Abstract: Metrological apparatus, particularly for measuring form, has a turntable 4 and a transducer 14 and associated stylus 12 mounted so that the tip 12a of the stylus is displaceable radially and vertically relative to the turntable 4. The problem of obtaining both high resolution and large range of operation is solved by using a transducer/stylus 12,14 having a resolution of 12 nanometers and a range of only 0.4 mm and mounting the transducer 14 on an arm 10 which is driven radially and/or vertically by a computer 30 in response to the output from the transducer 14 to cause the stylus 12 to follow the workpiece surface. High resolution data defining the rotary position of the turntable 4 and the radial and vertical position of the transducer 14 is provided by a photodetector/optical grating arrangement 23, 21, 31; 27, 19, 43 and associated interpolators 42, 44, 46.