  Accurate measurement of the frequency-dependent shift of the self-absorbed
radio core is required for multi-frequency analysis of VLBI data since absolute
positional information is lost as a result of phase self-calibration. We use
the cross-correlation technique of Croke & Gabuzda (2008) on the optically thin
jet emission to align our VLBA images. Our results are consistent with those
obtained from the phase-referencing method, as well as alignment by
model-fitted optically thin jet components. Physical parameters of the compact
jet regions, such as the magnetic field strength (B_core) and the distance of
the radio core to the jet origin (r_core), can be calculated from these
measurements. For the source Mrk 501, we find a magnetic field strength of
0.15\pm0.04 G in the 8.4-GHz core at a distance of 0.8\pm0.2 pc from the base
of the jet. By extrapolating our 4.6 to 15.4 GHz results for BL Lac (2200+420),
we estimate magnetic field strengths of the order of 1 G in the millimetre VLBI
core. Using our core-shift measurement between 1.6 and 4.8 GHz for 1803+784, we
find B_core(4.8 GHz) = 0.11\pm0.02 G and r_core(4.8 GHz) = 20\pm5 pc. The
phase-referencing observations of this source at 8.4 and 43 GHz by
Jim\'enez-Monferrer et al. (2008) imply B_core(43 GHz) = 1.0\pm0.4 G and
r_core(43 GHz) = 2.0\pm0.9 pc.
