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
