Ordnance Survey Map References

[Simon Ritchie]

The Ordnance Survey (OS) is the UK government body responsible for producing maps.  The Environment Agency lay out their lidar data using OS notation, so to use it you need to understand OS map references.  (If you already do, you can skip this note.)

OS map references are based on an imaginary oblong box drawn around the UK and aligned to the North.  The bottom left corner is out at sea to the South-West of Cornwall and every piece of land in the UK lies within this box.  This is the zero point of the system - its map reference is (0,0).  If you are on land in the UK, you are some distance to the East of the zero point and some distance to the North of it.  The OS map reference of your position is the number of metres to the East and the number to the North.  For example, the Meridian Line in Greenwhich Park is 538882m East of the zero point and 177346m to the North of it, so its map reference is (538882,177346).

Many people find those big numbers difficult to handle so the OS came up with an alternative system.  The big oblong is divided into 100 Km squares and each of those has a two-letter label.  For example, the bottom right hand corner of square TQ is at (500000,100000) in the old notation.  Within square TQ, the East component of every map reference starts with 5 and the North component starts with 1, so we drop those two digits and replace them with TQ, for example (538882,177346) becomes TQ38882.77346.

A similar system is used to divide up the OS map.  An OS map is divided into 1Km squares, divided by blue lines.  For example TQ1352 is the square whose bottom left corner lies at map reference (513000,152000) or TQ13000.52000.

If that doesn't make sense, try the official explanation: https://www.ordnancesurvey.co.uk/getoutside/guides/beginners-guide-to-grid-references/

Every mapping authority has a similar system for representing positions, for example American maps use the United States National Grid (USNG) system: https://usngcenter.org/