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

Transverse Mercator map projections

PreviousTransformation between OS Net v2001 and v2009 realisationsNextThe National Grid reference convention

Last updated 4 months ago

Was this helpful?

When features on the curved surface of the Earth are represented on a plane surface, distortions of distances, angles or both are inevitable. Originally the ‘plane surface’ was a map sheet; now it is often the plane coordinate system of GIS software. A map projection is any function that converts ellipsoidal latitude and longitude coordinates to plane easting and northing coordinates. OS maps use a type of projection known as the Transverse Mercator (TM). The same type of projection is used in a worldwide mapping standard known as Universal Transverse Mercator (UTM). The parameters of the National Grid TM and UTM projections are given in .

The TM projection can be thought of as a sheet of paper carrying the mapping grid (of eastings and northings), which is curved so as to touch the ellipsoid along a certain line. This line of contact is chosen to be a north-south central meridian. Points on the ellipsoid are projected onto the curved sheet, giving easting and northing coordinates for each point. The effect is to distort the distance between projected points, except on the central meridian, where the ellipsoid touches the mapping grid. This scale distortion effect increases east and west of the central meridian. The scale distortion can be measured by a local scale factor which is 1 on the central meridian and greater than one everywhere else.

To reduce the worst scale distortion effect in the extreme eastern and western regions of the mapping area, a scale reduction factor is introduced over the whole mapping area. Strictly this turns the projection into a “Modified TM”. This makes projected distances on the central meridian slightly too small, but lessens the scale distortion for points far to the east or west of the central meridian. With the overall scale reduction applied, there are now two lines (either side of the central meridian) on which the local scale factor is one. Inside these lines, the local scale factor is less than one (with a minimum on the central meridian), and outside these lines it is more than one. On the central meridian, the local scale factor is now equal to the scale reduction factor that was introduced.

The TM projection used for all OS maps has a central meridian at longitude 2° West and a central meridian scale factor of approximately 0.9996 (see for exact value). The two lines of true scale are about 180 km to the east and west of the central meridian. The stated scale of an OS map is only exactly true on these lines of true scale, but the scale error elsewhere is quite small. For instance, the true scale of OS 1:50 000 scale map sheets is actually between 1:49 980 and 1:50 025 depending on easting.

The UTM projections are a way of mapping the whole world in a systematic way by dividing the Earth by longitude into 60 zones, each 6 degrees of longitude wide. The 60 UTM zones each have a different central meridian. The zones relevant to British mapping are 29 (central meridian 9° W), 30 (central meridian 3° W) and 31 (central meridian 3° E). The scale on the central meridian is 0.9996 for all UTM zones. The International 1924 ellipsoid was commonly used in the UTM projection in Europe, but other ellipsoids can also be used with the WGS84 ellipsoid being increasing popular. When working with UTM coordinates always check which ellipsoid is being used.

gives formulae for conversion between latitude and longitude and TM eastings and northings. 'Datum, ellipsoid and projection information' gives the TM parameters for the National Grid and the British UTM zones.

Datum, ellipsoid and projection information
Datum, ellipsoid, and projection information
Converting between grid eastings and northings and ellipsoidal latitude and longitude