT for a Vertical CRS that means "ellipsoidal
> height"?
>
> To define a 3D CRS I just make a compound CRS with a horizontal and a
> vertical one.
> I could just remove any VCRS for ellipsoidal heights. However I cannot make
> the difference if the user actively selected "ellipsoidal height" for VCRS,
> or just forgot (or don't know) it.
>
> I don't want to "promoteTo3D" the 2D CRS  adding the third axis, because it
> removes the authority.

Interesting question!

First, beware that "ellipsoidal height" is incompletely specified, as
you neeed to specify the datum, both to know the ellipsoid, and to know
the ellipsoid position/orientation.  For example NAD83 ellipsoidal
height and ITRF2014 ellipsoidal heights are different at the same point.

I am not aware of any vertical-only ellipsoidal height CRS definitions.
I have only come across vertical-only CRSs such as NAVD88 and similar.

Probably there are no ellipsoidal height only CRS definitions because
when measuring ellipsoidal height, one obtains a 3D result.

I would suggest that when trying to add EH, you match the 2D CRS to see
if there is a corresponding 3D, with the same 2D component.  If the 2D
CRS is NAD83, WGS84, some ITRF, and probably some other satellite-era
national datums, then this can make sense.  If it's some CRS that it's
fundamentally* 2D only, then adding EH doesn't really make sense.

And, the user should have to specify the vertical datum; just checking
ellipsoidal height only makes sense if the 2D CRS is really a 2D view of
a 3D CRS and ellipsoidal height is unique.


* I realize original NAD83 isn't really 3D, but later ones are more 3D
  than not.
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