The British National Grid

The British National Grid sits in the same category as the US /learn/state-plane-coordinate-system support and most other national projected coordinate systems: a single, country-specific system designed for everyday mapping, surveying, and engineering work. Where the US system uses many per-state zones to keep distortion low, the UK uses a single grid covering the whole country — an approach that works because Great Britain is small enough (about 1,000 km north-south, 500 km east-west) for a single Transverse Mercator zone to maintain acceptable scale distortion. This article covers the grid's specification, the OSGB36 datum it is built on, the grid-reference format that is familiar to anyone who has navigated by an Ordnance Survey map, and the OSTN15 transformation that bridges to modern international reference frames.

The grid specification

The British National Grid is a Transverse Mercator projection with the following parameters, per the Ordnance Survey Guide to Coordinate Systems in Great Britain:

| Parameter | Value | |---|---| | Datum | OSGB36 | | Ellipsoid | Airy 1830 (a = 6,377,563.396 m, b = 6,356,256.909 m) | | Projection | Transverse Mercator | | Central meridian | 2°W | | Latitude of origin | 49°N | | Scale factor at central meridian | 0.9996012717 | | False easting | 400,000 m | | False northing | −100,000 m | | EPSG code | 27700 |

The false easting of 400,000 m and false northing of −100,000 m are chosen so that every point in Great Britain has positive easting between 0 and 700,000 and positive northing between 0 and 1,300,000. The unusual scale factor (0.9996012717 rather than a round number) was chosen to minimise scale distortion specifically for the British landmass — it makes the projection a secant Transverse Mercator with two lines of unit scale running roughly north-south through Britain.

The Airy 1830 ellipsoid is older and smaller than WGS 84. It was fitted in the early 19th century to British triangulation data, which means it matches Britain well but diverges from a globally representative ellipsoid. This is the fundamental reason OSGB36 coordinates and WGS 84 coordinates of the same physical point differ by up to 120 m at the corners of the country.

The letter grid system

The British National Grid divides Britain into 100 km squares each labelled with two letters. The labelling scheme works as a 5×5 grid of major squares (labelled with one letter each, starting with H in the northwest and ending with T in the southeast, skipping I), each of which is subdivided into a 5×5 grid of 100 km sub-squares (labelled with one letter each).

Familiar 100 km square labels:

| Code | Region | Approximate Easting / Northing | |---|---|---| | HU | Shetland | 400,000 / 1,100,000 | | NT | Edinburgh, Lothians | 320,000 / 670,000 | | NS | Glasgow, Strathclyde | 220,000 / 670,000 | | NN | Glencoe, Cairngorms | 220,000 / 770,000 | | NZ | Newcastle, Durham | 420,000 / 520,000 | | SD | Lake District, Lancashire | 320,000 / 470,000 | | SE | Yorkshire | 420,000 / 470,000 | | SK | Sheffield, Nottingham | 420,000 / 370,000 | | SP | Oxford, Birmingham | 420,000 / 270,000 | | SU | Salisbury Plain | 420,000 / 170,000 | | SX | Devon, Cornwall | 220,000 / 70,000 | | SY | Dorset | 320,000 / 70,000 | | ST | Bristol, Bath | 320,000 / 170,000 | | TQ | London, South-East | 520,000 / 170,000 | | TR | Kent | 620,000 / 170,000 | | TV | Sussex (Beachy Head) | 520,000 / 70,000 |

The letter prefix gives an immediate sense of where in the country a reference is — TQ is unambiguously London, NT is Edinburgh, HU is Shetland.

Grid references in practice

A grid reference combines the 100 km square code with numeric easting and northing within the square:

• 2-digit reference (e.g., SU 38 14): 10 km precision — useful for general region identification.
• 4-digit reference (e.g., SU 387 148): 100 m precision — matches a small built-up area or a specific landscape feature.
• 6-digit reference (e.g., SU 3876 1485): 10 m precision — enough to specify a particular building or field.
• 8-digit reference (e.g., SU 38765 14852): 1 m precision — used for engineering and survey work.

The reference can also be written purely numerically with no letter prefix, by giving the full easting and northing values relative to the grid's false origin:

SU 38765 14852
= Easting:  438,765 m
  Northing: 114,852 m

Either form unambiguously identifies the same point. Tourists, walkers, and outdoor activities typically use the letter form; engineers and software typically use the all-numeric form.

OSGB36 vs WGS 84

OSGB36 is older and smaller than WGS 84. The differences between the two are not constant — they vary across Great Britain:

• Central England (Birmingham region): OSGB36 latitude is about ~80 m south of WGS 84
• Cornwall (Penzance): ~120 m
• Shetland: ~80 m
• Hebrides: ~100 m

The pattern reflects how Airy 1830 was fitted to British triangulation. The ellipsoid matches the underlying terrain in the middle of the country but diverges toward the geographic extremes.

Per the Ordnance Survey documentation, the authoritative way to convert between OSGB36 and ETRS89 (and through ETRS89, WGS 84) is the OSTN15 transformation grid:

• A fine-resolution lookup grid that gives the OSGB36-to-ETRS89 shift at thousands of grid points across Britain.
• Software interpolates between grid points to compute the shift at any specific location.
• Accuracy: ~1 m typical, ~0.1 m in well-surveyed areas.

OSTN15 is implemented in PROJ (the geospatial transformation library used by QGIS, GDAL, and most modern GIS software), so any software using PROJ handles BNG via EPSG code 27700 with OSTN15 applied automatically.

For approximate conversion (within ~10 m), simple seven-parameter (Helmert) transformations are sometimes used. These work for casual purposes but produce systematic errors at the metre level; for survey-grade work, OSTN15 is the only acceptable method.

OSGM15 — the vertical companion

Ordnance Survey also publishes OSGM15 — the model that gives the geoid-ellipsoid separation across Great Britain. OSGM15 lets users convert between ellipsoidal heights (what GPS reports) and orthometric heights (above mean sea level, what Ordnance Survey maps show as “height above Newlyn datum”). The two transformations together — OSTN15 horizontally and OSGM15 vertically — give a complete 3D conversion between WGS 84 / ETRS89 and the British national reference.

The vertical datum used by Ordnance Survey is ODN (Ordnance Datum Newlyn), based on tide-gauge readings at Newlyn, Cornwall, during 1915–1921. The /learn/horizontal-vs-vertical-datum support covers the horizontal-vs-vertical distinction.

History

The British National Grid was established in the 1930s by the Davidson Committee, which reviewed Ordnance Survey's needs for a national reference system. The committee recommended:

• A single Transverse Mercator projection covering all of Great Britain
• A new triangulation based on geodetic-quality observations
• A grid reference system using letters and numbers

The recommendation became the British National Grid in 1936. The supporting triangulation — the "Retriangulation of Great Britain" — was conducted between 1936 and 1962 using ground-based optical triangulation and trig-pillar networks. About 6,500 trig pillars were erected across the country, many of which remain visible on hilltops today.

In the 1980s and 1990s the network was updated using satellite Doppler and GPS observations. The current operational network is OSNet, the modern GPS-based reference network with about 110 continuously operating reference stations across Britain. OSNet provides real-time differential corrections for survey-grade GPS work.

Trig pillars

The 6,500 trig pillars built between 1936 and 1962 are a distinctive piece of British landscape infrastructure. Each pillar is a concrete obelisk about 1.2 m tall, with a brass plate on top and a hollow centre where a theodolite or other survey instrument could be mounted. The pillars were placed on hilltops with clear lines of sight to other pillars, forming the visual network on which the retriangulation was based.

About 4,000 trig pillars remain standing in 2026, although most are no longer in active use. They have become a cultural icon: hikers “collect” pillar visits (a Trigpointing UK database records pillar conditions and visits), and the pillars frequently appear in landscape photography and on Ordnance Survey's own marketing material. The pillar at Ben Nevis (the highest point in the UK) is a particularly popular destination.

The modern OSNet network has made the trig pillars operationally obsolete, but the network of CORS stations now serves the same function from a small number of permanent installations rather than from a sky-spanning network of pillars.

A worked example

Take the Tower of London (approximately 51.5081°N, 0.0760°W in WGS 84). The BNG conversion:

OSGB36 latitude:  51.5077°N (about 5 m south of WGS 84)
OSGB36 longitude: 0.0744°W  (about 10 m east of WGS 84)
BNG easting:      533,648 m
BNG northing:     180,505 m
100 km square:    TQ (London)
Grid reference:   TQ 33648 80505 (6-digit reference)
                  TQ 336 805 (4-digit reference, 100 m precision)

The shift from WGS 84 to OSGB36 of ~10 m is typical for the London area — small compared to the corners of the UK but still operationally significant for survey work. The grid reference TQ 336 805 places the Tower in central London exactly where one would expect on an Ordnance Survey 1:25,000 Explorer map.

The seven-parameter approximation

For applications that need rough OSGB36-to-WGS 84 conversion without the precision of the OSTN15 grid, a seven-parameter Helmert transformation is sometimes used. The Helmert transformation applies a 3D translation, 3D rotation, and a scale factor — seven parameters in total — to convert between two reference frames.

The standard Helmert parameters for OSGB36 to WGS 84 (per the Ordnance Survey Coordinate Systems guide):

Translation: (446.448 m, -125.157 m, 542.060 m)
Rotation:    (0.1502", 0.2470", 0.8421")
Scale:       -20.4894 ppm

The transformation is convenient because it requires only seven parameters and no lookup grid, but it produces systematic errors of ~3 m in the average case and up to 7 m in worst cases. OSTN15 is far more accurate but requires the lookup grid. For survey-grade work, OSTN15 is mandatory.

Software representation

EPSG codes for the British National Grid:

| EPSG | System | Notes | |---|---|---| | 27700 | OSGB36 / British National Grid | Standard BNG | | 4277 | OSGB36 (geographic) | Latitude/longitude on Airy 1830 | | 4326 | WGS 84 (geographic) | International reference | | 4258 | ETRS89 (geographic) | European reference |

Modern GIS software handles BNG via EPSG:27700 with OSTN15 transformation. Web mapping libraries typically convert BNG to Web Mercator for display; the conversion is handled automatically by the underlying geospatial software stack.

Sources

• Ordnance Survey Guide to Coordinate Systems in Great Britain
• Ordnance Survey OSTN15 transformation
• Ordnance Survey OSNet GPS reference network
• UK Geospatial Commission
• EPSG registry

For closely related material, see /learn/etrs89-explained for the European reference frame, /learn/nad83-explained and /learn/state-plane-coordinate-system for the North American counterparts, and /learn/conformal-projections for the projection family the BNG's Transverse Mercator belongs to.