distance from water to the highest point on a vessel or lowest point on a bridge span, etc.
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The deck of this lift bridge on Canada's Welland Canal typically rests only a few metres above the water level. When a ship approaches, the deck is raised to provide sufficient air draft (or draught) for the vessel to pass through.
Air draft (or air draught) is the distance from the surface of the water to the highest point on a vessel (a ship or boat).