WGS84 vs OSGB36: Why Your Coordinates Are 100 m Out

A datum is a mathematical model of the Earth's shape and size. Because the Earth is not a perfect sphere, different datums use different approximations to describe its surface. Each datum defines a slightly different reference frame, which means the same physical location has slightly different latitude and longitude values depending on which datum is being used.

In everyday terms, a datum is like the reference grid that sits underneath all coordinates. If you change the datum, you change the grid, and the coordinates shift even though the physical location has not moved.

WGS84 (World Geodetic System 1984) is the global datum used by the GPS satellite system. It is the default for virtually all consumer GPS devices, smartphones, Google Maps, Apple Maps, OpenStreetMap, GeoJSON, GPX files, and most web mapping APIs.

When someone shares a latitude/longitude coordinate without specifying the datum, it is almost always WGS84. It is the assumed default in the digital mapping world.

WGS84 is designed to be accurate across the entire globe rather than optimised for any particular country. Its EPSG code is 4326.

OSGB36 (Ordnance Survey Great Britain 1936) is the traditional datum for mapping in Great Britain. It is the datum that underlies the British National Grid coordinate system, which is used by Ordnance Survey, UK planning authorities, land registries, utility companies, and most professional survey work in England, Scotland, and Wales.

OSGB36 was designed to be a best fit for the British Isles specifically, rather than for the whole globe. That local optimisation is why it gives slightly different latitude/longitude values compared to WGS84 for the same physical point.

OSGB36 is primarily used with projected coordinates (Easting/Northing in metres), but it also has its own geographic coordinate system (latitude/longitude in degrees). Its EPSG code is 27700 (for the projected BNG version).

The offset between WGS84 and OSGB36 varies across Great Britain, but it is typically between 70 and 120 metres. The direction of the shift is generally northeast when going from OSGB36 to WGS84, or southwest when going from WGS84 to OSGB36.

This means that if you take a latitude/longitude coordinate from a WGS84 source (such as a GPS device) and plot it on a system expecting OSGB36, the point will appear roughly 70 to 120 metres away from the true location. The reverse is also true.

An offset of 100 metres is large enough to put a point on the wrong building, the wrong side of a road, or the wrong field. In planning, surveying, and engineering contexts, that kind of error can have real consequences.

The problem is that coordinates from WGS84 and OSGB36 look identical. They are both latitude/longitude pairs with the same number of decimal places. There is no visible marker that tells you which datum they belong to. That invisibility is what makes datum mistakes so common and so frustrating.

Converting between WGS84 and OSGB36 requires a datum transformation, not just a simple number format change. The transformation adjusts the coordinate values to account for the different Earth models used by each datum.

CoordinateMapper handles this transformation automatically. When you convert between latitude/longitude (which defaults to WGS84) and British National Grid Easting/Northing or UK Grid references (which use OSGB36), the datum transformation is applied behind the scenes. You do not need to specify the datum manually.

If you are using other software, make sure it applies the correct transformation. A simple format change without datum transformation will leave the offset intact.

WGS84 and OSGB36 are different mathematical models of the Earth. The same physical location has different coordinate values in each datum, with an offset of approximately 70 to 120 metres across Great Britain. WGS84 is the global standard for GPS and web maps. OSGB36 is the UK standard for Ordnance Survey and professional mapping. Converting between them requires a datum transformation, not just a format change. Recording the datum alongside every coordinate is the single most effective way to prevent offset problems.