Overall, the analysis of the results obtained for stations absent of calibration problems shows that the VMF1 grids can be used to calculate the ZHD with an accuracy of 2– 4 mm for most stations. However, there are stations such as QAQ1 (Qaqortoq, Greenland) or TRAB (Trabzon, Turkey), for which the differences exceed the standard deviation of 4 mm, in some cases with a clear seasonal signal present in the corresponding residuals, and epochs for which the difference largely exceeds the centimetre level, even when using the Hopfield height reduction. In comparison, for the same type of height reduction, the precision of the estimates with ECMWF 025 is 1–2 mm for most stations (Table 1). A very small number of stations were found to reveal small but non negligible signals in the ECMWF-025 residual differences using the Berg reduction. This is illustrated in Figs. 11 and 12, for station MATE (Matera, Italy) for which the ECMWF-025 Berg differences, although small, have a clear annual signal (Fig. 12). In comparison, when using the Hopfield height reduction this seasonal signal disappears (Fig. 11), a clear indication that it is due to the annual variation of atmospheric pressure with temperature. Another type of problem was identified on VMF1: for some locations and epochs the ZHD values reveal much larger residuals with respect to those derived from in situ pressure data, evidencing that their accuracy is not uniform. This is illustrated on Figs. 10 and 11 for station