Forecasting legal coastline based on highest astronomical tide as a predictive model for land–sea integration

This study develops a highest astronomical tide (HAT)-referenced legal coastline framework to support the integration of terrestrial and marine cadastre in Bekasi Regency, Indonesia. Six Landsat image epochs acquired between 2015 and 2024 were processed and normalised to HAT derived from an 18.6-year tidal record at Pondok Dayung Tidal Station, selected as a regional reference because of its comparable tidal characteristics with the study area. Water bodies were delineated using the modified normalized difference water index (MNDWI), with scene-specific thresholds determined by the Otsu method and refined through morphological post-processing. This procedure produced a vertically consistent legal coastline for each selected observation epoch. The bounding-box area of the coastline decreased by 6.33 km 2 between 2015 and 2024, indicating a net contraction of the coastal spatial envelope despite localised erosion and accretion. A random forest regression model was used to hindcast the 2024 coastline and achieved an RMSE of 12.8 m, a mean displacement of 9.6 m, an MAE of 10.7 m, and an R 2 of 0.84. The resulting six-epoch dataset provides a consistent geospatial record of coastline change over a 10-year calendar period and supports scenario-based projections of future HAT-referenced coastline positions. The proposed legal coastline is intended as a scientific spatial reference for future harmonisation of land–sea administration, spatial planning, and coastal risk management; it does not replace existing statutory boundary definitions in Indonesia.