The value of _diffrn_data_frame.center_units
specifies the units in which the values of
are presented. The default is 'mm' for millimetres. The
alternatives are 'pixels' and 'bins'. In all cases the
centre distances are measured from the centre of the
first pixel, i.e. in a 2x2 binning, the measuring origin
is offset from the centres of the bins by one half pixel
towards the first pixel.
If 'bins' is specified, the data in
are used to define the binning scheme.