A marker system includes a structure having a first surface and a second surface, a first marker located on the first surface, and a second marker located on the second surface, wherein the first marker and the second marker are arranged so that they face approximately a same direction, and so that they can be simultaneously viewed by an optical device, and wherein a first distance between the first marker and the optical device is different from a second distance between the second marker and the optical device when the first and the second markers are simultaneously viewed by the optical device.
