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
  • The ODN datum
  • The ODN TRF
  • Relative accuracy of ODN bench marks

Was this helpful?

  1. Deep Dive
  2. A Guide to Coordinate Systems in Great Britain
  3. Ordnance Survey coordinate systems

Ordnance Datum Newlyn

PreviousNational Grid and the OSGB36 TRFNextThe future of British mapping coordinate systems

Last updated 4 months ago

Was this helpful?

Before satellite surveying methods became available in the 1970s, ellipsoid height was a rare and specialised coordinate type. The everyday vertical coordinate type was orthometric height, or more exactly mean sea level height relative to the national tide gauge datum, as realised in mainland Britain by the ODN heights of OS bench marks. Even though anybody with a GNSS receiver can now measure ellipsoid heights quite easily, Geoid‑related height coordinates such as ODN heights are much more useful for most applications because they are heights relative to a level surface.

The ODN datum

Like horizontal datums, vertical datums were ‘traditionally’ defined at a single initial point. In the case of horizontal datums, this is an astronomical observatory. In the case of vertical datums it is a coastal tide gauge. Tide gauge-based vertical datums are still used in most countries.

The tide gauge has a height reference benchmark, the height of which on the gauge scale is known. A long series of sea level records from the tide gauge is averaged to give the vertical offset of MSL from the bench mark. This is the MSL height of the bench mark.

In Britain, the MSL of the tide-gauge benchmark at Newlyn near Penzance in Cornwall is used – hence the name Ordnance Datum Newlyn. Its MSL height was established from continuous tide readings between 1915 and 1921. That adopted value has remained unchanged in the ODN system, even though MSL at the Newlyn gauge has slowly changed. Hence the height of a point above mean sea level given by OS is not affected by variations in mean sea level, and the proper description of an OS height is ‘orthometric height relative to Ordnance Datum Newlyn’.

The ODN TRF

A network of about 200 fundamental bench marks for height measurement was constructed across Britain in the first half of the twentieth century. These are underground chambers containing a height reference object, topped by a small pillar on which is another reference point. The idea is that the point on the top is easy to access for normal use, whereas the hidden underground mark is less susceptible to damage.

The orthometric height of each fundamental bench mark relative to Newlyn was determined by a network of precise levelling lines across Britain. You can find descriptions of the precise levelling technique in land surveying textbooks. This technique is capable of transferring orthometric height from one point to another with an error of less than 2 x √d millimetres, where d is the distance levelled in kilometres. Today, the accuracy of the precise levelling technique is rivalled by the combination of GNSS ellipsoid heighting with a precise gravimetric Geoid model that allows the ellipsoid height difference between two points to be easily converted to an orthometric height difference.

The ODN TRF was subsequently densified by less precise levelling to obtain the heights of about three-quarters of a million Ordnance Survey bench marks all over Britain. These lower-order bench marks are often seen cut into stone at the base of a building, church or bridge. About half a million of them are in existence and usable today. An important error source to bear in mind is possible subsidence of Ordnance Survey bench marks, especially in areas where mining has caused collapse of the ground. In these areas, cases of bench mark subsidence of several metres have been reported.

Relative accuracy of ODN bench marks

The following table is a statistical statement of ODN bench mark accuracy relative to other bench marks in the same area. The primary layer of the ODN TRF consists of the fundamental bench marks, which are considered error-free within the ODN coordinate system. The Ordnance Survey National Geoid Model (see ) relates ODN orthometric heights to the GNSS ellipsoid GRS80. The table below shows the expected maximum levelling error between bench marks on the same levelling line. This only describes the original measurement error. The Ordnance Survey bench mark network has not been maintained since the 1970s, so beware of possible bench mark subsidence.

Accuracy of Ordnance Survey bench mark levelling, where d is distance levelled in km

Type of benchmark
Maximum error

Fundamental

Error free

Geodetic

±2mm x √d

Secondary

±5mm x √d

Tertiary

±12mm x √d

National Geoid Model OSGM15