WGS 84, EPSG:4326 and EPSG:3857 — which coordinate system you need

You exported some data and the map "drifted" into the ocean off the coast of Africa, or stretched the wrong way. Almost always the cause is a mix-up between WGS 84, EPSG:4326 and EPSG:3857. Let's keep it simple: what they are and which coordinate system you actually need.

WGS 84 is a reference system (a datum)

WGS 84 (World Geodetic System 1984) is a model of the Earth's shape and the reference point coordinates are tied to. GPS works in WGS 84. On its own, WGS 84 says "where zero is and which ellipsoid", but it doesn't yet define how to lay coordinates out on axes. So "in WGS 84" is about the datum, not about specific X/Y numbers.

EPSG:4326 is WGS 84 in degrees (latitude/longitude)

EPSG:4326 is WGS 84 expressed in geographic coordinates: latitude and longitude in degrees. That's what you see as 55.75, 37.61 (Moscow). Such coordinates are handy as a universal "address" of a point on the planet and are used in GPS, GeoJSON and data exchange.

An important subtlety: in EPSG:4326 the axis order is formally latitude, then longitude (lat, lon). Yet many formats and libraries write lon, lat. Hence the classic mistake: the numbers are swapped and the point flies off — for example, from Moscow to the Indian Ocean. If the map "drifted", the first thing to check is the order of latitude and longitude.

EPSG:3857 is web mercator in meters

EPSG:3857 (Web Mercator, "Pseudo-Mercator") is a projection of the same WGS 84 data onto a plane, in meters. Google Maps, Yandex.Maps, OpenStreetMap, Leaflet and MapLibre are built on it. Coordinates look like large numbers such as 4187591, 7509137.

Mercator is convenient for web maps (north always up, tiles fit together) but distorts areas: the closer to the poles, the stronger the "inflation" (Greenland looks the size of Africa). Plus web mercator is clipped at about ±85.05° latitude — the poles don't fit on it.

So which one to choose

• Data for a web map (Leaflet, MapLibre, tiles) → EPSG:3857.
• Data exchange, GeoJSON, GPS, universal coordinates → EPSG:4326 (WGS 84 in degrees). By standard, GeoJSON is always in 4326.
• Measuring distances and areas → neither: take a projection in meters for your region (UTM or a local system). In degrees, meters "float"; in web mercator, areas are distorted.
• Cadastre and precise surveying in Russia → local coordinate systems (MSK). See the separate article MSK coordinate systems.

A broader breakdown of EPSG codes and choosing a projection is in the guide Coordinate systems and projections for GIS.

How to download a map straight in the coordinate system you need

Don't reproject coordinates by hand — on export in osm2cdr pick the coordinate system you need from 165 available (WGS 84 / EPSG:4326, web mercator / EPSG:3857, UTM zones, standard Gauss-Kruger zones for Russia and others). Select the area, choose the coordinate system and download the data straight in it — as GeoJSON, Shapefile, GeoTIFF or DXF. It's free and saves you the reprojection hassle.

In short

• WGS 84 — a datum (the GPS reference system), by itself without axes.
• EPSG:4326 — WGS 84 in degrees (lat/lon); mind the lat/lon order.
• EPSG:3857 — web mercator in meters, for web maps; distorts areas, clipped at ±85°.
• For web — 3857; for exchange/GPS — 4326; for measurements — a projection in meters.
• On export — a choice of 165 coordinate systems.