Edwards is a large dry salt lake, and although there are runway markings and instrumentation the who are can be used if necessary; essential for flight testing when problems could require pilots to land anywhere (also rocket-powered a/c land as gliders). I assume Groom Lake has the same characteristics. Bonneville salt flats, used for land speed records, is smoother as it floods annually, depositing a new surface, which like any truly flat surface ie height above sea level is constant (DVR6K please note) follows the curvature of the earth