According to Kouba (2009b), erroneous hydrostatic/wet mapping separation values can cause height errors up to 20% of the error in the a priori ZHD. Thus, for achieving sub-mm height accuracy, sub-cm accuracy of ZHD is required, which in turn means that pressure must be known better than the 5 hPa accuracy level. Such pressure accuracy can be obtained only from local measurements, or from a state-of-the-art NWM.