Ahhh no problem VicentLaw! When landing in bare terrain, normal surface + h should be enough inputs for a custom ground handling model, you will get some errors in the terrain triangles edges, but they should be small since surface normals don't vary greatly, it's a good aproximation. Another story is landing over objects though, h_obj must be accesible for any X,Y,Z coordinate for an accurate contact point detection.
Regards!