As used in UNCLOS Articles 6 and 47, “atoll” means a reef with or without an island situated on it surrounded by the open sea, that encloses or nearly encloses a lagoon.
In LOAC-governed situations under the “other rules of international law” clauses in UNCLOS, a different definition may apply. The same may be the situation if the UN Charter supersedes UNCLOS or if jus cogens norms apply.
Consolidated Glossary ¶ 9 defines “atoll” as “[a] ring-shaped reef with or without an island situated on it surrounded by the open sea, that encloses or nearly encloses a lagoon.” UNCLOS Article 121 defines “island” as:
1.… a naturally formed area of land, surrounded by water, which is above water at high tide.
2.Except as provided for in paragraph 3, the territorial sea, the contiguous zone, the exclusive economic zone and the continental shelf of an island are determined in accordance with the provisions of this Convention applicable to other land territory.
3.Rocks which cannot sustain human habitation or economic life of their own shall have no exclusive economic zone or continental shelf. Territorial Sea Convention Article 10 is similar to UNCLOS Articles 121(1) and 121(2) and refers to other provisions of that Convention.
UNCLOS Article 6 declares that for islands situated on an atoll or an island having a fringing reef, the baseline for measuring the territorial sea is the seaward low water line of the reef as shown by the appropriate symbol on charts the coastal State officially recognizes. Article 47(7) similarly says that for computing the ratio of water to land when establishing archipelagic waters of an archipelagic State as UNCLOS defines that State in Article 46(a), atolls and waters within them may be included as part of the land area of an archipelagic State. Under Article 47(1), an archipelagic State may draw straight archipelagic baselines joining outermost points of the outermost islands and drying reefs of the archipelago, defined in Article 46(b), provided that within such baselines are included the main islands and an area in which the ratio of the area of the water to the area of the land, including atolls, is between 1 to 1 and 9 to 1.