LogoLogo
OS Docs HomeOS NGDOS APIsOS Download ProductsMore than MapsContact Us
  • More than Maps
  • Geographic Data Visualisation
    • Guide to cartography
      • Introduction to cartography
      • Types of maps
      • Symbology
      • Colour
      • Text on maps
      • Generalisation
      • Coordinate reference systems
      • Projections
      • Scale
      • Map legends
      • Map layout
      • Relief representation
      • North arrows
    • Guide to data visualisation
      • Introduction to data visualisation
      • GeoDataViz design principles
      • Types of visualisation
      • Thematic mapping techniques
      • Data visualisation critique
      • Accessible data visualisation
      • Ethical data visualisation
      • Software
      • Data
    • GeoDataViz assets
      • GeoDataViz basemaps
      • Stylesheets
      • GeoDataViz virtual gallery
      • Equal area cartograms
      • How did I make that?
        • Apollo 11 Landing
        • North York Moors National Park, 70 years
        • Snowdonia National Park, 70 years
        • Great Britain's National Parks
        • Great Britain's Islands
        • Great Britain's AONB's and National Scenic Areas
        • Famous shipwrecks of Pembrokeshire
        • Trig pillars today
        • Britain's most complex motorway junctions
      • #30DayMapChallenge
  • Data in Action
    • Examples
  • Demonstrators
    • 🆕Product Viewer
    • Addressing & location demonstrators
      • Address Portfolio overview
      • Which address product should you use?
      • AddressBase
      • AddressBase Core
      • AddressBase Plus
      • AddressBase Premium
      • Address Classifications
      • Addressing Lifecycle
      • OS Emergency Services Gazetteer
      • What are Vertical Streets?
      • Why are there differences in boundaries?
    • Contextual demonstrators
    • Customer best practice
      • Channel Shift
      • Data Management and OS Data Hub
      • End User Licence vs Contractor Licence
      • 🆕 IDs vs Spatial Relationships
      • Why we should capture good quality addresses at source
      • Why we Snap and Trace
    • Network Demonstrators
      • OS Detailed Path Network
      • OS Multi Modal Routing Network
        • OS Multi Modal Routing Network
      • Water Networks overview
      • OS MasterMap Highways Network and OS NGD Speeds
      • OS MasterMap® Highways Network and OS Open Roads™
    • OS MasterMap Generation APIs
      • Using the OS Features API
      • Using the OS Features API Archive
      • Using the OS Downloads API
      • Using OS APIs in ESRI Software
    • 🆕OS NGD (National Geographic Database)
      • OS NGD Address
      • OS NGD Boundaries
      • 🆕OS NGD Buildings
        • 🆕Building and Building Access Feature Types
        • Building Part and Building Line Feature Types
      • 🆕OS NGD Geographical Names
      • OS NGD Land
      • OS NGD Land Cover enhancements
      • 🆕OS NGD Land Use
      • OS NGD Land Use enhancements
      • 🆕OS NGD Structures
        • 🆕OS NGD Structures
        • Field Boundaries
      • 🆕OS NGD Transport Features
      • 🆕OS NGD Transport Network
      • OS NGD Transport RAMI
      • OS NGD Water Features
      • OS NGD Water Network
      • OS NGD API - Features
      • Ordering OS NGD data
      • Change only updates
      • OS NGD Versioning
      • Creating a topographic map from OS NGD Data
      • Analytical styling for OS NGD data
    • OS MasterMap® demonstrators
    • 🆕Product & API Comparisons
      • 🆕Comparison of Water Network Products
  • Tutorials
    • GeoDataViz
      • Thematic Mapping Techniques
      • Downloading and using data from the OS Data Hub
      • How to download and use OS stylesheets
      • How to use the OS Maps API
      • Creating a bespoke style in Maputnik
    • GIS
      • Analysing pavement widths
      • Basic routing with OS Open Data and QGIS
      • Walktime analysis using OS Multi-modal Routing Network and QGIS
      • Creating 3D Symbols for GIS Applications
      • Constructing a Single Line Address using a Geographic Address
      • Creating a Digital Terrain Model (DTM)
      • Visualising a road gradient using a Digital Terrain Model
      • Visualising a road gradient using OSMM Highways
    • 🆕APIs
      • 🆕Using OS APIs with EPC API
      • 🆕OS APIs and ArcGIS
  • Deep Dive
    • Introduction to address matching
    • Guide to routing for the Public Sector
      • Part 1: Guide to routing
      • Part 2: Routing software and data options
      • Part 3: Building a routable network
    • Unlocking the Power of Geospatial Data
    • Using Blender for Geospatial Projects
    • A Guide to Coordinate Systems in Great Britain
      • Myths about coordinate systems
      • The shape of the Earth
      • What is position?
        • Types of coordinates
        • We need a datum
        • Position summary
      • Modern GNSS coordinate systems
        • Realising WGS84 with a TRF
        • The WGS84 broadcast TRF
        • The International Terrestrial Reference Frame (ITRF)
        • The International GNSS Service (IGS)
        • European Terrestrial Reference System 1989 (ETRS89)
      • Ordnance Survey coordinate systems
        • ETRS89 realised through OS Net
        • National Grid and the OSGB36 TRF
        • Ordnance Datum Newlyn
        • The future of British mapping coordinate systems
        • The future of British mapping coordinate systems
      • From one coordinate system to another: geodetic transformations
        • What is a geodetic transformation?
        • Helmert datum transformations
        • National Grid Transformation OSTN15 (ETRS89–OSGB36)
        • National Geoid Model OSGM15 (ETRS89-Orthometric height)
        • ETRS89 to and from ITRS
        • Approximate WGS84 to OSGB36/ODN transformation
        • Transformation between OS Net v2001 and v2009 realisations
      • Transverse Mercator map projections
        • The National Grid reference convention
      • Datum, ellipsoid and projection information
      • Converting between 3D Cartesian and ellipsoidal latitude, longitude and height coordinates
      • Converting between grid eastings and northings and ellipsoidal latitude and longitude
      • Helmert transformation worked example
      • Further information
  • Code
    • Ordnance Survey APIs
    • Mapping
    • Routing with pgRouting
      • Getting started with OS MasterMap Highways and pgRouting
      • Getting started with OS MasterMap Highways Network - Paths and pgRouting
      • Getting started with OS NGD Transport Theme and pgRouting
      • Getting started with OS NGD Transport Path features and pgRouting
  • RESOURCES
    • 🆕Data Visualisation External Resources
Powered by GitBook

Website

  • Ordnance Survey

Data

  • OS Data Hub
On this page

Was this helpful?

  1. Deep Dive
  2. A Guide to Coordinate Systems in Great Britain

Ordnance Survey coordinate systems

PreviousEuropean Terrestrial Reference System 1989 (ETRS89)NextETRS89 realised through OS Net

Last updated 4 months ago

Was this helpful?

Let’s now take a look at the coordinate systems used by OS for mapping Great Britain. There are three coordinate systems to consider:

  • OS Net, a modern 3-D TRF using the ETRS89 datum (as described in ). This coordinate system is the basis of modern Ordnance Survey ‘control survey’ (the surveyor’s jargon for adding local points to a TRF for mapping purposes), and is the basis of definition of all Ordnance Survey coordinates. A subset of OS Net has been ratified as the official densification of ETRF89 in GB.

  • The National Grid, a ‘traditional’ horizontal coordinate system, which consists of: a traditional geodetic datum (see ) using the Airy 1830 ellipsoid; a TRF called OSGB36 (Ordnance Survey Great Britain 1936) which was observed by theodolite triangulation of trig pillars; and a Transverse Mercator map projection (see ) allowing the use of easting and northing coordinates. This coordinate system is important because it is used to describe the horizontal positions of features on British maps. However, its historical origins and observation methods are not of interest to most users and will be skipped over in this booklet. National Grid coordinates are nowadays determined by GNSS plus a transformation rather than theodolite triangulation.

  • Ordnance Datum Newlyn (ODN), a ‘traditional’ vertical coordinate system, consisting of a tide‑gauge datum with initial point at Newlyn (Cornwall), and a TRF observed by spirit levelling between 200 fundamental bench marks (FBMs) across Britain. The TRF is densified by more than half a million lower-accuracy bench marks. Each bench mark has an orthometric height only (not ellipsoid height or accurate horizontal position). This coordinate system is important because it is used to describe vertical positions of features on British maps (for example, spot heights and contours) in terms of height above ‘mean sea level’. Again, its historical origins and observation methods are not of interest to most users. The word ‘Datum’ in the title refers, strictly speaking, to the tide‑gauge initial point only, not to the national TRF of levelled bench marks.

Because triangulation networks need hilltop stations whereas levelling networks need low‑lying, easily accessible routes, there are hardly any common points between the height bench mark network and the OSGB36 horizontal network. OS Net provides a single 3-D TRF that unifies ODN and OSGB36 via transformation software (see ). Using transformation techniques, precise positions can be determined by GNSS in ETRS89 using OS Net and then converted to National Grid and ODN coordinates. This is the approach used today by Ordnance Survey.

European Terrestrial Reference System 1989 (ETRS89)
We need a datum
Transverse Mercator map projections
Geodetic transformations