An instrument system that includes a first optical fiber, a second optical fiber and a controller is provided. The first optical fiber is operatively coupled to an elongate body that is adapted to be placed inside a patient. The second optical fiber is operatively coupled to the patient, to an actuating element adapted to actuate the elongate body, or to a portion of an imaging system adapted to identify a location of the portion relative to the elongate body. The controller is operatively coupled to the first optical fiber and the second optical fiber and is adapted to receive a first signal from the strain sensor provided on the first optical fiber, receive a second signal from the strain sensor provided on the second optical fiber; and determine a position or orientation of the elongate body based on the first signal and based on the second signal.