Abstract: The present invention includes a method and apparatus which determines the tree-dimensional (x, y, z coordinate) position of the receiving position of a GPS antenna on an earth-moving machine or a vehicle, determines the tilt of the machine or vehicle in real time (tilt may vary from point to point), and utilizes this data to calculate the three-dimesional (x, y, z coordinate) position of a point of the earth's surface disposed beneath the machine or vehicle. In a preferred embodiment, the present invention derives and displays the distance between the actual elevation of this point and the desired surface elevation.