Baseline (sea)

The baseline is the fundamental reference line in the law of the sea. Under Article 5 of UNCLOS, the normal baseline is the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state. This rule reflects customary international law and was already recognized in the 1958 Geneva Convention on the Territorial Sea and the Contiguous Zone. The baseline is not merely a geographic concept; it has direct legal consequences, as all maritime zones are measured seaward from it. The coastal state's sovereignty extends to the territorial sea (up to 12 nautical miles), while sovereign rights over resources extend to the EEZ (up to 200 nautical miles). The baseline also determines the outer limit of internal waters, which lie landward of the baseline and are subject to full territorial sovereignty.

In certain geographic conditions, states may employ straight baselines that depart from the low-water line. Article 7 of UNCLOS permits straight baselines where the coastline is deeply indented or fringed with islands, or where there is a delta or other natural conditions that make the coastline highly unstable. The method involves drawing straight lines connecting appropriate points, which may enclose waters that are sufficiently linked to the land domain to be considered internal waters. However, the use of straight baselines is subject to limitations: they must not depart appreciably from the general direction of the coast, and the waters enclosed must be sufficiently linked to the land. States must clearly indicate baselines on charts or lists of geographic coordinates. The International Court of Justice and arbitral tribunals have addressed disputes over excessive straight baselines, such as in the 2012 Bangladesh/Myanmar case, where the tribunal cautioned against drawing baselines that cut off the territorial sea of another state.

Special rules apply to particular coastal features. For islands situated on atolls or having fringing reefs, the baseline is the seaward low-water line of the reef (Article 6). Bays are governed by Article 10: a bay is a well-marked indentation whose area is at least as large as that of a semicircle whose diameter is the line across its mouth. If the mouth does not exceed 24 nautical miles, a closing line may be drawn, and the waters landward are internal. For river mouths, the baseline is a straight line across the mouth between points on the low-water line of the banks (Article 9). These provisions aim to balance the interests of coastal states with the freedom of navigation and other uses of the sea. The rules also apply to archipelagic states, which may draw straight archipelagic baselines joining the outermost points of the outermost islands, subject to specific criteria regarding water-to-land ratios and lengths of lines (Article 47).

Beyond the mainstream rules, several lesser-known aspects merit attention. The concept of the baseline has historical roots in the 'cannon shot' rule, which tied the breadth of the territorial sea to the range of coastal artillery. The low-water line is not a static feature; it shifts with tides and coastal erosion, leading to practical challenges in charting. Some states, such as the United Kingdom, have used 'baseline points' that are not always on the mainland but on outlying rocks or islands. The 2012 Bangladesh/Myanmar arbitration also addressed the use of straight baselines on a deltaic coast, noting that the coastline must be 'highly unstable' to justify such lines. Another edge case involves 'historic bays' such as the Chesapeake Bay, where the United States claims internal waters based on historic title, a doctrine recognized in customary law. Additionally, the baseline can affect the delimitation of maritime boundaries between opposite or adjacent states, as seen in the 1969 North Sea Continental Shelf cases. The role of baselines in the context of sea-level rise is a growing concern, as rising waters may alter the low-water line and thus the extent of maritime zones, a topic debated in international forums.

Territorial sea

A belt of coastal waters extending up to 12 nautical miles from the baseline, over which the coastal state exercises sovereignty.

Exclusive economic zone (EEZ)

An area beyond and adjacent to the territorial sea, extending up to 200 nautical miles from the baseline, where the coastal state has sovereign rights over natural resources.

Contiguous zone

A zone extending up to 24 nautical miles from the baseline, where the coastal state may exercise control to prevent and punish infringement of its customs, fiscal, immigration, or sanitary laws.

Internal waters

Waters landward of the baseline, over which the coastal state has full sovereignty, subject to certain rights of innocent passage for foreign vessels in straits.

Archipelagic baseline

Straight lines drawn joining the outermost points of the outermost islands of an archipelagic state, used to measure maritime zones.