🆕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 corresponding feature types within 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 description attribute 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 watertype attribute.

  • The physicalcontainment attribute 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 feature types are directly comparable and contain identical geometries.

The OS NGD Water Network Collection also contains an additional feature type, Water Link Set, which provides a single geometry for a complete named watercourse.

Comparison map

Below is a pair of interactive maps showing the similarities between the OS MasterMap Water Network Layer (top) and OS NGD Water Network Collection (bottom). Clicking on a feature in either map will open a popup displaying the shared attributes. Full details on the attribution can be found below in the Attribution comparison table.

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:

• OS NGD file formats and naming
• Getting Started with OS NGD API – Features
• Getting Started with OS NGD API – Tiles

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:

• Downloading with OS Select+Build
• Getting Started with CSV
• Getting Started with GeoPackage

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:

• watercourseName
• watercourseNameAlternative
• localName
• localNameAlternative

In the OS NGD Water Network Collection, the watercourse names can be found in the below attributes:

• name1_text
• name2_text
• namesecondary1_text
• namesecondary2_text
• nametertiary1_text
• nametertiary2_text

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.

A very small percentage of OS NGD Water Links (<0.01%) have their primary, secondary, or tertiary names ordered incorrectly, meaning they are not assigned in decreasing geometry length order. See OS NGD Water Theme current known data issues for more detail.

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:

OS MasterMap Water Network Layer Attribute(s)	Purpose	OS NGD Water Network Collection Attribute(s)	Notes
startNode / endNode	The HydroNode coincident with the first / last vertex for this WatercourseLink.	startnode / endnode	Attributes serves the same purpose; now uses OSIDs as opposed to TOIDs.
form	The nature of the watercourse being represented by the feature.	description	Attribute serves the same purpose with new additional context provided in watertype (inland vs. tidal), and the description metadata attributes: description_evidencedate , description_updatedate and description_source
level	A description of the relationship of the watercourse to ground level.	physicallevel	Attribute serves the same purpose with new additional context provided in physicalcontainment (for example, In Culvert, In Tunnel or In Natural Subterranean Passage)
flowDirection	Direction of water flow in the segment relative to digitisation of segment geometry.	flowdirection	Attribute serves the same purpose. There are some minor differences between the code list values and their definitions.
primacy	Value indicating the relative importance of the WatercourseLink within any larger watercourse it is part of.	primacy	Attribute serves the same purpose.
provenance	The origin of the geometry.	geometry_source / capturespecification	These attributes split the source and capture specification, rather than combining them into a single attribute.
catchmentName / catchmentID	The name of the catchment area where the WatercourseLink falls, assigned by a responsible body.	catchmentname / catchmentid	Attributes serve the same purpose. ⚠️ Approximately 20% of OS NGD Water Link features have incorrect null catchment values (see current known data issues)
width / gradient / length	Measurements relating to the WatercourseLink	width / gradient / geometry_length	Attributes serve the same purpose; however, enhancements to width are due to be supplied to the OS NGD Water Network Collection in September 2025.
levelOfDetail	A calculated value indicating the relative cartographic importance of a WatercourseLink to aid selection and depiction when styling the data.	levelofdetail	Attribute serves the same purpose. There are some minor differences between the code list values and their definitions. ⚠️ Approximately 20% of OS NGD Water Link features have incorrect null level of detail values (see current known data issues)
permanence	Describes if the watercourse contains water year-round (in normal conditions) or only contains water intermittently.	permanence	Attribute serves the same purpose, now provided as a code list value as opposed to a boolean value.
hydroNodeCategory	Nature of the HydroNode.	description	Attributes serves the same purpose; some minor differences in the values (see hydronodecategoryvalue & waternodedescriptionvalue)

There are also three attributes that are not currently populated or used in OS MasterMap Water Network Layer. These are also not included in the OS NGD Water Network Collection.

1. managedNavigation
2. watercourseNumber
3. fictitious

All 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:

• OS NGD Core Principles
• OS NGD Water Network Collection

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.