EPSG:4326 vs EPSG:27700 — WGS 84 vs British National Grid
EPSG:4326 (WGS 84) is global longitude/latitude in degrees. EPSG:27700 (OSGB36 / British National Grid) is the Ordnance Survey grid for Great Britain: a Transverse Mercator projection in meters, on the older OSGB 1936 datum. The British National Grid does not share WGS 84's datum, so converting between them needs a datum shift as well as a projection change — and the OSGB36 datum differs from WGS 84 by up to ~100+ meters in places, so ignoring the shift produces a large, location-dependent error. A London point is about (-0.13, 51.51) in 4326 but around (530000, 180000) meters in 27700.
| Property | EPSG:4326 | EPSG:27700 |
|---|---|---|
| EPSG code | EPSG:4326 | EPSG:27700 |
| Name | WGS 84 | OSGB36 / British National Grid |
| Type | Geographic | Projected |
| Units | degrees | metres |
| Datum | WGS 84 | OSGB 1936 |
| Area of use | Worldwide | United Kingdom (Great Britain and Isle of Man) |
| proj4 | +proj=longlat +datum=WGS84 +no_defs | +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +units=m +no_defs |
When to use EPSG:4326
Use EPSG:4326 for global interchange, web maps, GeoJSON and GPS data, or when sharing UK data with non-UK tools.
When to use EPSG:27700
Use EPSG:27700 for Ordnance Survey data, UK cadastral and engineering work, and any analysis measured in meters on the national grid. It is the standard for authoritative British mapping.
Converting between EPSG:4326 and EPSG:27700 is a reprojection, not a relabel. Convert and reproject a file or detect your CRS.
Frequently asked questions
What is EPSG:27700?
EPSG:27700 is the OSGB36 / British National Grid — the Ordnance Survey's projected coordinate system for Great Britain. It's measured in meters using a Transverse Mercator projection on the OSGB 1936 datum.
Why can't I just relabel British National Grid data as WGS 84?
Because EPSG:27700 differs from WGS 84 in both projection (meters vs degrees) and datum (OSGB36 vs WGS 84). The datum difference alone reaches 100+ meters in places, so a proper reprojection with datum transformation is required — relabeling would place data well off.
How accurate is converting between 27700 and 4326?
A high-accuracy conversion uses Ordnance Survey's OSTN15 transformation grid for centimeter-level results. A simple seven-parameter (Helmert) transform is accurate to within a couple of meters — fine for mapping, not for survey work.
Why are my UK coordinates large numbers like 530000, 180000?
Those are British National Grid eastings and northings in meters (EPSG:27700). They're projected grid coordinates, not WGS 84 degrees, and must be reprojected before a degree-based map or GeoJSON tool will place them correctly.