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