Abstract: An autonomous machine comprises driving means for moving the machine along a surface of an area, and a navigation system. The navigation system causes the machine to follow a boundary of the area, storing path information on the path travelled by the machine as the machine follows the boundary and compares when the machine has returned to a previously visited position in the area. The navigation system compares the latest section of the path travelled by the machine with information representing a section of the path previously stored in the memory and decides when the new path information and previously stored path information are substantially the same.