A system for determining a shape of a bendable instrument, comprising: when the instrument is in a neutral shape, means for moving a first control element of the instrument a first amount to remove slack from the first control element and means for moving a second control element of the instrument a second amount to remove slack from the second control element without bending the instrument from the neutral shape, wherein the first control element is used to exert a force to bend the instrument in one direction and the second control element is used to exert a force to bend the instrument in a direction opposite the one direction; sensing means for sensing a first calibration position of the first control element and a second calibration position of the second control element, the first and second calibration positions defined with the instrument in the neutral shape after moving the first and second control elements (820, 830) the respective first and second amounts; and means for bending a segment of the instrument from a non-bent position by moving the first control element a first movement amount from the first calibration position and by moving the second control element a second movement amount from the second calibration position; characterised in that the system further comprises: measuring means to measure the first movement amount the first control element moved from the first calibration position and to measure the second movement amount the second control element moved from the second calibration position; means to determine a first measured position from the first movement amount and to determine a second measured position from the second movement amount; processing means to calculate a first position delta by subtracting the first calibration position of the first control element from the first measured position of the first control element; and to calculate a second position delta by subtracting the second calibration position of the second control element from the second measured position of the second control element; said processing means being used to determine a resulting shape of the instrument based on a relationship modelling the resulting shape of the instrument based on a sum of the first position delta and the second position delta. when the instrument is in a neutral shape, means for moving a first control element of the instrument a first amount to remove slack from the first control element and means for moving a second control element of the instrument a second amount to remove slack from the second control element without bending the instrument from the neutral shape, wherein the first control element is used to exert a force to bend the instrument in one direction and the second control element is used to exert a force to bend the instrument in a direction opposite the one direction; sensing means for sensing a first calibration position of the first control element and a second calibration position of the second control element, the first and second calibration positions defined with the instrument in the neutral shape after moving the first and second control elements (820, 830) the respective first and second amounts; and means for bending a segment of the instrument from a non-bent position by moving the first control element a first movement amount from the first calibration position and by moving the second control element a second movement amount from the second calibration position; measuring means to measure the first movement amount the first control element moved from the first calibration position and to measure the second movement amount the second control element moved from the second calibration position; means to determine a first measured position from the first movement amount and to determine a second measured position from the second movement amount; processing means to calculate a first position delta by subtracting the first calibration position of the first control element from the first measured position of the first control element; and to calculate a second position delta by subtracting the second calibration position of the second control element from the second measured position of the second control element; said processing means being used to determine a resulting shape of the instrument based on a relationship modelling the resulting shape of the instrument based on a sum of the first position delta and the second position delta.