A baseline, as defined by the United Nations Convention on the Law of the Sea, is the line (or curve) along the coast from which the seaward limits of a state's territorial sea and certain other maritime zones of jurisdiction are measured, such as a state's exclusive economic zone. Normally, a sea baseline follows the low-water line of a coastal state. This is either the low-water mark closest to the shore or an unlimited distance from permanently exposed land, provided that some portion of elevations exposed at low tide but covered at high tide (such as mud flats) is within 3 nautical miles (5.6 kilometres; 3+1⁄2 statute miles) of permanently exposed land. When the coast is deeply indented, has fringing islands or is highly unstable, straight baselines may be used.

Measurement of baseline

The following methods are used to measure a baseline under United Nations Convention on the Law of the Sea 1982.

Normal baseline

Except where otherwise provided in this Convention, the normal baseline for measuring the breadth of the territorial sea is the low-water line along the coast as marked on large-scale charts officially recognized by the coastal State.

Reefs

In the case of islands situated on atolls or of islands having fringing reefs, the baseline for measuring the breadth of the territorial sea is the seaward low-water line of the reef, as shown by the appropriate symbol on charts officially recognized by the coastal State.

Straight baselines