Analysis of a relative offset between vertical datums at the North and South Islands of New Zealand

The leveling networks realized by 13 different local vertical datums were jointly readjusted at the South and North Islands of New Zealand. The relation between these two leveling networks and the Word Height System was then defined using GPS-leveling data and the EGM08 global geopotential model. In this study, we investigate the relative offset between these two vertical datum realizations. This is done based on comparison of the geometric geoid/quasigeoid heights (obtained from GPS and newly adjusted leveling data) with the regional gravimetric geoid/quasigeoid solutions. Moreover, oceanographic and geodetic models of mean dynamic topography (MDT) are used to assess the relative offset between these two vertical datum realizations through the analysis of regional spatial variations of mean sea level (MSL). The comparison of GPS-leveling data with regional gravimetric solutions reveals large systematic distortions (exceeding several decimeters across New Zealand) between the geometric and gravimetric geoid/quasigeoid heights attributed mainly to systematic errors within regional gravimetric solutions. The presence of a significant offset between the vertical datum realizations at the North and South Islands is not confirmed. The MSL difference between tide gauges in Wellington and Dunedin of ∼24 cm is estimated based on the analysis of MDT models.