1. A system comprising:an elongate instrument comprising: an elongate body having a proximal end, a distal end, and a lumen extending from the proximal end to the distal end, a splayer coupled to the proximal end of the elongate body, and one or more control wires extending through the elongate body and coupled to the distal end of the elongate body with a control ring, wherein the one or more control wires are configured to control movement of the distal end of the elongate body;
a shape-sensing fiber, wherein at least a first portion of the fiber, including a first end, is positioned within the lumen of the elongate body, wherein at least a second end of the fiber is fixed to an anchor slide, wherein the anchor slide fixes the second end of the fiber to a reference structure in one or more degrees of freedom at a fixed point separate from and proximal to the elongate instrument, wherein the reference structure facilitates the calibration of the fiber to a known structure of the reference structure, and wherein every location on the fiber is measurable relative to the fixed point to determine a relative position and shape of the fiber, a distal end of the shape-sensing fiber coupled to a termination having light absorptive properties, wherein the fiber and the termination are coupled to each other within the control ring;
at least one computer-readable memory having stored thereon executable instructions; and
one or more processors in communication with the at least one computer-readable memory and configured to execute the instructions to cause the system to at least:
register the fiber during use of the elongate instrument by comparing the shape of the fiber to a model,
wherein the fiber is fixed in six degrees of freedom at the fixed point, with the position of the fiber relative to the elongate body known in all six degrees of freedom at the fixed point, such that an absolute measurement of position, orientation, and shape of the elongate body is quantifiable from the relative position and shape of the fiber.