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
