[Therion] 3D model height

[Benedikt Hallinger]

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 at 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
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