Image Number 3 for United States Patent #4724653.
A levelling and shifting machine and a transmitter system (1) are used, the latter being installed on a carriage parked on the track and transmitting a spreading beam in the horizontal plane for levelling and a spreading beam in the vertical plane (Fr) for shifting, these beams defining an absolute measuring base. The receivers for levelling and shifting, which are installed on a measuring carriage of the machine, are designed for self-centering relative to the line of incidence of one of the said beams or the other during each measurement. In a curve of the track (3), the vertical beam (Fr) defines a cord of this curve, and the set position of the receiver defines the current value of the pitch of the curve (fm.sub.0, fm.sub.1 etc). A computer calculates the desired value of the pitch (f.sub.0, f.sub.1 etc.) and the variation (y.sub.0, Y.sub.1 etc.) between the two values, the latter variation determining the shifting correction.The measuring interval (G') covered by the machine without a change in the position of the transmitter (1) is selected greater than the length (G) of the cord, and the initial measuring point (A.sub.0) is selected on the secant passing through the cord beyond the point of intersection of the beam (Fr) and the track (3), so that the sum of the maximum pitches towards one side and the other is compatible with the travel of the receiver on its measuring carriage.