EPSG:4326 vs EPSG:3857: The Ultimate Mapping Difference の座標変換
In web mapping and 地理情報システム (GIS), the two most commonly confused codes are EPSG:4326 and EPSG:3857. Mixing them up causes data to render completely off the map, often dropping your points off the coast of Africa (Null Island, 0,0).
- EPSG:4326 = A 3D Globe represented by degrees of Latitude and Longitude.
- EPSG:3857 = A flat 2D Map represented by meters of X and Y (Web Mercator (EPSG:3857, valid -85.05° to +85.05°)).
EPSG:4326（WGS84 地理座標）
EPSG:4326 uses the World Geodetic System 1984 (WGS84) ellipsoid as its model of the Earth. It does not project the Earth onto a flat surface. Coordinates are stored as angles (degrees, minutes, seconds, or decimal degrees).
- Unit: Degrees
- Axes: Latitude (-90 to +90), Longitude (-180 to +180)
- Primary Use: GPS hardware output, storing data in databases (like PostGIS), standard data exchange (GeoJSON, KML).
If your coordinate looks like `-80.123, 25.456`, it is almost certainly EPSG:4326.
EPSG:3857（Webメルカトル投影）
EPSG:3857 takes the WGS84 ellipsoid and projects it onto a flat square using the Mercator projection. The units are physical distances (meters) from the Equator and Prime Meridian.
- Unit: Meters
- Axes: X (Easting) and Y (Northing), typically ranging from -20,037,508 to +20,037,508 meters
- Primary Use: Drawing square map tiles in a web browser (Google Maps, Mapbox, Leaflet, OpenLayers).
If your coordinate looks like `-8919379.12, 2931215.34`, it is EPSG:3857 (or a similar meter-based grid like UTM).
一般的なデータ障害
Almost all modern web maps store the raw spatial data as EPSG:4326 (Lat/Long) in the database, but display the map tiles to the user using EPSG:3857 (Web Mercator (EPSG:3857, valid -85.05° to +85.05°)).
距離と面積の歪み
EPSG:3857 is conformal (preserves shape locally) but violently distorts area and distance at higher latitudes. Measuring a polygon's area or a line's length directly on an EPSG:3857 geometry without applying a geodesic correction will yield massive errors.
Always use EPSG:4326 or a local projection (like UTM or State Plane) if you need to calculate physical distances mathematically.
Convert your Web Mercator (EPSG:3857, valid -85.05° to +85.05°) coordinates back to GPS Latitude/Longitude instantly:
—EPSG:3857 to EPSG:4326 ConverterFAQ
Is EPSG:4326 the same as WGS84?
For most practical mapping purposes, yes. WGS84 is the geodetic datum. EPSG:4326 is the official registry code that points to the geographic 3D coordinate system based exactly on that WGS84 datum.
Why does Google Maps use 3857?
Because the Mercator projection makes all grid lines perfectly square and orthogonal at all latitudes, it is much easier and faster for a computer graphics engine to render pre-calculated 256x256 pixel image tiles continuously across the screen.
See also: Web Mercator (EPSG:3857, valid -85.05° to +85.05°) Distortion | 測地系(データム)のシフトについて | GIS Pro Development
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