A pair of numbers like 48.8584, 2.2945 only points to a real place if you know the coordinate reference system (CRS) they belong to. The same numbers in a different system can land hundreds of metres away, or nowhere sensible at all. Understanding CRS is what separates confident GIS work from mysterious alignment bugs.
Two ingredients: datum and projection
A CRS combines two ideas:
- A datum is a model of the Earth's shape and where it sits, defining where latitude and longitude zero out. WGS84 is the global datum GPS uses. National datums such as NAD83 or ETRS89 fit their region more tightly.
- A projection is a recipe for flattening that curved model onto a flat map. Every projection distorts something: area, shape, distance or direction. You choose one whose distortion is acceptable for your task.
A geographic CRS (like plain lat/long) uses angles on the datum directly. A projected CRS (like UTM or Web Mercator) adds a projection so you can work in metres on a flat grid.
EPSG codes: short names for CRSs
Describing a full CRS in words is tedious, so each one gets a number from the EPSG registry. Instead of "WGS84 geographic coordinates in degrees" you write EPSG:4326. These codes are the universal shorthand in GIS software, spatial databases and web maps.
The two codes you will see constantly
- EPSG:4326 is WGS84 latitude and longitude in degrees. It is how GPS reports position and how GeoJSON stores coordinates. Think of it as the storage and interchange system.
- EPSG:3857 is Web Mercator, the projected system used by Google Maps, OpenStreetMap, Mapbox and virtually every slippy web map. It measures in metres and lets tiles line up into a seamless grid.
A huge amount of practical GIS is simply moving data between 4326 (for storage) and 3857 (for display), plus the occasional UTM zone (for local measurement).
Why Web Mercator is loved and hated
Web Mercator made global slippy maps possible: it is simple, tiles cleanly, and preserves shape locally so cities look right. But it badly distorts area near the poles, which is why Greenland looks as big as Africa on most web maps. For anything involving area or distance measurement, do not measure on Web Mercator. Use a geodesic method or an appropriate local projection instead.
Reprojecting: moving between systems
Converting coordinates from one CRS to another is called reprojection. Done correctly it is lossless in position (within the datums' accuracy), but it must account for both the projection maths and any datum shift. Getting the datum wrong is a classic source of errors that put data tens or hundreds of metres off.
Our EPSG lookup and reprojector lets you search for a CRS by code or name and transform coordinates between any two systems, and the coordinate converter handles the everyday lat/long, UTM and MGRS cases.
Practical advice
- Store in 4326. Keep your canonical data in WGS84 lat/long and reproject on demand.
- Never measure area on Web Mercator. Use a geodesic calculation or a local equal-area or UTM projection.
- Always record the CRS. A dataset without a stated CRS is a future headache. When in doubt, note the EPSG code alongside the data.
Try it: look up any CRS and reproject coordinates with the free EPSG / CRS Lookup.