[PROJ] Anybody from Thailand?

[Kristian Evers]

Javier,

In case you are not aware, the UN Global Geodetic Center of Excellence
(UN-GGCE) is hosting a number of "Geodesy Capacity Development Workshops"
this year. I believe there's one for each major region, the Europe workshop
is in February. The programme for the workshop is quite extensive, with one
of the sessions focusing on the ISO and EPSG registries. I'm sure it's not
a silver bullet that magically fixes everything, but it should bring
attention to the matter for all attending people. The focus of the
workshops, I believe, is mainly on strengthening the capabilities of
geodesists in lesser developed countries. So maybe, a year from now it'll
be a little bit easier to have the conversation about the EPSG registry
with local authorities. The UN-GGCE might also be a good place to ask for a
point of contact in a specific country.

/Kristian

On Tue, 7 Jan 2025 at 16:27, Javier Jimenez Shaw via PROJ <
proj at lists.osgeo.org> wrote:

> 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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