Survey difference maps at 30 GHz. Top: difference between survey 1 and 2. Middle: difference between survey 1 and 3. Note that the 1−2 difference map has sharper features, due to the different scanning strategy between odd and even surveys that prevents optical systematics from being cancelled in the subtraction. Bottom: the mask used to distinguish between CMB-dominated regions and Galactic regions when computing the numbers reported in Table 8 (30 GHz case).