Coordinate Reference Systems Explained

A coordinate reference system (CRS) is the set of rules that gives a pair of numbers a real-world meaning. Without one, the numbers 500000, 4649776 or 51.5, -0.12 are only digits. A CRS says what the numbers measure, in which units, from where, and on which model of the Earth.

The parts of a CRS

• Datum: the model of the Earth and its origin, such as WGS 84.
• Coordinate system: the axes and units, for example degrees of latitude and longitude, or metres of easting and northing.
• Projection (for projected systems): the method used to flatten the surface, such as Mercator or the transverse Mercator used in UTM.

Geographic vs projected

Geographic CRSs use angles on the Earth's surface: latitude and longitude in degrees. They are global and what GPS reports. Distances between points are not simple to compute directly, because a degree of longitude varies in length; see how long a degree is.

Projected CRSs place coordinates on a flat plane with units such as metres. They are convenient for measuring distance and area, and for engineering and survey work, but each one is only accurate within the region it was designed for. Examples include Web Mercator and UTM zones.

EPSG codes

Because there are thousands of systems, a registry assigns each one a number, known as an EPSG code. Some you will often see:

Code	Meaning
EPSG:4326	WGS 84 latitude and longitude, in degrees
EPSG:3857	Web Mercator, in metres, used for web map tiles

Software uses these codes to know how to interpret and convert data.

Why mismatches cause problems

If two layers use different CRSs and the software does not convert between them, features can appear shifted or in the wrong place entirely. A classic sign is a map layer that sits correctly but in the wrong country, or one that is a few metres to hundreds of metres off. Typical causes:

• Treating degrees as metres, or the reverse
• Assuming a local datum is WGS 84
• Swapping the order of axes, see latitude first or longitude first

Good practice

1. Always record the CRS along with the data.
2. Convert everything to a single CRS before combining or comparing.
3. For general-purpose data exchange, use WGS 84 latitude and longitude.
4. Use a local projected system when you need precise measurements over a small area.

Location-code services typically take WGS 84 latitude and longitude as input, which keeps integration simple and unambiguous.