What Is WGS 84? The Datum Behind GPS

WGS 84, the World Geodetic System 1984, is a standard model of the Earth's size, shape and orientation that defines where latitude and longitude lines sit. GPS uses it, and so do almost all web maps and consumer apps. When you see a pair of decimal coordinates without any further label, they are very likely on WGS 84.

What a datum is

The Earth is not a perfect sphere. It is slightly flattened at the poles and has an irregular surface. To give coordinates meaning, you need an agreed mathematical model, called a datum, that fixes:

• The shape and size of a reference ellipsoid that approximates the Earth
• Where its centre sits relative to the real Earth
• How it is oriented in space

Different countries and eras have used different datums, each fitted to their own region. Two datums can give slightly different coordinates for the very same place, sometimes by tens or hundreds of metres.

Why WGS 84 became the default

WGS 84 is a global datum, centred on the Earth's centre of mass, rather than fitted to one region. It was created for the GPS system, and has been refined since its introduction. Because satellite navigation produces positions in this system, adopting the same datum everywhere avoids conversion. This is why a phone, a web map and a routing service can exchange latitude and longitude without further work.

EPSG:4326

In databases and map software, coordinate systems are labelled with EPSG codes. EPSG:4326 means geographic coordinates (latitude and longitude, in degrees) on WGS 84. If a tool asks you which coordinate system your data uses and you have normal phone or web-map coordinates, EPSG:4326 is usually the answer. It is different from EPSG:3857, which is the Web Mercator projected system.

When a datum mismatch matters

If you combine data from a source that uses a local datum with WGS 84 data and do not convert, features can appear shifted. Older surveys, national grids and some engineering datasets are typical sources of this problem. The shift is often a few metres up to a few hundred, which is easily noticeable at street scale. See coordinate reference systems explained for how to deal with it.

WGS 84 and location codes

A code system based on coordinates needs a datum to be meaningful. UPCS codes are generated from latitude and longitude as chosen on the map, which are the same kind of coordinates your phone's GPS reports. The code refers to a 3 m by 3 m cell, so a datum shift of a few metres would be on the same scale as the cell itself, which is another reason to keep all inputs on a single, common reference system.