The legal coastline as a unifying framework for land and marine cadastre

BACKGROUND AND OBJECTIVES: Coastal zones represent dynamic interfaces where terrestrial and marine spaces meet, often creating challenges for governance, spatial planning, and cadastral clarity. In Bekasi Regency, the nonexistence of a unified reference line between land and sea has given rise to overlapping jurisdictions, ambiguous land tenure, and complications in carrying out integrated coastal management. The study aimed to define a legal coastline founded on the high-water spring datum, serving as a unified framework for land and marine cadastre, while ensuring conformity with the International Hydrographic Organization S-4 and C-13 standards and Indonesian Law Number: 32/2004 on coastal zone governance. METHODS: The research integrated extensive tidal data from Muara Bendera, orthophotos derived from unmanned aerial vehicles, the extraction, and global navigation satellite system surveys to delineate the High-Water Spring shoreline. Unmanned aerial vehicle images were collected at both high and low tide to capture the full range of shoreline positions, while real time kinematic measurements provided precise ground control for validation. The shoreline was subsequently modified to align with the high-water spring vertical datum as per the guidelines set forth by International Hydrographic Organization, and it was cross-validated across various coastal morphologies, including sandy beaches and mangrove-lined areas. FINDINGS: Unmanned aerial vehicle mapping achieved strong performance on sandy coasts (Muara Beting: mean positional error 2.06 meters, overall validation score 75.6/100) but struggled in mangrove-dominated areas (Muara Bendera: mean error 34.63 meters, score 37.7/100), demonstrating the importance of RTK ground validation in complex coastal environments. Despite these variations, the integrated workflow proficiently created a continuous high-water spring-based coastline for Bekasi Regency that aligns with both scientific and legal standards. CONCLUSION: The study concludes that adopting a high-water spring based legal coastline provides a stable and authoritative boundary to unify land and marine cadastre. It addresses spatial overlaps while simultaneously reinforcing the framework for administrative governance, spatial planning, and coastal zone management. While unmanned aerial vehicle photogrammetry offers a practical solution for sandy coast, mangrove habitats demand a combination of remote sensing techniques and ground-based surveys.