15. A control circuit according to claim 1 wherein said means for generating grid signals generates a plurality of output signals representing characteristics of the grade of the area of land after grading.
15. A control circuit according to claim 1 wherein said means for generating grid signals generates a plurality of output signals representing characteristics of the grade of the area of land after grading. 20. A control circuit according to claim 15 wherein one of said output signals represents the value of the height of a blade above the predetermined reference level to obtain the grade, said blade and the receiver being mounted on a movable vehicle.