Default TerrainProvider ? Fallback if no terrain is available


following question:

I have a terraindb, which covers a region. I see that, if I move out of the region, several calculations are done according the WGS84 Ellipsoid. Is this due to that the WGS84 Ellipsoid is the first entry in my list of terrain providers ?

If I have a global terrain is it possible to use this instead of the WGS 84 Ellipsoid as fallback, if the active terrain doesn’t delivers data ?

My problem ist the calculation of the camera height above ground:
In the cases where no terrain is avaiable I get the ellipsoid-height (nearly 0.0). Is it possible to define a global default terrain instead ?