Coordinate positioning machine

A method of calibrating a coordinate positioning machine, a geometry of the machine being characterized by a parametric model and the calibration method being intended to determine a new set of model parameters that characterizes the geometry of the machine better than an existing set of model parameters, the method comprising:

• controlling the machine into a pivot pose in which a target point associated with a moveable part of the machine and a pivot point associated with a fixed part of the machine are separated from one another by a known separation;
• determining an error value for the pivot pose based on the known separation and a separation expected for the pivot pose from the existing set of model parameters;
• controlling the machine into a plurality of different target poses;
• for each target pose, (i) measuring a separation between the target point and the pivot point and (ii) determining an error value for that target pose based on the measured separation and a separation expected for that target pose from the existing set of model parameters;
• determining an overall error measure from the error values;
• and
• determining a new parameter set that would result in a lower overall error measure than for the existing set of model parameters.