SURFACE MEASUREMENT APPARATUS AND METHOD

A metrological apparatus has a workpiece support surface (16) and a mover (9) to carry out a measurement by effecting relative movement in a measurement direction, X, between the workpiece support surface and a stylus (11) such that the stylus is deflected as a stylus tip of the stylus follows surface variations. A transducer (39) provides a measurement data set in a measurement coordinate system representing the deflection, a, of the stylus at measurement points in the measurement direction, X. A rotation device (16) effects relative rotation of the workpiece support surface and the mover about a rotation axis. A data processor is provided to determine a location of intersection of a first measurement data set representing a measurement along a measurement path on a calibration component surface which is not symmetric about the rotation axis and a second measurement data set representing a measurement along a measurement path on the calibration component surface after rotation of 180 degrees about the rotation axis and to determine the frame of reference of the apparatus using the determined intersection.