Integrating the continuous hydrographic datum and DEM for maritime boundary delimitation

The delimitation of maritime boundaries plays a significant role in preserving a country’s sovereignty and jurisdiction. Their determination justify a country’s rights, allowing coastal states to prevent illegal infringements and trespassing by foreign vessels. A proper delimitation of maritime boundaries also helps to preserve a country’s commercial fishing rights, marine navigation and shipping routes and underground resource exploration, such as that of minerals, oil and gas. The maritime baseline is established based on a combination of maritime basepoints, and represents the low-water line along the coast.

There are three types of maritime baseline: normal, straight and straight archipelagic. The United Nations Convention on the Law of the Sea (UNCLOS) has established several maritime zones with their respective distances from the coast that are applicable to all coastal states, as illustrated in Figure 1.

To this day, the maritime basepoint is still determined based on a limited number and sparse distribution of tide gauge stations along the coast. However, as previous studies show, the resulting hydrographic datum is only valid in coastal areas around the tide gauge stations, and is inaccurate at distances a few tens of kilometres away from the tide gauge stations and in offshore areas. Such limitations raise uncertainty in the establishment of the maritime basepoints and baselines, particularly when there is a large distance between tide gauge stations. Another challenge regarding the uncertainty in maritime boundary delimitation is the overlapping claim by neighbouring states, particularly in the territorial sea (TS) and exclusive economic zone (EEZ) regions. Unresolved maritime boundaries can heighten tensions and increase the potential for military confrontations, as states may engage in aggressive stances.

Another issue is legal ambiguities, where inconsistent enforcement practices may result from varying interpretations of international law, particularly the UNCLOS. There are also environmental concerns, as disputed areas may suffer from a lack of attention to environmental protection efforts, worsening problems such as pollution and habitat destruction. Coastal development is another issue. Natural changes in coastlines due to natural processes can lead to outdated baselines, complicating urban planning and coastal infrastructure development. Technological challenges also arise as advancements in mapping and surveying may outpace legal frameworks, leading to disputes over which data should be used to establish baselines. Last but not least is the issue of international cooperation: uncertainty can hinder regional cooperation on issues such as search and rescue operations, environmental protection and joint resource management.

Variety of data sources

A continuous hydrographic datum (CHD), which integrates the hydrographic datum from multiple sensors, can mitigate this problem. In this study, a CHD was developed based on the hydrographic datum derived from tide gauge stations, multi-mission satellite altimeter (TOPEX/Poseidon, Jason-1, Jason-2, Jason-3, GEOSAT Follow On (GFO), ERS-1, ERS-2, ENVISAT-1, Cryosat-2, SARAL, Sentinel-3A and Sentinel-6), satellite-derived bathymetry, a hydrodynamic model accessible via Tide Model Driver (TMD) and shipborne bathymetry using single-beam echosounder (SBES) and multibeam echosounder (MBES) data. The data sources used to develop the CHD are shown in Figure 2.

Common reference surface

Several CHDs have been developed in the past decades, comprising bathymetry with reference to the Ellipsoid (BATHYELLI, 2005), Vertical Offshore Reference Frame (VORF, 2009), Continuous Vertical Datum for Canadian Water (CVDCW, 2010), Vertical Reference Frame for the Netherlands (NEVREF, 2018), Saudi Continuous Chart Datum (SCCD, 2019) and the Malaysia Vertical Separation (MyVSEP) model (2022), as illustrated in Figure 3.

However, the derived hydrographic datum for each sensor must first be referenced to a common reference surface, which is either World Geodetic System 1984 (WGS84) or Geodetic Reference System 1980 (GRS80) globally, or the local mean sea level (MSL). This article proposes the utilization of the local MSL as a reference surface, meaning that the derived lowest astronomical tide (LAT) and highest astronomical tide (HAT) from every sensor are referenced to MSL, denoted as  and . As a way of integrating the derived hydrographic datum from every sensor, this study proposes using a spatial interpolation technique (with a grid size of ≤ 10km). Methods such as ordinary kriging, minimum curvature Spline, inverse distance weighting (IDW) and many other spatial interpolation techniques are recommended.

Comprehensive foundation

Several reasons justify the establishment of a CHD. First, the dynamic ocean environment is constantly changing due to tides, currents and climatic variations. A CHD can accommodate these fluctuations, providing real-time data that enhances navigational safety. Second, with the increase in maritime activity with the rise of shipping traffic, fisheries and offshore energy exploration, the demand for accurate hydrographic data has surged. Thus, CHD allows for updated information that supports efficient route planning and resource management. Third, climate change plays a significant role. Rising sea levels and changing weather patterns demand adaptive management strategies. A CHD can assist in monitoring these changes, providing essential data for climate research and policy-making. Fourthly, the CHD can be utilized in several applications. A CHD provides up-to-date depth information, which is crucial for safe navigation, particularly in shallow or congested waters. It also plays a key role in environmental monitoring as accurate hydrographic data supports the management of marine ecosystems, enabling better assessments of habitats and biodiversity. Fifthly, in infrastructure development, a CHD aids in the planning and maintenance of maritime infrastructure such as ports, bridges and offshore platforms. Lastly, in the disaster response, real-time data from CHD can enhance preparedness and response strategies for natural disasters such as tsunamis and hurricanes.

Digital elevation bathymetry model

To pinpoint the location of the lowest water line along the coast for the delimitation of maritime boundaries, the established CHD must also be integrated with a high-accuracy digital elevation model (DEM), for example obtained using light detection and ranging (Lidar) or interferometric synthetic aperture radar (IfSAR), or an open-source DEM obtained using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Shuttle Radar Topography Mission (SRTM), Advanced Land Observing Satellite World 3D (ALOSW3D) or TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) data. In a previous study by Indonesia, the Digital Elevation Bathymetry Model (DEBM) was developed in 2022 to update the position of maritime basepoints and baselines, particularly for the region without bathymetry data. Its data sources comprise elevation and bathymetry data. The elevation data used includes the National Digital Elevation Model (DEMNAS), which was developed based on IfSAR (5m resolution), TerraSAR-X (5m resolution) and ALOS PALSAR (11.25m resolution). The spatial resolution is 0.27 arcs per second. The reference surface for DEMNAS is Earth Gravitational Model 2008 (EGM2008). As for the bathymetry data, the tidal data was acquired from Indonesian National Bathymetry Data (INBD), which is based on the General Bathymetric Chart of the Ocean (GEBCO) combined with local bathymetry data. In addition, the DEBM employs SBES and satellite-derived bathymetry (SDB) data using four bands: blue (0.49μm), green (0.56μm), red (0.665μm) and near-infrared (0.842μm) images from Sentinel 2A. It also incorporates hourly tide gauge station data from the Mailepet station (1° 33’ 49.7” S and 99° 11’ 49.2" E), obtained from the Geospatial Information Agency (BIG) from 2011 to 2022.

The DEBM is one of the most important examples of integrating bathymetry data from multiple sensors and DEM for the delimitation of maritime boundaries. There are several advantages to integrating the CHD and DEM. To begin with, there is the accurate depth and elevation data. By combining real-time hydrographic data with high-resolution elevation models, decision makers can achieve a more comprehensive understanding of the underwater terrain and water column dynamics, leading to more accurate boundary definitions. Next is the adaptability to environmental changes. A CHD accounts for fluctuations in sea levels and tidal changes, while the DEM provides static topographical data. Their integration allows for the adaptive management of boundaries in response to climate change and shifting oceanographic conditions. Another advantage is the enhanced spatial analysis. By using geographic information systems (GIS), combined datasets can facilitate complex spatial analyses, including identifying natural features that serve as boundaries, such as underwater ridges or valleys. Lastly is the legal and diplomatic clarity. Accurate and updated data can help resolve disputes over maritime boundaries by providing clear, objective evidence based on scientifically valid measurements.

Conclusion

This article emphasizes the importance of developing a CHD as well as its integration with a DEM for the delimitation of maritime boundaries. It also summarizes and reviews nations that have successfully developed a CHD using multiple sensors, including BATHYELLI, VORF, CVDCW, NEVREF, SCCD and MyVSEP. This article also highlights several DEMs (Lidar, IfSAR, SRTM, ASTER, ALOSW3D and TanDEM-X) that can be used for integration with the CHD. In conclusion, the proposed can improve the conventional approach of only using a limited number of sparsely distributed tide gauge stations, leading to a more continuous, consistent and justified establishment of maritime basepoints and baselines for coastal states.

References

Dewi, R. S., Rachma, T. R. N., Sofian, I., Rimayanti, A., & Artanto, E. (2022). Integrating  Multisource of Bathymetry Data for Updating Basepoint and Baseline Positions of Maritime Boundary. Geographia Technica, 17(1), 18–32. https://doi.org/10.21163/GT_2022.171.02

Hasan, Md. M., Jian, H., Alam, Md. W., & Chowdhury, K. M. A. (2019). Protracted maritime boundary disputes and maritime laws. Journal of International Maritime Safety, Environmental Affairs, and Shipping, 2(2), 89–96. https://doi.org/10.1080/25725084.2018.1564184

Hamden, M. H (2022). Development Of Quasi-Seamless Hydrographic Separation Models Based on Satellite Altimetry and Coastal Tide Gauges in Malaysia. [Unpublished doctoral dissertation]. Universiti Teknologi Malaysia.

National Oceanic and Astronomic Administration (NOAA) (2023). NOAA’s Participation in the U.S. Extended Continental Shelf Project. Retrieved 25 September 2024, from https://oceanexplorer.noaa.gov/okeanos/explorations/ex1810/ecs/welcome.html