Q. What is the difference between WGS84 and ETRS89?
Q. How do I change my coordinates in Latitude and Longitude to grid Eastings and Northings or vice versa?
Q. What is the National Grid coordinate system and what do the letters signify?
Q. How do I calculate the azimuth (true bearing) between two points?
Q. How do I calculate the difference between Grid North and True North (Convergence) at a location?
Q. How do I calculate the local scale factor?
Q. How do I calculate the t-T correction?
Q. What coordinate system is used in Ireland and other parts of Europe?
Q. How do I relate coordinates from the old and new transformation models?

Q. What is the difference between WGS84 and ETRS89?

A. Most people who are familiar with GPS have heard of the WGS84 (World Geodetic System 1984) coordinate system. This is a global coordinate system designed for use anywhere in the world. WGS84 coordinates are usually expressed as latitude, longitude and ellipsoid height.

WGS84 was designed for navigation applications, where the required accuracy is one metre or lower. A high-accuracy version of WGS84 known as ITRS (International Terrestrial Reference System) has been created in a number of versions since 1989, and this is suitable for international high-accuracy applications (it is used mostly by geoscientists). However, there is a problem with trying to use a global coordinate system for land surveying in a particular country or region. The problem is that the continents are constantly in motion with respect to each other, at rates of up to 12 centimetres per year. There are in reality no fixed points on Earth. In common with the rest of Europe, Great Britain is in motion with respect to the WGS84 coordinate system at a rate of about 2.5 centimetres per year. Over a decade, the WGS84 coordinates of any survey station in Britain change by a quarter of a metre due to this effect, which is unacceptable for precise survey purposes.

For this reason, the European Terrestrial Reference System 1989 (ETRS89) is used as the standard precise GPS coordinate system throughout Europe. ETRS89 is based on ITRS (the precise version of WGS84), except that it is tied to the European continent, and hence it is steadily moving away from the WGS84 coordinate system. In 2000, the difference between the ITRS (precise WGS84) coordinates of a point and the ETRS89 coordinates is about 25cm, and increasing by about 2.5 cm per year. The relationship between ITRS and ETRS89 is precisely defined at any point in time by a simple transformation published by the International Earth Rotation Service.

The ETRS89 coordinate reference system is used as a standard for precise GPS surveying throughout Europe. Using ETRS89 you can ignore the effects of continental motion: to a high degree of accuracy, the ETRS89 coordinates of a survey station stay fixed, as long as there is no local movement of the survey station. ETRS89 has been officially adopted as a standard coordinate system for precise GPS surveying by most national mapping agencies in Europe.

Q. How do I change my coordinates in Latitude and Longitude to grid Eastings and Northings or vice versa?

A. You must first determine if you require a conversion or a transformation, i.e. are your coordinate systems based on the same datum.

If you wish to convert GPS derived coordinates (based on the WGS84 or ETRS89 ellipsoid) to Ordnance Survey National Grid coordinates (based on the Airy 1830 ellipsoid) or vice versa, then you require a transformation. The Coordinate Converter facility (available from the GPS Website homepage) will perform this operation on either a single position or using a batch file for multiple coordinate input. You have to register to use this facility.

If you wish to convert the format of your coordinates from Latitudes and Longitudes to grid Eastings and Northings on the same datum then you can use the spreadsheet available here.

For more information on datums and coordinate systems see the online guide available here.

Q. What is the National Grid coordinate system and what do the letters signify?

A. For a full description of the National Grid and information on its use see our online guide here.

Q. How do I calculate the azimuth (true bearing) between two points?

A. Click here for a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, pprojection functions and t-T correction.

Q. How do I calculate the difference between Grid North and True North (Convergence) at a location?

A. Click here for a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, pprojection functions and t-T correction.

Q. How do I calculate the local scale factor?

A. Click here for a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, pprojection functions and t-T correction.

Q. How do I calculate the t-T correction?

A. Click here for a spreadsheet that contains many utility programs for working with coordinates including azimuth calculations, calculating convergence, pprojection functions and t-T correction.

Q. What coordinate system is used in Ireland and other parts of Europe?

A. For a Website containing technical details on mapping systems and transformations used outside Great Britain, click here.

Q. How do I relate coordinates from the old and new transformation models?

A.  The new and definitive Ordnance Survey transformation and geoid model OSTN02 / OSGM02 is an improvement over the previous models OSTN97 / OSGM91. The horizontal transformation accuracy has been improved from 0.2m to 0.1m rms and the vertical accuracy has been improved from 0.05m to 0.02m rms. This means that coordinates transformed using both new and old models are very likely to differ.

If you have coordinates in OSGB36 which were transformed using the previous transformation (OSTN97 / OSGM91) and wish to relate them to coordinates transformed using the new transformation (OSTN01 / OSGM02) they should first be back-transformed into ETRS89 using OSTN97 / OSGM91 and then forward transformed again into OSGB36 using the new transformation OSTN02 / OSGM02. An executable is available which will do this back transformation for users who no longer have access to OSTN97 / OSGM91. Click here to download the executable file.

Users who have archived their coordinates in ETRS89 will not have to back transform to relate old and new datasets. The archive coordinates can be simply transformed again using the new OSTN02 / OSGM02 transformation models.