Moreover, it has to be considered that ZTD values
always represent only a kind of mean (or virtual) troposphere
condition above the site, since delays from arbitrary directions and elevation angles are mapped into zenith. Thus, in the
presence of strong troposphere asymmetries, ZTD can differ
from its true value by a few mm. Station 3017 is chosen to
display the temporal characteristics of a comparison between
GPS derived ZTDs and those obtained from ray-tracing (Fig. 4).