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@lists.osgeo.org> wrote: > Javier Jimenez Shaw via PROJ <proj@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. > _______________________________________________ > PROJ mailing list > PROJ@lists.osgeo.org > https://lists.osgeo.org/mailman/listinfo/proj >
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