Coordinate & Mapping Resources

The OS National Grid

The Ordnance Survey National Grid (also called the British National Grid, BNG) is the flat, metre-based coordinate system that covers Great Britain. It is a Transverse Mercator projection with a true origin south-west of the Isles of Scilly and a false origin placed so that every location in GB has positive coordinates: 400 km west and 100 km south of the true origin.

From that false origin, eastings increase to the east and northings increase to the north, both in metres. The grid is divided into 100 km squares, each given a two-letter code (for example SU, TQ, SE, NZ). A grid reference is just a shorthand for a full easting/northing using those letters.

Reading a grid reference

A grid reference has three parts: the 100 km square letters, then equal numbers of easting and northing digits. Take SU 387 148:

• SU — the 100 km square (its south-west corner is at easting 400000, northing 100000);
• 387 — easting within the square, to the nearest 100 m: 38 700 m → full easting 438700;
• 148 — northing within the square: 14 800 m → full northing 114800.

So SU 387 148 = easting 438700, northing 114800. Add digits for more precision: SU 3874 1483 is to 10 m, SU 38740 14830 is to 1 m. Always read easting before northing — “along the corridor, then up the stairs”.

Grid-reference precision

A “6-figure” grid reference means three digits each side (100 m); a “10-figure” reference means five digits each side (1 m). Quoting more digits does not add accuracy that was not in the underlying position — a coordinate scaled off a 1:2500 map is not a 1 m position however many digits you write.

Easting & northing

Full easting and northing values (no letters) are what CAD and GIS software actually store. They run from roughly:

• easting: 0 to ~700000 m (west coast of Cornwall to the east coast of Kent / Norfolk);
• northing: 0 to ~1300000 m (Isles of Scilly to Shetland).

Because both are always positive and northing exceeds easting for most of the country, a transposed pair is usually obvious. In drawings and coordinate schedules, state the system explicitly — “OSGB36 / British National Grid, EPSG:27700, values in metres” — so there is no ambiguity on import.

OSGB36 vs WGS84 (EPSG:27700 vs EPSG:4326)

Two different reference frames are in play, and confusing them puts positions out by up to ~100 m:

OSGB36 is not a simple offset from WGS84 — it is stretched and rotated relative to the global frame because it was surveyed by triangulation before satellites. That is why a proper transformation, not a rubber-sheet fudge, is needed.

The datum transformation — OSTN15 / OSGM15

Ordnance Survey publishes the definitive transformation between ETRS89 (effectively WGS84 for GB) and OSGB36 National Grid:

• OSTN15 — a horizontal “rubber sheet” grid of easting/northing shifts, accurate to a few centimetres, that models the distortions in the 1936 network;
• OSGM15 — the geoid model that converts ellipsoidal height to orthometric height above Ordnance Datum Newlyn (ODN).

A 7-parameter Helmert transformation (the old “OSTN02 approximate” or a single set of Bursa-Wolf parameters) gets you to ~1 m; only OSTN15 gets you to survey accuracy. Most GIS and libraries such as proj4 / PROJ can use the OSTN15 grid-shift file when configured; a plain Helmert is the common default and is fine for planning-scale work but not for setting out.

Height & vertical datum

Horizontal and vertical are separate problems. GNSS gives ellipsoidal height (height above the WGS84/GRS80 ellipsoid). UK levels, drawings and flood data use ODN — orthometric height above mean sea level as realised at Newlyn. The difference (the geoid–ellipsoid separation) across GB is roughly +45 to +55 m and varies with location; OSGM15 models it. Terrain data from different sources (LIDAR DTM, OS Terrain, ASTER/SRTM) may use different vertical references — check before differencing.

What3Words

What3Words divides the world into a grid of 3 m × 3 m squares, each with a fixed three-word address such as ///filled.count.soap. It is not a coordinate system for design, but it is increasingly used in highways incident management and site-access instructions because a field team can act on three words immediately — no risk of a transposed digit. Treat it as a human-readable pointer that resolves to a lat/long, not as survey data.

Converting between formats

• Grid reference ↔ easting/northing — arithmetic only (add the 100 km square offset, pad the digits). No datum change.
• Easting/northing ↔ lat/long — a projection and a datum transformation. Use OS tools, proj4/PROJ with OSTN15, or a GIS.
• Bulk conversion — ogr2ogr -s_srs EPSG:4326 -t_srs EPSG:27700, or the OS Coordinate transformation tools / API for OSTN15 accuracy.
• CAD import — export a CSV of easting/northing from CivilsRoute; import into Civil 3D via the Survey Database or MicroStation via OpenRoads, with the coordinate system set to British National Grid.

Source links

• OS — A guide to coordinate systems in Great Britain
• OS — OSTN15 / OSGM15 transformation
• OS Net — GNSS correction network
• EPSG:27700 — British National Grid · EPSG:4326 — WGS84
• PROJ · proj4js
• What3Words
• Grid Reference Finder — quick lookups and conversions.

Coordinate conversions in browser tools are planning-grade. Setting-out and survey deliverables must use a controlled OSTN15/OSGM15 transformation and be checked against known control.