apps.garmin.com/.../3d55e7f6-d14b-4db9-b79d-bc90b4018299
I develop this app and the data provided are from open ToPo. It's possible use different satellite and the accuracy are really high. I advise to try different satellite to obtain a good 3D points.
Then with data obtained you can calibrate altimeter on device settings, not possible in app...
Next step I have in mind to develop widget that show altitude with 24 hours graph.