One major part in the error budget of GPS measurements is the imperfect modeling of the tropospheric delay. By processing a global network of 195 stations we have compared two different mapping techniques: (1) the commonly used Niell hydrostatic mapping function (NMF) and (2) the isobaric hydrostatic mapping function (IMF) based on numerical weather fields. The two solutions reveal significant differences in the derived zenith total delay (ZTD) parameters and site positions. The largest differences occur in Antarctica, where the annual mean heights differ by up to 15 mm. We infer that the significant differences are related to model deficiencies in NMF since a) IMF improves the repeatability in station heights in high southern latitudes significantly, and b) using IMF reduces the dependence of the solution on the elevation cut-off angle by about 20%. In conclusion, the use of mapping function (MF) parameters based on meteorological data is strongly recommended for global GPS analyses.