Projections and EPSG codes in plain English

A coordinate reference system (CRS) is the rulebook that says what a pair of numbers means: degrees on the globe, or metres on a particular flat grid.

The same point can be written very differently depending on the rulebook. The numbers alone don't tell you which one was used.

Each CRS has an ID number from the EPSG list. Two you will meet constantly: EPSG:4326 is ordinary GPS latitude and longitude (WGS 84). EPSG:3857 is Web Mercator, in metres, used by web map tiles.

Countries have their own grids too, such as British National Grid (EPSG:27700) and Irish Transverse Mercator (EPSG:2157).

Software reads coordinates using the CRS it is told. If a file says nothing, or says the wrong thing, metres get read as degrees or the other way round, and the data lands in the wrong place.

Two classic signs. Your data shrinks to a tiny speck near 0, 0 off West Africa: degrees were read as metres. Or the software refuses it as out of range: metres were read as degrees, since no latitude is 530,000.

Reprojecting rewrites every coordinate from one CRS into another. Do it when the people or software you are sending to expect a certain system, for example a council asking for Irish Transverse Mercator.

GeoJSON, KML and GPX always use plain latitude and longitude, so they have nothing to choose.