Hi Frans!

The GPS will be a direct read from the GPS unit in the phone - GPS isn't great at height measurements, though (only lon/lat), so there may be a +/- 10 to 20m

It's also based on an ellipsoid (WGS84) model of the planets surface (so, yes, an adjustment may be a good idea for this as that model won't match the actual planets shape). iirc geoid information isn't passed through the API, so the difference can't be calculated (yet)

I'll put this down on the feature list though.