Comparison of Water Network Products
Comparing OS MasterMap Water Network Layer to the OS NGD Water Network Collection
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.
Overview
The purpose of this page is to provide a technical summary of things to consider when migrating from OS MasterMap Water Network Layer to the OS NGD Water Network Collection. It covers data supply and formats, as well as a comparison of the attribution of the two products.
Key features
The OS MasterMap Water Network Layer product includes the following key features:
Flow and connectivity
Three-dimensional geometry
Catchment information, including name
The names of watercourses, including language alternatives
Average widths
All these key features are also present in the OS NGD Water Network Collection. OS NGD data also offers the following additional improvements (with several planned enhancements due to be delivered soon):
It provides greater analytical ability from more detailed attribution, for example:
The Water Link
descriptionattribute has been enhanced to provide a more detailed indication of what the link represents. Drains, Leats and Overflows are now classified and included.The Inland or Tidal nature of a Water Link is described by the
watertypeattribute.The
physicalcontainmentattribute describes whether underground watercourses are contained in a culvert, a tunnel or a natural subterranean passage, in addition to the existing level attribution.
The names of watercourses (where present) are given at primary, secondary and tertiary levels, including language alternatives.
The enhancement to improve the width of watercourses, due in September 2025, offers improved methods of calculation and greater coverage across Great Britain to support hydrological modelling.
The Water Link Set Feature Type provides a single geometry for a complete named watercourse, making it easier to understand whole rivers.
Feature types
OS MasterMap Water Network Layer has two feature types:
Watercourse Link
Hydro Node
In the OS NGD Water Network Collection, there are corresponding feature types for:
Water Link
Water Node
These are directly comparable and contain identical geometries.
The OS NGD Water Network Collection also contains a Water Link Set Feature Type, which provides a single geometry for a complete named watercourse.
Currency
Both OS MasterMap Water Network Layer and the OS NGD Water Network Collection have an update frequency of quarterly (January, April, July and October).
Data supply
File formats
OS MasterMap Water Network Layer is available in:
Geography Markup Language (GML)
GeoPackage
Vector tiles
It is also currently supplied in the OS Features API and the OS Vector Tile API (although access to the product will be removed from both APIs in March 2026).
The OS NGD Water Network Collection is available in:
GeoPackage
CSV (comma-separated values)
GeoJSON
Vector tiles
The download service of OS Select+Build supplies GeoPackage and CSV formats, OS NGD API – Features supplies GeoJSON, and OS NGD API – Tiles supplies vector tiles.
The OS NGD Documentation site contains further guidance on the differing formats and APIs available:
Data ordering
Through the OS Data Hub, it is currently only possible to order OS MasterMap Water Network Layer in one of the following options:
Geography Markup Language (GML): Individual 5km² tiles, available for the whole of Great Britain or covering a specified area of interest
GeoPackage: Single file containing full Great Britain coverage
Vector tiles: Single MBTiles file containing full Great Britain coverage
It is not possible to order a true area of interest as the data is supplied in GML 5km² tiles. It is also not possible to order a true Change-Only Update (COU); instead, you can only choose to receive the 'changed chunks', which resupply an entire 5km² tile if any change has occurred within it.
All OS NGD download data is supplied through OS Select+Build in the OS Data Hub. OS Select+Build offers a much more tailored experience to ordering and downloading OS data with an improved range of options, including:
True areas of interest are supplied, as opposed to 5km² tiles, removing need to deduplicate adjoining 'hairy' tiles.
In addition to updates being delivered as full refreshes, it is also possible to take COUs in CSV format for full Great Britain coverage, so you'll receive only the updated features.
With OS Select+Build, it is also possible to filter your supply on the description, catchmentname, and catchmentid attributes, so you receive only the features that you require.
OS NGD documentation contains guidance on downloading with OS Select+Build and using and loading OS NGD data in both CSV and GeoPackage formats:
Unique identifiers
In OS MasterMap Water Network Layer persistent managed identifiers are supplied called TOIDs (Topographic Identifiers). These are also part of the inspireId and the localId attributes. TOIDs are strings of up to twenty characters, starting with 'osgb'; the remaining characters are digits (0 to 9).
The OS NGD Water Network Collection uses a new identifier called the OSID (OS Identifier) to uniquely identify features. The OSID is a Universally Unique Identifier (UUID) and requires no centralised authority to ensure uniqueness. It also provides the unique referencing of a Water Link to its corresponding startnode and endnode based on the Water Node's osid attribute.
The new Water Link and Water Node Feature Types also contain a toid attribute, which allows linking back to OS MasterMap Water Network Layer.
After 31 March 2026, when we cease to maintain or publish updates to OS MasterMap Water Network Layer, new TOIDs will cease to be generated and therefore will not appear in new OS NGD Water Network Collection features.
Attribution comparison
OS NGD data has been enriched with attribution to ensure that it's straightforward to navigate and query. Attribute names have also been simplified to make them easier to understand.
Watercourse name attribution
The various names of the watercourses are currently found in these OS MasterMap Water Network Layer attributes:
In the OS NGD Water Network Collection, the watercourse names can be found in the below attributes:
There is also additional attribution for the language of the names (suffixed with _language; for example, name1_language) and also identifiers of the corresponding Water Link Set in the nameid, namesecondaryid and nametertiaryid for each named set of Water Links.
Lifecycle and metadata
Both the Watercourse Link and Hydro Node Feature Types contain attributes relating to their lifecycle, including reasonForChange, versionId and beginLifespanVersion; comparable information can be found in the changetype, versiondate, versionavailablefromdate and versionavailabletodate OS NGD attributes.
Attribute comparison table
The following table provides a list of the OS MasterMap Water Network Layer attributes and their equivalents in the OS NGD Water Network Collection:
The HydroNode coincident with the first / last vertex for this WatercourseLink.
Attributes serves the same purpose; now uses OSIDs as opposed to TOIDs.
The nature of the watercourse being represented by the feature.
A description of the relationship of the watercourse to ground level.
Direction of water flow in the segment relative to digitisation of segment geometry.
Identical attribute.
Value indicating the relative importance of the WatercourseLink within any larger watercourse it is part of.
Identical attribute.
The origin of the geometry.
These attributes split the source and capture specification, rather than combining them into a single attribute.
The name of the catchment area where the WatercourseLink falls, assigned by a responsible body.
Identical attributes.
Measurements relating to the WatercourseLink
Identical attributes; however, enhancements to width are due to be supplied to the OS NGD Water Network Collection in September 2025.
A calculated value indicating the relative cartographic importance of a WatercourseLink to aid selection and depiction when styling the data.
Identical attribute.
Describes if the watercourse contains water year-round (in normal conditions) or only contains water intermittently.
Identical attribute.
Nature of the HydroNode.
The OS MasterMap Water Network Layer attributes above use the names from the GML format; please see the Attribute naming differences between GML and GeoPackage formats page for the GeoPackage equivalents.
Help and support
Supporting links
The OS NGD Documentation site has further information that you may find useful. Some particularly relevant sections that may aid you with migration are:
Contact us
If you have any questions, need assistance, or want to provide feedback, please don't hesitate to reach out to us at os.uk/contact-us.
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