The navigation constraint can be one of:
(0) Camera navigation is unconstrained. In this mode, the camera may pass above and below the elevation surface. Subsurface navigation is allowed in this mode.
(1) Camera navigation is constrained to remaining above the elevation surface. The altitude of the camera may not fall below the elevation of the surface. If the elevation is added or updates resulting in the camera altitude being below the elevation the camera will be pushed up to be at the surface.