Fig. 3 illustrates the differences between each of the GNSS-derived ZTD and the corresponding values from ERA Interim, for a set of eleven stations which belong simultaneously to IGS and EPN. The small dots represent daily means while the large dots represent monthly means. In agreement with the results presented in Section 2.3, both IGS and U.Porto show very stable and homogeneous differences for the whole period. The mean and standard deviation of the differences between U.Porto and ERA Interim ZTD is 3.3 mm and 12.2 mm while the corresponding values for the difference between IGS and ERA Interim are 3.5 mm and 12.1 mm, respectively. Considering the individ-ual GNSS sites, the standard deviation of the differences for both IGS and U.Porto ZTD range from 8 mm to 14 mm, with a mean value of 12 mm.