No, the issue is the difference between the altitudes of the mean sea level (geoid) and the ellipsoidal coordinates.
Balázs. On Mon, Feb 22, 2021 at 10:22 PM Benedikt Hallinger <b...@hallinger.org> wrote: > As far as i understood, when you tag your data to be in a certain > coordinate system that should include all three data points when the > coordinate system defines them. > If you want an altitude offset, i would expect that we must manually > convert. > > As an example: i tag my data to be wsg89, which defines altitude pegged to > a certain reference. > Assume alt is 0. > When i now switch coordinate systems to something really (artifically) > weird, like the origin pegged to the peak of mt. Everest, i would expect > all three coordinates to be correctly transformed (making especially > altitude very negative). > If i now want to say that a point is at the same level as everests peak, i > would have to apply a manual transformation step. > But that’s not the fault of wgs89 mir the target cs, the reason was that > my initial calibration was never in wgs89! > > What do i get wrong in this thought process? > > > > Am 22.02.2021 um 21:37 schrieb Tarquin Wilton-Jones via Therion < > therion@speleo.sk>: > > > > > >> > >> If there is a real need for a switch between converting and not > >> converting the heights, let us know. > > > > At least here in the UK, a lot of rough and ready surveys seem to use > > Garmin handheld GPS units like the 66sr. These use barometric altitude, > > which you calibrate to a local benchmark. That means you get local > > mapping heights that do not need to be converted, while the GPS > > coordinates would need to be converted. > > > > Anyone relying on this approach (which already is rather poor from an > > accuracy perspective) would indeed need you to perform conversion on x > > and y but not z, while anyone using a proper GPS unit which outputs > > ellipsoid heights would need you to convert x, y, and z. I therefore see > > a need for it to be controllable. > > > > Personally, I use real GPS ellipsoid heights, which I manually convert > > using continental drift calculations and the higher quality > > OSTN15+OSGM15 transformation (since these then remain correct in spite > > of continental drift). I do not rely on proj for that, and have built my > > own tool instead, since proj does not have access to the data required > > for it. > > > > Tarquin > > _______________________________________________ > > Therion mailing list > > Therion@speleo.sk > > https://mailman.speleo.sk/listinfo/therion > _______________________________________________ > Therion mailing list > Therion@speleo.sk > https://mailman.speleo.sk/listinfo/therion >
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