Abstract: A method of determining the coordinates of the center of curvature of the curved surface of a workpiece with respect to a predetermined frame of reference the origin of which coincides with the center of relative rotation between the workpiece and a sensor of a surface measurement instrument. The method requires sensor signals to be derived at only three points along the traverse of the sensor and there are no constraints on the position or symmetry of the three points thereby allowing the method to be used on workpieces having only a partial arc. Two output signals representing the x and y coordinates of the center are produced from signals representing the angular position of the three points and from the sensor signals at the said three points.