An EPSG code is a short integer that uniquely identifies a coordinate reference system (CRS), datum, ellipsoid, or projection within the EPSG Geodetic Parameter Dataset. Quoting a single number such as EPSG:4326 unambiguously communicates an entire set of geodetic parameters, removing guesswork about how coordinates map to the Earth.
Why it matters
In GIS, the same x,y pair means nothing without a CRS. EPSG codes give software, file formats, and people a compact, machine-readable way to declare that CRS. GDAL, QGIS, PostGIS, PROJ, and web mapping libraries all accept EPSG codes, so they are the lingua franca for reprojection and data exchange. Mislabeling or omitting the code is the root cause of layers that fail to overlay.
Origin and structure
The dataset was created by the European Petroleum Survey Group (EPSG), now maintained by the IOGP Geomatics Committee. Codes are arbitrary identifiers, not ranges with strict meaning, though some clusters exist by convention.
Concrete examples
EPSG:4326— WGS 84 geographic (latitude/longitude in degrees), the GPS and GeoJSON default.EPSG:3857— WGS 84 / Pseudo-Mercator, used by web tile maps.EPSG:32633— WGS 84 / UTM zone 33N, a projected system in metres suitable for area and distance work.EPSG:4269— NAD83 geographic, common for North American data.
Common pitfall
An EPSG code names a CRS but does not by itself fix axis order. EPSG:4326 is formally defined as latitude, longitude, yet many tools and the GeoJSON spec store longitude first. Always confirm coordinate order rather than assuming. Also remember that a projected EPSG code (metres) and a geographic one (degrees) are not interchangeable: slope, buffer, and area results are wrong if you analyse in degrees.