Heightmap vs. sphere/box intersection?

Hi, im am using a 3rd party terrain engine which exposes elevation, vertex elevation, normals and provides a intersect ray function. Are there any fast methods to check spheres/boxes(no AABBs) against the heightfield or do i have to rebuild a list of triangles out of the terrain and do "normal" triangle-box/sphere intersection tests? I am sure this has been covered many times, but i haven''t found anything ''cause the search option is still disabled. Thank you very much. - Stephan