Abstract. Vertical land motion (VLM) at the coast is a substantial contributor to relative sea level change. In this work, we present a refined method for its
determination, which is based on the combination of absolute satellite altimetry (SAT) sea level measurements and relative sea level changes
recorded by tide gauges (TGs). These measurements complement VLM estimates from the GNSS (Global Navigation Satellite System) by increasing their spatial
coverage. Trend estimates from the SAT and TG combination are particularly sensitive to the quality and resolution of applied altimetry data as well as
to the coupling procedure of altimetry and TGs. Hence, a multi-mission, dedicated coastal along-track altimetry dataset is coupled with
high-frequency TG measurements at 58 stations. To improve the coupling procedure, a so-called “zone of influence” (ZOI) is defined, which confines
coherent zones of sea level variability on the basis of relative levels of comparability between TG and altimetry observations. Selecting 20 %
of the most representative absolute sea level observations in a 300 km radius around the TGs results in the best VLM estimates in terms of
accuracy and uncertainty. At this threshold, VLMSAT-TG estimates have median formal uncertainties of
0.58 mm yr−1. Validation against GNSS VLM estimates yields a root mean square (rmsΔVLM) of VLMSAT-TG and
VLMGNSS differences of 1.28 mm yr−1, demonstrating the level of accuracy of our approach. Compared to a reference
250 km radius selection, the 300 km zone of influence improves trend accuracies by 15 % and uncertainties by 35 %. With
increasing record lengths, the spatial scales of the coherency in coastal sea level trends increase. Therefore, the relevance of the ZOI for
improving VLMSAT-TG accuracy decreases. Further individual zone of influence adaptations offer the prospect of bringing the accuracy of
the estimates below 1 mm yr−1.
Sinking Tide Gauge Revealed by Space-borne InSAR: Implications for Sea Level Acceleration at Pohang, South Korea
