In Eq. (1), a priori ZHD is evaluated from available sur-face pressure using the modified Saastamoinen model (Davis et al., 1985) and mfh and mfw are known mapping functions relating the zenith delays with those in the slant direction. For each observation, a combined ZTD value is obtained, as the sum of the a priori ZHD and the esti-mated ZWD. In principle, an error in the a priori ZHD will be absorbed by the estimated ZWD. However, for very low elevation angles, the hydrostatic and wet mapping func-tions will differ significantly and an error in ZHD cannot be absorbed by ZWD, which then induces errors in station position, mainly in height (Kouba, 2009b). Thus, in spite of the small variability of the ZHD field when compared with the variability of the wet component, precise GNSS posi-tioning and ZTD estimation require ZHD to be known with high accuracy.