Method and apparatus for keeping a machine member of an agricultural machine at a substantially constant height above a field during its movement through the field. A wheel-supported main frame supports, for vertical movement, an auxiliary frame carrying the machine member and also carrying an ultrasonic transducer spaced at a constant distance away from the machine member. The machine member is set at an initial position relative to the surface of said field. Thereafter the position of said machine member relative to said main frame is determined, and an electrical signal corresponding thereto generated. The height of said transducer above the field is determined ultrasonically and an electrical signal corresponding thereto generated. From these electrical signals the amount and direction of divergence of the actual position of the machine member relative to the field surface from its initial set position is determined, and an electrical correction signal corresponding thereto is produced, causing movement of the auxiliary frame relative to the main frame.