1. An apparatus comprising: a kinematic chain comprising a proximal end and a distal end flexibly coupled to the proximal end; a shape sensing optical fiber associated with the kinematic chain, wherein a shape sensing segment in the optical fiber is defined in the fiber between the proximal end and the distal end, and wherein a three-dimensional reference frame is defined at a start location of the shape sensing segment; an interrogator that outputs strain information associated with shape sensing segment; and an electronic data processor that receives the strain information and outputs a Cartesian position of an end point of shape sensing segment with respect to the reference frame.