Measured offsets in right ascension (left) and declination (right) for high signal-to-noise sources of LOFAR with respect to FIRST (plotted as points). The colourscale shows the value of the given offset. The top panels show the distribution of the offsets. The offsets, particularly in declination, show a variation across the field of view. The background colourscale shows a fitted plane used for correcting the LOFAR catalogued sources.