The importance of knowing ZHD with high accuracy
has been addressed by several authors. For example, Tregoning
and Herring (2006) and Kouba (2009b) studied the
effect of a priori ZHD errors in precise GNSS positioning,
designated as hydrostatic/wet mapping errors. A priori
zenith hydrostatic delay errors project into GNSS height
estimates with typical sensitivities of up to 0.2 mm/hPa,
depending on the elevation angle cut-off. Tregoning and
Herring (2006) show that the ZHD errors can cause height
errors of up to 10 mm and seasonal variations of up to
2 mm in amplitude, and errors in zenith delay estimates
of about half the magnitude of the height errors.