  Many important techniques for investigating the properties of extragalactic
radio sources, such as spectral-index and rotation-measure mapping, involve the
comparison of images at two or more frequencies. In the case of radio
interferometric data, this can be done by comparing the CLEAN maps obtained at
the different frequencies. However, intrinsic differences in images due to the
frequency dependence of the radio emission can be distorted by additional
differences that arise due to source variability (if the data to be compared is
obtained at different times), image misalignment, and the frequency dependence
of the sensitivity to weak emission and the angular resolution provided by the
observations (the resolution of an interferometer depends on the lengths of its
baselines in units of the observing wavelength). These effects must be
corrected for as best as possible before multi-frequency data comparison
techniques can be applied. We consider the origins for the afore-mentioned
factors, outline the standard techniques used to overcome these difficulties,
and describe in detail a technique developed by us, based on the
cross-correlation technique widely used in other fields, to correct for
misalignments between maps at different frequencies.
