[PROJ] Anybody from Thailand?

[Greg Troxel]

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.