1. An terrain contour matching navigation system for an aircraft, the system comprising: an inertial navigation system configured to output a navigation solution including a horizontal position of the aircraft and a vertical inertial altitude of the aircraft; a clearance altimeter configured to output a clearance altitude of the aircraft above a terrain; a memory programmed with a contour map; and a processor coupled to the inertial navigation system, the clearance altimeter and the memory; wherein the processor is programmed with a reference basket selector to select a reference basket area from the memory; wherein the processor is programmed to calculate a first single sample point comprising the horizontal position of the aircraft and an altitude sample, the altitude sample based on a difference between the vertical inertial altitude and the clearance altitude; wherein the processor is programmed with a correlation function that correlates the first single sample point over the reference basket area to determine a position in the reference basket area corresponding to a predicted position of the aircraft and further determine a correlation quality; wherein when the correlation quality does not achieve a pre-determined quality threshold, the processor is programmed to calculate an additional sample point and the correlation function correlates the additional sample point over the reference basket area to determine the position in the reference basket area corresponding to the predicted position of the aircraft and further determine the correlation quality; and wherein when the correlation quality does achieve a pre-determined quality threshold, the processor is programmed with a position error output function that calculates a horizontal position error and vertical position error based on the position in the reference basket area corresponding to the predicted position of the aircraft.