A sensing apparatus for detecting a translation of a body relative to a surface, the apparatus comprising: a rolling component for contact, in use, with the surface, the rolling component being retained by, and able, in use, to rotate independently of the body; one or more indicator means associated with the rolling component and rotatable therewith; and one or more transducers for producing one or more signals in response to a rotation of the indicator means relative to the one or more transducers wherein, in use, the rolling component rolls upon the surface in response to a relative translation of the body to the surface, thereby causing the positional orientation of the indicator means to change with respect to the transducers.