OS MasterMap Networks - Water Layer
Demonstrating the power of Ordnance Survey Networks products
The OS MasterMap Water Network Layer product (also referred to as OS MasterMap Networks – Water Layer) will become End of Life on 31 March 2026. As of this date, Ordnance Survey will cease to maintain or publish updates, and access to the product will be removed from the OS Data Hub, OS Features API and OS Vector Tile API. More information is available on the End of Life Product Notices page.
To use this demonstrator you agree to the terms of this Licence
Introduction
The OS MasterMap® Networks - Water Layer is a digital representation of Great Britain’s watercourses. It includes rivers, streams, lakes, lochs and canals as a series of watercourse network lines. Watercourses that are underground or below structures are also included where Ordnance Survey capture processes or accepted sources can infer the connection. The product also includes three-dimensional coordinates of network features.
The OS MasterMap Networks - Water Layer is supplied in a range of formats to suit your needs – GML 3.2, GeoPackage, or Vector Tiles (MB Tiles).
The product is released quarterly in January, April, July and October
What information does the OS MasterMap Networks - Water Layer contain?
The OS MasterMap Networks - Water Layer is a vector product, comprised of lines - water network features, and points – water nodes.
The lines (shown highlighted on this interactive map) are known as WatercourseLink features and are derived from Ordnance Survey’s MasterMap Topography Layer. They sit along the centre of the real world watercourse - such as rivers, streams, lakes, lochs, and canals. They aren’t continuous line features; instead they are split in the following circumstances:
- Where two or more watercourses meet.
- Where the real-world-based attribution of a watercourse changes.
- Where the name of a watercourse changes or ceases to apply.
- They may be split where they pass into a culvert or tunnel or onto an aqueduct.
The points (shown highlighted on this interactive map) are known as HydroNode features. These represent the source, sink, or junction of a watercourse. They also occur where any real-world attribution changes. They can only occur at the end point of a WatercourseLink feature.
Where does the OS MasterMap Networks - Water Layer data come from?
The data in the OS MasterMap Networks - Water Layer is derived from the OS MasterMap Topography Layer, and OS MasterMap Imagery Layer. Surveyors use these datasets in conjunction to then infer the centre lines of a watercourse to create the WatercourseLink layer.
Water network links will be captured for all water features that meet the requirements for topographic capture, except for areas of static water with no inlet or outlet that do not form a linear feature, e.g. dew ponds.
Links may also be captured for pipes and culverts. These can be captured or inferred by Ordnance Survey, or obtained from external sources.
All network features in the WatercourseLink layer will start and end with a Hydro Node. HydroNodes can be classified as junction, Outlet, source, or Pseudo. Nodes which do not have a determined category will be given a value of 'null'. The table below outlines the requirements for each type of node classification.
Two watercourses merging
Two watercourses meeting
Where two or more watercourses meet, the network lines will be merged to form one.
The resulting link will be attributed with the name of the watercourse which the other has merged with.
Attribution change
Where the attribution of the watercourse changes within a section, it will result in a new watercourse link segment being created.
For example, here is a sluice where a watercourse goes from an inland river to a tidal river, meaning the WatercourseLink is broken at the point of change by a HydroNode. HydroNodes which occur at the point of a name or attribution change on the watercourse will be classified as 'pseudo'.
Underground watercourses
Underground and below structures
Where a watercourse flows underground, surveyors will use the visible information they have to infer the information on the watercourse. For example, where a watercourse flows through an underground tunnel, surveyors will record each end of the tunnel and create a connecting link between them.
For those watercourses which flow underground, we feed in hydrological data from the Environment Agency to populate the attribution of these.
These features are populated with 'underground' in the 'level' attribute.
Three-dimensional coordinates attributed to the OSMM Network - Water Layer
The OS MasterMap Network - Water Layer is heighted by interpolation from a bare earth terrain comprising mass points and breaklines. The highest resolution height information available was used in this process. This terrain was captured by Ordnance Survey recently and is the basis for new products that include a height component. Please note that this is a static dataset and therefore is not maintained.
The data used to height the network is of a resolution and quality that in some cases results in height attribution that contradicts the flow attribution. This is typically in areas with little change in elevation. In these cases, the calculated gradient attribute will not be supplied.
Attribution of the OS MasterMap Network - Water Layer
The OS MasterMap Network - Water Layer provides a detailed centre line following the curve of the waterway precisely. It includes the coordinates of watercourse sources and where they meet, exactly. Each feature in the dataset contains the below attributes. Here, we will go through each one and explain what they are referring to. Please note, the casing of attribute headers differs slightly between formats of the data - these differences are shown in the tables below:
WatercourseLink:
HydroNode:
OS Open Rivers™
In addition to the Premium OS MasterMap Network - Water Layer, we also publish an opensource dataset - OS Open Rivers™.
OS Open Rivers™ is a generalised product which is automatically derived from Ordnance Survey large-scale data. The nominal usage scale is 1:25 000, with a recommended viewing scale range of 1:15 000 to 1:30 000.
This product shows the high-level view of watercourses in Great Britain, including freshwater rivers, tidal estuaries, and canals. It is updated every 6 months, in April and October.
It is also made up of a WatercourseLink network (highlighted on the interactive map to the left), and a HydroNode point layer (highlighted on the interactive map below). The WatercourseLink feature represents the alignment of a watercourse.
The HydroNode layer explicitly represents the start, end, and junctions of watercourses, and places where related real-world attribution changes have been recorded, for example, the point where a watercourse becomes tidal. The HydroNode is coincident with the ends of related links.
Both the WatercourseLink and HydroNode layers have a reduced attribution when compared to OS MasterMap Network - Water Layer, but still include key information such as flow direction, length, form, and name.
Where does the OS Open Rivers™ data come from?
The data contained within this product is derived from the OS MasterMap Water Network product. It is generalised at a scale between 1:15 000 and 1:30 000 to create a product which is suitable for smaller scale mapping and more of an overview of Great Britain's water network.
Which parts of the OS MasterMap Water Network come through into OS Open Rivers™?
Only features from the OS MasterMap Network - Water Layer with a Primary function (primacy = 1) are then generalised and appear in the OS Open Rivers™ product. Features with a Secondary function (primacy = 2) are only available in the OS MasterMap Network - Water Layer.
This primacy attribute is derived by assessing a combination of the flow, width, gradient and length of the network WatercourseLink feature to identify the most likely course of the main flow of a watercourse at bifurcations.
Attribution of OS Open Rivers™
Please note, the casing of attribute headers differs slightly between formats of the data - these differences are shown in the tables below:
WatercourseLink:
HydroNode:
Comparing OS MasterMap Network - Water Layer and OS Open Rivers™
Use the interactive swipe map below to see the difference in geometry, features, and attribution between these two products for yourself.
OS MasterMap Network - Water Layer (left) and OS Open Rivers™ (right).
Using the OS MasterMap Network - Water Layer for flood risk analysis
The OS MasterMap Network - Water Layer can be used for a wide variety of practical applications.
Here, the data has been styled to show direction of water flow, and highlight the watercourse name.
In order to get a clearer picture of the main watercourse, compared to its tributaries and any other parts of the network, we have styled the data based on the 'width' attribute of each line segment in WatercourseLink.
Next we have added flood risk area polygons, which is an open dataset available from the Environment Agency.
We can use these in conjunction with the watercourse widths to illustrate the primary watercourse running through Exeter, allowing us to build up a picture of where the most at-risk areas of flooding are.
Finally, bringing in building footprints and intersecting these with the flood catchment polygons highlights those buildings which are most at risk.
This could be used by risk mitigation organisations to prioritise homes for evacuation in the case of emergency. It would also be useful in building up a risk model for building insurance purposes.
This story map has been created using the following data:
- OS MasterMap Network - Water Layer - WatercourseLink
- OS MasterMap Network - Water Layer - HydroNode
- OS Open Rivers - WatercourseLink
- OS Open Rivers - HydroNode
- OS Open Zoomstack - Local Buildings
- Environment Agency Flood Risk Areas
Further detail about these can be found on the OS website product pages .
Documentation about the layers shown here can be found at:
Overview: OS Open Rivers | OS MasterMap Networks - Water
Getting Started Guides: OS MasterMap Networks - Water
Technical Specification: OS Open Rivers | OS MasterMap Networks - Water
The logo in the top left of this story map will take you to the main OS website homepage.
More story map examples like this one can be found on the More Than Maps site .
