The hydrostatic component, with a m

The hydrostatic component, with a mean value at sea
level of 2.3 m, accounts for nearly 90% of the total zenith
tropospheric delay and is usually modelled with high accuracy
from in situ measurements of surface pressure, from a
Numerical Weather Model (NWM), for example from the
European Centre for Medium-Range Weather Forecasts
(ECMWF) or the US National Center for Environmental
Prediction (NCEP), or from alternative sources, such as
the Global Pressure and Temperature model (GPT)
(Boehm et al., 2007) or the Vienna Mapping Functions 1
(VMF1) ZHD datasets, (Boehm et al., 2006a). The wet
component, accounting only for nearly 10% of the total
path delay, is much more variable, difficult to model, and
is estimated along with the position parameters in most
geodetic applications.