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