[PROJ] Anybody from Thailand?

[Javier Jimenez Shaw]

The problem is how do you "identify" the elevation without an vertical
reference. If I say that the height/elevation/z is 345.678 m.... what is
the reference?
A geoid model express the transformation from a 3D CRS to a compound (2D +
vertical) CRS. Without a way to name the vertical CRS, what are we talking
about?
I know that many countries only have one. So it is "the one and only". But
there are many countries, so there are many "one and only" vertical CRS...
that is a contradiction ;)

They just have to register theirs in EPSG (I'm sure they have one).
I have already talked with some countries about this topic: Israel,
Colombia, Brazil, ... They are usually so busy to pay attention. Maybe I am
lucky now with Brazil, that has a new management.

My point with the email was to find something with the right contacts.
Knowing the right person makes everything much easy.

Let's see.

On Tue, 7 Jan 2025 at 13:38, Greg Troxel via PROJ <proj at lists.osgeo.org>
wrote:

> Javier Jimenez Shaw via PROJ <proj at lists.osgeo.org> writes:
>
> > Could anybody explain why a vertical coordinate reference system is
> needed
> > to properly use a geoid model?
>
> (This is all a little fuzzy, and I hope that fuzz doesn't matter.)
>
> Restating what I think is agreed: A geoid model is some combination of
> numbers and formulas, that when given a lat/lon, tells you the
> difference between HAE in some datum and some kind of height.
>
> A gravimetric geoid model relates the zero of equipotential surface to
> zero HAE.
>
> A hybrid geoid model relates the zero of an orthometric datum (that is
> probably not an equipotential surface), such as NAVD 88, to zero HAE.  A
> hybrid geoid model does not make sense without a vertical CRS.
>
> A pure gravimetric geoid model needs only a W_0 (U_0), and we don't tend
> to label that as a CRS.  I would expect you'd need to complete the CRS
> by talking about dynamic vs orthometric height, and you'd need dynamic
> to not make a semantic mess where you can go up 100m, sideways along an
> equipotential, down 100m and back, and not end up where you started.
>
> https://www.ngs.noaa.gov/GEOID/
> NGS says their gravimetric geoids:
>
>   Converts heights from ITRFxx to the NGS geoid supprface
>   (not NAVD 88 or other Vertical datums)
>
> An example is "EGM2008" which I see as a gravimetric geoid model,
> converting WGS84 HAE to "WGS84 Orthometric Height".
> https://spatialreference.org/ref/epsg/3855/
>
> So, if the Thai geoid is basically functionally like EGM2008, but
> restricted to Thailand and more accurate, then I can see that they don't
> feel the need to name/publish a vertical CRS.  It would just be a
> locally-more-accurate transformation.
>
>
> So the question is what TGM2017 is defined to be, both the geodetic
> datum for the HAE input, and the output vertical.
>
> When you say "why a vertical CRS is needed", do you count "meters above
> the W_0 surface" as a CRS?   If so, is this EPGS:3855?
>
>
>
>
>
> Separately, the idea that Thailand has no national vertical datum,
> especially from before GNSS, is hard to believe.  There could be a
> regional datum for a few countries that is in EPSG but I'd expect your
> search tool to turn that up.
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