Summary and Background Terminology describing marine areas Baseline
24.The marine area around the UK coast is sub-divided into a number of zones. These are measured from a “baseline”. This is usually the low water mark around the coast. But there may be straight baselines across the mouths of bays, and all rocks, reefs etc above the sea at low water but submerged at other times extend the baseline if they are within 12 nautical miles (“nm”) of the mainland or an island. The UK baseline is delineated in the Territorial Waters Order in Council 1964 (as amended by the Territorial Sea (Amendment) Order 1998, SI 1998/2564).