The Ordnance Survey National Grid, commonly called the British National Grid, is the most popular and official grid reference system used in the UK [1][2]. It is the standard system for Great Britain and is mandated by the UK government for geospatial data [2]. The system is used by national mapping agencies, emergency services, hikers, government bodies, and appears on all Ordnance Survey maps [1][3].

The grid divides Great Britain into large squares, with the primary reference system using two-letter codes (such as TQ for London or NN for Scottish Highlands) to identify 100-kilometre squares, followed by numerical coordinates for eastings and northings [1]. Grid references can be expressed at varying levels of precision: four-figure references identify a one-kilometre square, six-figure references pinpoint a 100-metre square (the standard for general navigation and hiking), and eight-figure or ten-figure references provide precision down to ten metres or one metre respectively [1][3].

While other coordinate systems exist—such as ETRS89 and WGS84, which are widely used for GPS and web mapping—these are not the primary traditional map grid reference system on Ordnance Survey maps [2]. The British National Grid remains the primary language for physical maps and land management in Great Britain. It is important to note that Northern Ireland uses a separate but similar system called the Irish Grid Reference System [1]. When people refer to the most popular grid system in the UK, they are referring to the OS National Grid used in Great Britain.