CASC station (Cascais, Portugal) is an example (Fig. 7) of a station with a very stable pressure data set, located in a region of the globe where the pressure has relatively low variability and virtually no seasonal signal. In this case, the VMF1 grids can be used to calculate the ZHD with an accuracy of 1.6 mm (1r) while the accuracy of the estimate with ECMWF-025 SLP is 1.0 mm (see Table 1). In Fernandes et al. (2010) a comparison between ZHD computed from in situ pressure and from VMF1, at a set of European GNSS stations, is shown. For those stations, the standard deviation of the differences between the ZHD values was 3.4 mm. The present study shows that this result cannot be extended to regions in the world with large variability in the pressure and temperature fields. The importance of the procedure adopted in the height reduction is illustrated in Figs. 8 and 9 for station DAEJ (Daejeon, South Korea), which has a fairly good pressure data set but located in an area in which the seasonal signal of pressure has large amplitude. The first figure refers to the results using the Hopfield (1969) formulation and the second one to the corresponding results using the Berg (1948) expression for the height dependence of atmospheric pressure. For simplicity, the ZHD values computed using these different height reduction methods are designated by Hopfield and Berg values, respectively. Considering the Berg height reduction, the differences with respect to VMF1 grids reveal a strong seasonal signal with peak to peak amplitude larger than 2 cm (Fig. 9). On the contrary, in the ZHD calculated using the Hopfield height reduction the seasonal signal almost disappears for this station. The ZHD values from ECMWF-025 have a difference relative to the corresponding in situ values with a small residual signal with a very weak, almost unnoticeable, seasonal signal for the Berg results and even smaller for the Hopfield values. For this station, the standard deviation of the differences is 6.0 mm for VMF1 and 1.3 mm for ECMWF-025, for the Berg reduction. The corresponding statistical parameters for the Hopfield reduction are 3.0 mm and 1.0 mm for VMF1 and ECMWF-025, respectively (Table 1).