Coordinate reference systems

Let’s start with the shape of the Earth. Unfortunately, it’s not a perfect sphere as that would make life too simple. It’s actually an uneven and slightly squashed sphere which is flattened at the poles and bulges around the equator. This irregular and lumpy shape is represented by a surface known as the geoid. The geoid is related to the naturally differing force of gravity and is close to mean sea level and represents the best averaged shape of the Earth, ignoring the topography of mountain heights and sea depths.

The shape of the geoid however is too complicated for the calculations required to make a map, so simpler mathematical models, which best estimate the geoid, have been created and are known as reference ellipsoids. These models can be designed to achieve a best fit to the geoid globally or to best fit the Earth’s surface at a specific location for mapping a single country.

In addition to an ellipsoid there are other parameters that, when all collectively adopted, constitute a “Coordinate Reference System” (CRS). There are many CRSs but they all aim to achieve the same thing - a model used for describing positions on the Earth. As such the coordinates of a physical point on the Earth will differ depending on the underlying CRS or, vice versa, the same coordinates will appear in different locations depending on the underlying CRS.

For example, the diagram below shows the physical position of three points that all have the same coordinates but in three different coordinate systems (OSGB36, WGS84 and ED50). Each one of these coordinate systems is widely used and fit for its purpose, and none of them is wrong. The differences between them are just a result of the different conventions (ellipsoid, etc…) adopted by the coordinate systems.

The physical position of three points that all have the same coordinates in three different coordinate systems.

To go between different CRSs a coordinate transformation must be used. This is a mathematical process that can range from a simple shift in values to a complex “rubber sheet” operation that aims to model variances in one or both of the CRSs. For example, ETRS89 coordinates from a GNSS positioning process should pass through the OSTN15 transformation model (a “rubber sheet” process) in order to become OSGB36 National Grid coordinates.

Coordinate systems

Now that we have our model of the Earth (the CRS) we need some coordinates to describe positions within the CRS. A coordinate system allows us to identify a position on the Earth’s surface and relate it to a position on a map, and vice versa. The two most common coordinate systems are latitude/longitude or a national grid system.

Latitude and longitude are theoretical lines, superimposed on a 3D model of the Earth. Lines of latitude run parallel to the Equator whereas lines of longitude run from pole to pole. Together, these form what we call a ‘graticule.’ Latitude is measured as the number of degrees, minutes, and seconds a place is north or south of the equator whilst longitude is the number of degrees, minutes, and seconds a place is east and west of an arbitrary ‘zero’ line – almost always the Greenwich Prime Meridian.

The graticule – lines of latitude and longitude

Often, to make navigation and defining a position easier, many countries have developed their own National Grid Coordinate System which comprises a regular grid, projected up from the curved surface of the ellipsoid.

For Great Britain, Ordnance Survey created the British National Grid as a way of defining locations across GB. This coordinate system only applies to GB and is used on all OS maps. The origin of this grid (bottom left corner) is just west of the Isles of Scilly meaning that all grid references on the GB landmass are positive. Coordinates are stated as the distance east (Eastings) of the origin, followed by the distance north (Northings) of the origin. A position in the UK can be easily communicated by providing a grid reference using this national grid system.

Coordinate systems of Great Britain: the graticule shown in grey and the British National Grid shown in blue

Always remember: Along the corridor (eastings) then up the stairs (northings).

Tip: On the OS Maps App, if you hold your finger on the location you want a grid reference for, it will display the National Grid reference.

For those wondering what happened to Northern Ireland, Ordnance Survey is the national mapping agency for Great Britain only. Ordnance Survey Northern Ireland (OSNI) is responsible for mapping in Northern Ireland and they use the Irish National Grid to define positions.

To make referencing map squares a little easier the UK is split into 100km squares, each with a two-letter identifier. These 100km square are split into ten 10x10km squares and split again into 1x1km squares. These squares are the blue lines on OS Landranger and Explorer maps. A map tile reference can be given using the 100km square ID and the eastings and northings within the 100km square.

British National Grid

CRSs are often identified with a unique number – a SRID (Spatial Reference System Identifier). A common SRID Authority is the EPSG registry (https://epsg.org/home.html). The SRID is usually in the format “EPSG::xxxx”. CRSs in common use in Great Britain, and their EPSG codes, are:

• OSGB36 National Grid, EPSG::27700. The CRS used for OS map data.

• ETRS89, EPSG::4258 (2D) and EPSG::4937 (3D). The CRS commonly used across Europe for data collected using GNSS positioning methods.

• WGS84, EPSG::4326 (2D) and EPSG::4979 (3D). The native datum of the US GPS GNSS system. Data collected using GPS can often be flagged as WGS84 even though it may be ETRS89. The difference between ETRS89 and WGS84 can be ignored for most purposes.

For a much more in-depth discussion of coordinate systems, we recommend reading our guide to the coordinate systems of Great Britain

Vertical coordinate reference systems

We’ve defined how horizontal coordinate systems work but maps also show height. For this, we need to define where zero is. Generally, it’s mean sea level. For GB, zero is defined as mean sea level measured at the Newlyn tide gauge in Cornwall between 1915 and 1921 and is known as Ordnance Datum Newlyn (ODN). All heights on OS maps, are vertical heights above (or below) this known benchmark.

What does this all mean for the map I’m creating?

What does this mean for your maps? Well, if you’re creating a map in a GIS which will be used for navigation, you’ll probably want to include some way that the user can define where they are, and someone else to be able to work out where this is. As such, in your GIS, it’s wise to pick a sensible coordinate reference system – one that works for the locality you’re mapping. For this kind of map you will probably want to define this information somewhere on your map so that it is clear which coordinate system the map uses.

It is often helpful to include the grid and/or a graticule on your map in some way to help users define their location.

Technical information note

Also, it may be that some of the data you are importing into your GIS has been collected in a certain coordinate reference system. You’ll need to make sure you import your data in the right coordinate reference system in order for them to load in their correct geographical position. If you have lots of data in difference coordinate systems, you may need to apply some transformations to ensure that all data is in the same CRS.

If you’re creating a thematic map where defining exact location is not important, the coordinate system you use won’t matter so much. Make sure you think carefully about the projection you use though if you’re creating a small-scale map.