Thank-you for your reply,
I'll try to rephrase:
I think that OnlineMapsElevationManagerBase.GetElevation use the longitude-latitude of a point to retrieve its elevation from the loaded map (but maybe I'm wrong).
Let's say that I'd like to place a plane over the map, and I know that the plane is flying at the altitude of 350 meters.
I don't care about its longitude-latitude, and I don't care about the underlying map 3d model, because the elevation accuracy of the map 3d model change on the basis of the zoom (the more is zoomed, the more the altitude is accurate, at least with bing, mapbox, etc, from what I can see).
So, maybe my underlying map is not so accurate, but I'd like to place my plane at its real height.
It should be easy to place my plane at 350m if the altitude of the map 3d model was the real one in world space, but the lowest elevation of the map is always at 0 (with BottomMode = minValue).
I hope I explained myself better.