Correlation between the Type 1 and Type 2 (top) and Type 1 and Type 3 (middle) J = 1 → 0 CO maps at high Galactic latitudes 25°< | b | < 90°. Correlation between the Type 1 and Type 2 J = 2 → 1 CO maps is shown on the bottom plot. The solid line corresponds to perfect correlation (y = x). Below the dashed horizontal line the colour scale gives the logarithm of the number of pixels in a given bin. The best linear fits (solid red line) are computed from the individual pixels plotted above the dashed line. The best-fit slopes and uncertainties are: a) 0.93 ± 0.17; b) 0.87 ± 0.20; and c) 0.96 ± 0.12. The intercepts are compatible with zero within the error bars.