Southeast Asia is one of the most vulnerable regions globally to coastal flooding, where the risks of absolute sea‑level rise (SLR) and land subsidence severely overlap. Accurate assessment of coastal SLR and vertical land motion (VLM) is critical for disaster risk reduction. However, conventional altimetry products degrade within 10–15 km of the coast due to land‑contaminated waveforms and inappropriate corrections, while tide‑gauge (TG) and GNSS networks remain spatially sparse, hindering coastal observations. This study applies the X‑TRACK/ALES v3.0 coastal altimetry product to overcome these limitations and evaluate its performance in this region.
We integrated X‑TRACK/ALES with conventional gridded (C3S) and along‑track (X‑TRACK) altimetry product, 53 TG records, and 61 GNSS stations. A spatiotemporal filtering strategy suppressed high‑frequency noise while preserving coastal signals. To validate altimeter‑derived trends, we adopted the indirect "VLM = ALT – TG" approach and compared the results against GNSS using four spatial matching strategies across multiple overlapping time windows. Virtual stations were constructed within 50 km of each X‑TRACK/ALES median‑trend point. For VLM reconstruction, we paired each TG with the best‑correlated X‑TRACK/ALES observation and computed the ALT−TG difference.
Comparison of the three products reveals substantial discrepancies in coastal SLR estimates: 4.22 mm/yr from C3S, 3.45 mm/yr from X‑TRACK, and significantly lower at 2.36 mm/yr from X‑TRACK/ALES. The conventional products exhibit a systematic positive bias, with the largest discrepancies over the coral‑reef‑dense Indonesian archipelagos, likely due to radar‑signal contamination and mission‑bias corrections differences. Critically, joint validation demonstrates that X‑TRACK/ALES‑derived VLM trends agree closely with independent GNSS observations, while conventional products underestimate subsidence due to overestimated SLR. Using the X‑TRACK/ALES, we reconstruct a VLM dataset for 53 coastal sites, complementing sparse GNSS networks in both space and time and reveals nonlinear VLM phenomena at multiple localized sites.
Our validation confirms that X‑TRACK/ALES provides more accurate SLR estimates than conventional products along the Southeast Asian coast. Earlier studies using older products versions suggested underestimation; this discrepancy likely arises from subsequent version upgrades with more stringent data editing, while GPS imaging—an interpolation technique—cannot resolve local VLM variability, undermining previous validations. The validation is limited by the spatial sparsity and temporal overlap of collocated TG–GNSS records. As observing networks expand, coastal altimetry products warrant sustained evaluation. Given that local rapid subsidence now dominates RSLR in many low-elevation coastal cities, accurate VLM observations are essential for coastal adaptation—underscoring the critical value of our dataset.