A coordinate like 51.5 degrees north, 0.1 degrees west looks simple, but it is meaningless without a coordinate reference system, the framework that defines what those numbers refer to. The same numbers in different systems can point to places tens or hundreds of metres apart, which is why layers sometimes refuse to line up in QGIS even though each one looks correct on its own.

The Earth is not a perfect sphere, so geodesists model it with an ellipsoid, a squashed sphere that approximates the planet's shape, and a datum, which anchors that ellipsoid to the Earth's surface through surveyed control points. The most common global datum is WGS84, used by GPS, while many countries maintain local datums fitted more precisely to their own territory. Coordinates measured on different datums for the same physical point differ, sometimes significantly.

A projection is the second step: flattening the curved Earth onto a flat map. Every projection distorts something, area, shape, distance or direction, and the choice depends on what matters for your map. The ubiquitous Web Mercator, EPSG:3857, powers web maps but exaggerates areas near the poles, which is why Greenland looks as large as Africa on many online maps. Equal-area projections preserve area for thematic mapping, while conformal projections preserve local shape for navigation.

EPSG codes are the shorthand for all of this. EPSG:4326 means WGS84 latitude and longitude in degrees, the default for GPS data and many web services. EPSG:3857 is Web Mercator in metres. National systems have their own codes, such as EPSG:27700 for the British National Grid. When you see an EPSG code, you are looking at a complete recipe: datum, ellipsoid, projection and units.

QGIS handles most of this automatically through on-the-fly reprojection: layers in different systems are reprojected for display so they align on screen. Problems arise when a layer lacks a defined system, or carries the wrong one. A layer that loads in the wrong ocean has almost always been assigned the wrong system, not bad data, and fixing the assignment, not reprojecting, is the cure. Use the layer properties to check and correct the assigned system first.

For analysis, reproject to a common system explicitly rather than relying on display reprojection. Distance and area calculations in degrees are meaningless, so reproject to a local projected system in metres before measuring. QGIS's reproject tool creates a new layer in the target system, and that layer is the one to analyse and share.

A practical habit prevents most headaches: always check a new layer's coordinate system on load, note it in your project documentation, and choose your project system deliberately at the start, usually a local projected system for local work or WGS84 for global data. Five minutes of system hygiene at the start saves hours of misalignment detective work later.

Coordinate systems feel abstract until they bite, and then they feel very concrete indeed. Learn the difference between a datum and a projection, recognise the common EPSG codes, and treat system assignment as part of data quality, not an afterthought, and your layers will line up every time.

As a final check on any project, open your project's properties and confirm the project coordinate system, then list each layer's system alongside it. Mismatches that survive to the analysis stage produce wrong areas, wrong buffers and wrong overlays, and a one-minute audit at the start is far cheaper than discovering the error in a delivered report. A short note in the project readme recording the project system and each layer's original system will save the next person who opens the project real confusion.