Water
Water features are defined as features that contain, delimit, or relate to real-world objects containing water.
Physical water features shown in OS MasterMap Topography Layer include:
Mean High Water (springs) and Mean Low Water (springs)
Swimming Pools, Ponds, Lakes, and Lochs
Moats, Bridges, and Footbridges
Reservoirs, Rivers, Canals, and Streams
Drains and Ditches
Foreshore Features
Floating Objects (they are only shown when they are fixed and attached to permanent detail)
Shake Holes and Swallow Holes (in mountain and moorland areas; limits of numerous shake holes are shown, and the area described as ‘area of shake holes’)
Sluices (except those found in sewage works) and Culverts
Stepping Stones
Taps (which take the form of drinking fountains or that form the communal water supply), Drinking Fountains and Water Troughs (public)
Tidal Gauges
Waterfalls (only if formed by natural features) and Weirs
Bollards, Capstans, and Mooring Posts
Breakwaters and Groynes
Perches, Pilot Beacons, and Navigational Beacons
Pumps, Wells, Spouts, Springs, and Fountains
Taps, water troughs and drinking fountains are no longer captured under the current specification.
The image below shows a sample of real-world objects in the Water theme, including a pond, a river, flow arrows, sluices, and drains:
Non-physical features shown in the Water theme are:
The highest point in a river to which normal tides flow; this is described as the Normal Tidal Limit (NTL). The point is shown and annotated with text.
Low Water Level (LWL); this is the point to which mean tides (or mean spring tides in Scotland) flow at low water. The point is shown and annotated with text.
Textual descriptions of all water features.
Flow arrows, which are symbols used to indicate the direction of flow of non-tidal moving water.
As water is a dynamic element within the landscape, certain survey principles and constraints are imposed on the representation of water within OS MasterMap Topography Layer.
Rivers, streams, and drains are shown at their true scale width. A single line is normally used where their width is less than:
1.0m in urban areas
2.0m in rural, mountain and moorland areas
OS MasterMap Topography Layer does not contain polygons of the open sea. Where inland water bodies meet the sea, the following principles are applied:
Ordnance Survey shows high and low water marks of a mean average tide, that is, an average tide halfway between spring and neap tides in England and Wales, and of average spring tides in Scotland.
In tidal rivers, the point to which mean tides (or spring tides in Scotland) flow at high or low water is included.
Lakes and ponds are surveyed at normal winter level; reservoirs are shown at top water level, that is, spill over level. All water features are described. Continuous topographical water features that extend into private gardens are shown. Where a river flows under another object, typically a bridge, the part of the river beneath the object is not supplied. This is why there are gaps in rivers when the theme is viewed on its own. An example of such a gap in a river is shown below:
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