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This code list is used in association with the Associated Structure attribute which is present within multiple OS NGD feature types across various OS NGD themes. The code list indicates whether the feature is part of a larger structure (such as a dam, bridge, or breakwater) that performs a specific function, and, if so, what that function is.

Label	Definition
Aqueduct	A long high structure (resembling a bridge) made of stone, metal, or brick, which carries a water supply or canal over a large depression in the land.
Breakwater	A very large wall of stone that is 1m wide or more, extending from shore to sea to protect a harbour or beach from the force of the waves.
Bridge	A structure that is built over a river, railway, road, or ravine to permit the flow of traffic (road, rail, canal, or pedestrian) or to carry a facility, such as a pipeline. A bridge originally built to allow railways, roads, tracks, or canals to pass over obstructions that has been converted for use by pedestrians, cyclists, or ridden horses is represented as a Bridge and not as a Footbridge.
Clapper Bridge	An ancient form of bridge, allowing pedestrians and, in some cases, horse-drawn traffic to cross a river or stream. The bridge consists of large, flat, stone slabs supported on stone piers.
Canal Tunnel	A long narrow passage underground through which a canal passes.
Dam	A structure built across a river or stream to control the flow and level of the water and to create a body of water upstream for consumption and / or power generation.
Footbridge	A built structure which passes over an obstruction and is primarily for pedestrian use.
Leisure Pier	A structure extending into the water, used as a platform for recreational purposes.
Lift Bridge	A bridge that can be swung upwards to allow the passage of river or canal traffic beneath, or to close an adjacent gateway.
Natural Subterranean Passage	A natural underground passage.
Pedestrian Tunnel Or Subway	A passage for pedestrians, cyclists, or horses being ridden that passes under roads or railways below the general level of the ground.
Rail Tunnel	A long narrow passage underground or underwater through which a railway network passes.
Road Tunnel	A long narrow passage underground or underwater through which a road passes
Sluice	A set of structures, including a passage, that carries a current of water and has a mechanical barrier (a sluice-gate) that can be opened and closed to control the flow of water.
Swing Bridge	A structure designed to carry road, rail, or foot traffic over another feature, all or part of which can be moved in the horizontal plane to allow the passage of large vehicles beneath.
Tanker Berthing	A mooring for large vessels designed for the carriage of liquid cargo in bulk.
Transporter Bridge	A structure that moves a suspended section of roadway over a river, allowing the passage of tall ships beneath.
Tube Or Metro Tunnel	A long narrow passage underground or underwater through which a tube or metro network passes.
Underground Conduit	An underground conduit, typically containing pipes or cables.
Underground Flood Relief Channel	An underground flood relief channel.
Underpass	A means of pedestrian access under a road or railway.
Viaduct	Multiple arches carrying a road or railway over multiple features, typically to allow continuation of a route at a higher level.
Watercourse Tunnel	A water tunnel not designed for transport.