In view of the discontinuities present in the EPN products, the quality of the GNSS-derived ZTD was inspected by comparison with an external and independent data source. To this end, the products from ERA-Interim, the latest global atmospheric reanalysis data set produced by ECMWF, was used. Being a reanalysis product, it has the advantage over the operational model of providing a uniform product with the same level of accuracy for the whole period while using the most recent advances in data assimilation and modeling. The products used in this study have been downloaded from the ERA interim website at ECMWF, provided at 1.5◦ x 1.5◦ spatial sampling and at 6 h intervals. The fields used in this study are the mean sea level pressure (SLP), the total column water vapour (TCWV) and the 2 metre temperature (2T).