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. 21. A control circuit according to claim 15 wherein one of said output signals represents the value of the transmitter slope to obtain the grade.