A Guide to Coordinate Systems in Great Britain

An introduction to mapping coordinate systems and the use of GNSS datasets with Ordnance Survey mapping.

Who should read this guide?

This guide is aimed at people whose expertise is in fields other than geodesy, who need to know the concepts of coordinate systems in order to deal with coordinate data, and who need information on using mapping coordinate systems in Great Britain. It explains:

• The basic concepts of terrestrial coordinate systems.

• The coordinate systems used with Global Navigation Satellite Systems (GNSS) and in OS mapping.

• How these two relate to each other.

Although this guide deals with the GPS system (WGS84), the concepts and techniques can also be applied to other GNSS, for example, Russian GLONASS, European Galileo and Chinese BeiDou Navigation Satellite System (BDS).

The subject of geodesy deals, amongst other things, with the definition of terrestrial coordinate systems. Users of coordinates are often unaware that this subject exists, or that they need to know some fundamental geodetic concepts in order to use coordinates. This guide explains these concepts.

If you work with coordinates of points on the ground and would like to know the answers to any of the following questions, or if you don’t understand the questions outlined below, this guide is a good place to start:

• How do geodesists define coordinate systems that are valid over large areas? What is difficult about this task, anyway? Why can’t we all just use one simple coordinate system for all positioning tasks?

• What exactly is WGS84? How accurate is it? How does WGS84 relate to map coordinates? Why are there other GNSS coordinate systems that seem to be very similar to WGS84? Why are there so many acronyms used to describe GNSS coordinate systems?

• How is the Ordnance Survey National Grid defined? How does OSGB36 relate to the National Grid? Why does it seem to be difficult to relate the National Grid coordinates to GNSS coordinates? How are grid references converted to latitude and longitude coordinates?

• Why do coordinate systems use ellipsoids? Why are there so many different ellipsoids? Why is it so difficult to convert coordinates from one ellipsoid to another? Is an ellipsoid the same thing as a datum? What is the difference between height above mean sea level and height above an ellipsoid

• Why are transformations between different coordinate systems not exact? How can GNSS coordinates be related precisely to the National Grid and mean sea level (orthometric) heights?

Index

This guide includes the following sections:

• Myths about coordinate systems
• The shape of the Earth
• What is position?
• Modern GNSS coordinate systems
• Ordnance Survey coordinate systems
• From one coordinate system to another: geodetic transformations
• Transverse Mercator map projections
• 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