Absolute sea-level change determination from integrated geospatial data at the Ba Ria-Vung Tau tide-gauge station, Vietnam

Abstract

This study presents an integrated geospatial framework to estimate absolute sea level change (ASL) at the Ba Ria-Vung Tau tide-gauge station along the southern coast of Vietnam by combining long-term tide-gauge observations (1999–2024), GNSS measurements, and national second-order geometric leveling. We preprocessed tide-gauge records using the interquartile range (IQR) method to remove outliers before analysis. We estimated relative sea level (RSL) trends using a generalized least squares (GLS) model incorporating seasonal components and ARMA-structured residuals to account for temporal autocorrelation. The GLS-ARMA model provides a strong representation of the observed sea-level time series (R² ≈ 0.91), yielding a statistically significant mean RSL rise of approximately 3.6 mm.yr-1 over the study period. We inferred vertical land motion (VLM) from GNSS observations at the nearby CORS VTAU station using Precise Point Positioning. We connected the GNSS station to the tide-gauge benchmark through high-precision second-order geometric leveling. The GNSS-derived vertical component indicates land subsidence on the order of -1.8 mm.yr-1, suggesting that vertical land motion contributes substantially to the observed relative sea-level signal. After accounting for land subsidence, the remaining component reflects an estimated ASL rise of approximately 1.8 mm.yr-1, which is physically plausible when interpreted in regional and global contexts. However, owing to the limited length of the GNSS observation record, the estimated VLM and ASL rates remain preliminary and indicative. Despite this limitation, the results underscore the importance of explicitly accounting for vertical land motion in coastal sea-level assessments. The proposed framework provides a practical and transferable approach for initial ASL estimation at tide-gauge stations in data-limited regions. It highlights the need for longer GNSS time series, periodic leveling, and cross-validation with satellite altimetry to improve reliability for coastal flood-risk analysis and coastal zone management.