Rotary encoder apparatus

A rotary encoder apparatus comprising:

• i) a scale member on which is provided at least one curved scale track that is centred on and extends at least partially annularly around a scale-track axis, the at least one curved scale track comprising substantially radially-extending scale features;
• and
• ii) a scale reading apparatus configured to obtain readings of the substantially radially-extending scale features, at first and second radially-offset locations;
• in which in use the scale reading apparatus and the scale member are configured to rotate relative to each other about an axis of rotation which is substantially parallel to the scale-track axis, and wherein the rotary encoder apparatus is configured to use the readings of the scale member's radially-extending scale features obtained at both the first and second radially-offset locations to determine, at least one of:
• a) information about the arrangement of the scale reading apparatus and/or the scale member other than, or in addition to, information about their relative rotational arrangement about the axis of rotation;
• and
• b) a compensated measurement of the relative position of the scale reading apparatus and the scale member about the axis of rotation, wherein the compensated measurement is compensated for position errors about theaxis of rotation caused by a change in the relative arrangement of the scale reading apparatus and the scale member other than, or in addition to, a change in their relative rotational arrangement about the axis of rotation.