Monitoring absolute vertical land motions and absolute sea-level changes from GPS and tide gauges data over French Polynesia

In this study, we estimate the absolute vertical land motions at three tidal stations with collocated Global Navigation Satellite System (GNSS) receivers over French Polynesia during the period 2007–2020, and obtain, as ancillary results, estimates of the absolute changes in sea level at the same locations. To verify our processing approach to determining vertical motion, we first modeled vertical motion at the International GNSS Service (IGS) THTI station located in the capital island of Tahiti and compared our estimate with previous independent determinations, with a good agreement. We obtained the following estimates for the vertical land motions at the tide gauges: Tubuai island, Austral Archipelago −0.92 ± 0.17 mm/yr, Vairao village, Tahiti Iti: −0.49 ± 0.39 mm/yr, Rikitea, Gambier Archipelago −0.43 ± 0.17 mm/yr. The absolute variations of the sea level are: Tubuai island, Austral Archipelago 5.25 ± 0.60 mm/yr, Vairao village, Tahiti Iti: 3.62 ± 0.52 mm/yr, Rikitea, Gambier Archipelago 1.52 ± 0.23 mm/yr. We discuss these absolute values in light of the values obtained from altimetric measurements and other means in French Polynesia.