4. A control circuit according to claim 1 including input means for inputting a control box elevation signal representing the distance between a predetermined point on the receiver and the surface of the land and wherein said means for storing stores said control box elevation signal.
1. In a system for surveying the contour of an area of land including a stationary transmitter for generating a rotating laser beam in a plane above the area of land to be surveyed, and a receiver movable about the area of land and responsive to the rotating laser beam for generating an elevation signal representing the elevation of the land at the receiver location relative to a predetermined reference level, a control circuit connected to receive the elevation signal from the receiver, said control circuit comprising: input means for inputting a direction signal representing a predetermined side of the area of land; means for generating grid signals representing a predetermined grid pattern having a plurality of grid points representing locations on the area of land at which an elevation reading is to be taken; and means for storing said direction signal and the elevation signal of a selected one of said grid points when the receiver is located at the corresponding location on the area of land.