Computing Australia's Maritime Boundaries
Last updated:7 June 2023
Delineating the Territorial Sea Baseline
Critical to the determination of all maritime boundaries is the determination of the Territorial Sea Baseline (TSB) around Australia and its remote offshore territories. Essentially, the TSB is the line of low water along the coastline. However, United Nations Convention on the Law of the Sea (UNCLOS) allows for the TSB to jump across bays (bay closing lines), rivers (river closing lines) and between islands, as well as along heavily indented areas of coastline (straight baselines) under certain circumstances.
Article 5 of UNCLOS defines the baseline as 'the low-water line along the coast as marked on large scale charts officially recognised by the coastal State'. 'Low-water' is not defined and Australia has elected to use Lowest Astronomical Tide (LAT) as this is the datum used on hydrographic charts.
The TSB was originally determined in the early 1970s based on small scale mapping supported by some aerial photography. This data has now been completely revised using the most appropriate and current source information available, such as State mapping data, hydrographic charts and satellite imagery.
Determining Lowest Astronomical Tide
Lowest Astronomical Tide (LAT) is the lowest level to which sea level can be predicted to fall under normal meteorological conditions.
It would be a laborious and costly exercise to determine an exact LAT along all of Australia's vast coastline and therefore a best estimate is usually based on current charting, topographic maps, satellite imagery, and other sources.
Australia's TSB is generally consistent with hydrographic charts. However, Australian charts are prepared for the purposes of safe navigation and may not depict all physical features relevant to the generation and delineation of Australia's maritime zone. Physical changes to the coast and improved methods of locating the LAT line in the future may lead to differences between the TSB and the information depicted on charts in certain areas.
The difference between high tide (usually mean high water - MHW) and LAT can be significant. The above image of North Reef shows the difference between these two lines. Accurate determination of LAT can therefore present some difficulties, particularly in areas of large tidal range and gradually sloping foreshores. The image above is an illustration of a large tidal range.
Use of Straight Baselines
Under Article 7 of UNCLOS, outer points of deeply indented coastlines, or fringing islands, may be linked by straight baselines. There are 393 straight baselines around Australia and its offshore territories. The co-ordinates of the terminal points of these baselines were originally proclaimed in 1983, for the Seas and Submerged Lands Act (1973). Many of these co-ordinates were based on small scale topographic maps (the best available information at the time).
Geoscience Australia has updated these baselines against more recent and accurate information supplied by the individual State and Territory mapping agencies as well as the Australian Hydrographic Office and other Commonwealth agencies and new terminal points were proclaimed in February 2006. The Australian Maritime Boundaries (AMB) data release contains the revised straight baselines as are currently proclaimed.
Computing the outer limit
The computation of the outer limit of the various maritime zones is a complex task. It requires the selection of appropriate basepoints on the TSB from which to compute an envelope of arcs. In addition, allowance must be made for sections of the boundary derived from straight line segments, such as straight baselines.
A rigorous computation of the location of the outer limit line requires geodetic calculation on the spheroid rather than simple line buffering.
In order to assist with these computations, Geoscience Australia has used specialised software known as MarZone. This software was developed by Melbourne University's Department of Geomatics under contract to Geoscience Australia. This software automatically selects the most appropriate basepoints for a given maritime zone (i.e. 3, 12, 24, 200 or 350 nautical miles (M)) and computes the limit line using rigorous geodetic computations. It also assists with the Article 76 requirements for the computation of the Extended Continental Shelf.