OS NGD Water
Introduction to the theme
The OS National Geographic Database (NGD) Water Theme contains both the topographic and network representations of water features across Great Britain. The theme is made up of two collections (Water Features and Water Network), which in turn are made up of a combined total of nine feature types.
The OS NGD Water Features Collection contains topographic feature types that have geometry, feature classification (for example, a drain or an open reservoir) and other additional information attributed:
Inter Tidal Line features represent watercourses that extend across the inter tidal zone.
River Basin District Catchment features define a river basin or several river basins, together with coastal waters, as published by the authoritative national water bodies.
Tidal Boundary features show the alignment of the limit of tidal water at specific states of the tide.
Areas of permanent water are listed as Water features – these are a polygon topographic representation only.
Features associated with permanent water which are too small to be captured as a Water feature but are considered to be of interest for capture are listed as Water Point features (for example, a waterfall).
Waterbody Catchment features represent the unit of surface water being the whole (or part) of a stream, river or canal, lake or reservoir, as published by the authoritative national water bodies.
Narrow water features which are depicted by a line geometry rather than a polygon geometry are not included in the OS NGD Water Features Collection; however, they can be obtained from the OS NGD Water Network Collection.
The OS NGD Water Network Collection provides a three-dimensional structured link and node network of watercourses for Great Britain. This collection includes the feature types of Water Link, Water Link Set and Water Node:
A Water Link feature is a network representation of the general alignment and connectivity of permanent water (including inland rivers, tidal water, lakes, canals, and so on); each link is attributed with additional information (for example, flow direction and catchment name).
A Water Link Set feature represents a body of water with an accepted name that the water network flows through; it provides a single geometry for a complete named watercourse. Water Link Set features will be provided with a cross reference table, referencing the Water Link Set back to the Water Links of which it is comprised.
A Water Node feature depicts the location where a connected area of permanent water ends, where an area of permanent water is connected to two or more other areas of permanent water, or where a recorded attribute changes for an area of permanent water.
Data structure
The OS NGD Water Theme is made up of two collections: OS NGD Water Features and OS NGD Water Network. The OS NGD Water Features Collection is comprised of six feature types: Inter Tidal Line, River Basin District Catchment, Tidal Boundary, Water, Water Point, and Waterbody Catchment. The OS NGD Water Network Collection is made up of three feature types: Water Link, Water Link Set, and Water Node.
Unique identifiers
With the exception of the River Basin District Catchment, Waterbody Catchment, and Water Link Set features, two unique identifiers are provided with each feature within the OS NGD Water Theme:
OSID (Ordnance Survey Identifier): The primary identifier and unique key for this theme.
TOID (Topographic Identifier): An additional secondary identifier which can aid further data linking. TOIDs are an optional attribute and therefore will not always be provided with every feature.
River Basin District Catchment, Waterbody Catchment and Water Link Set features do not have TOIDs allocated to them; however, they do have OSIDs allocated to them.
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