{"id":16627,"date":"2026-04-08T10:00:00","date_gmt":"2026-04-08T08:00:00","guid":{"rendered":"https:\/\/www.sigterritoires.fr\/?p=16627"},"modified":"2026-03-15T16:47:35","modified_gmt":"2026-03-15T15:47:35","slug":"epsg-4326-vs-3857-2","status":"publish","type":"post","link":"https:\/\/www.sigterritoires.fr\/index.php\/en\/epsg-4326-vs-3857-2\/","title":{"rendered":"[EN]EPSG:4326 vs EPSG:3857: Understanding the Difference Between the Two Most Common GIS Coordinate Systems"},"content":{"rendered":"\n

\n\n\n\n

When working with geographic data in QGIS or in web mapping applications, two coordinate systems appear very frequently: EPSG:4326<\/strong> and EPSG:3857<\/strong>.<\/p>\n\n\n\n

These two systems are used in many contexts, but they do not represent coordinates in the same way.<\/p>\n\n\n\n

In this article, we will look at:<\/p>\n\n\n\n

\n
what EPSG:4326 and EPSG:3857<\/strong> are<\/li>\n\n\n\n
the differences between them<\/li>\n\n\n\n
when to use one or the other.<\/li>\n<\/ul>\n\n\n\n
\n\n\n\n
\n
\n
Contenu <\/p>\nToggle<\/span>
1. What Is an EPSG Code?<\/a><\/li>
2. EPSG:4326: Latitude \/ Longitude Coordinates<\/a><\/li>
3. EPSG:3857: The Web Mercator Projection<\/a><\/li>
4. The Main Differences<\/a><\/li>
5. Usage in QGIS<\/a>
EPSG:4326<\/a><\/li>
EPSG:3857<\/a><\/li><\/ul><\/li>
6. FAQ<\/a>
Data does not align with the basemap<\/a><\/li>
The coordinates seem incorrect<\/a><\/li><\/ul><\/li>
Conclusion<\/a><\/li><\/ul><\/nav><\/div>\n\n\n\n\n
\n\n\n\n
<\/span>1. What Is an EPSG Code?<\/span><\/h1>\n\n\n\n
An EPSG code<\/strong> identifies a geographic coordinate reference system.<\/p>\n\n\n\n
These codes are defined in the EPSG Geodetic Parameter Dataset<\/strong>, a reference database used in GIS software.<\/p>\n\n\n\n
Each code corresponds to a specific coordinate system.<\/p>\n\n\n\n
For example:<\/p>\n\n\n\n
Code<\/th> Description<\/th><\/tr><\/thead>
EPSG:4326<\/td> WGS84 \u2013 latitude and longitude<\/td><\/tr>
EPSG:3857<\/td> Web Mercator \u2013 projection<\/a> used for web maps<\/td><\/tr>
EPSG:2154<\/td> Lambert 93<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n
\n\n\n\n
<\/span>2. EPSG:4326: Latitude \/ Longitude Coordinates<\/span><\/h1>\n\n\n\n
The EPSG:4326<\/strong> system is based on the WGS84 geodetic system<\/strong>.<\/p>\n\n\n\n
Coordinates are expressed as:<\/p>\n\n\n\n
\n
latitude<\/strong><\/li>\n\n\n\n
longitude<\/strong><\/li>\n<\/ul>\n\n\n\n
The units are degrees<\/strong>.<\/p>\n\n\n\n
Example:<\/p>\n\n\n\n
Latitude : -20.3484\nLongitude : 57.5522\n<\/code><\/pre>\n\n\n\nThis system is commonly used for:<\/p>\n\n\n\n \nGPS devices<\/li>\n\n\n\n many geographic databases<\/li>\n\n\n\n GeoJSON files.<\/li>\n<\/ul>\n\n\n\n \n\n\n\n<\/span>3. EPSG:3857: The Web Mercator Projection<\/span><\/h1>\n\n\n\nThe EPSG:3857<\/strong> system is a projection known as Web Mercator<\/strong>.<\/p>\n\n\n\n Coordinates are expressed in meters<\/strong>.<\/p>\n\n\n\n Example:<\/p>\n\n\n\n X : 6404700\nY : -2309000\n<\/code><\/pre>\n\n\n\nThis projection is used by most web maps:<\/p>\n\n\n\n \nOpenStreetMap<\/li>\n\n\n\n Google Maps<\/li>\n\n\n\n Leaflet<\/li>\n\n\n\n OpenLayers.<\/li>\n<\/ul>\n\n\n\n \n\n\n\n<\/span>4. The Main Differences<\/span><\/h1>\n\n\n\nEPSG:4326<\/th> EPSG:3857<\/th><\/tr><\/thead> coordinates in degrees<\/td> coordinates in meters<\/td><\/tr> latitude \/ longitude<\/td> projected planar coordinates<\/td><\/tr> used for GPS data<\/td> used for web maps<\/td><\/tr> suitable for storing geographic coordinates<\/td> suitable for map display<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n \n\n\n\n<\/span>5. Usage in QGIS<\/span><\/h1>\n\n\n\nIn QGIS, these two systems are very common.<\/p>\n\n\n\n <\/span>EPSG:4326<\/span><\/h2>\n\n\n\nOften used for:<\/p>\n\n\n\n \nimporting GPS coordinates<\/li>\n\n\n\n storing geographic data<\/li>\n\n\n\n working with GeoJSON files.<\/li>\n<\/ul>\n\n\n\n<\/span>EPSG:3857<\/span><\/h2>\n\n\n\nOften used for:<\/p>\n\n\n\n \ndisplaying web map basemaps<\/li>\n\n\n\n working with OpenStreetMap<\/li>\n\n\n\n publishing web maps.<\/li>\n<\/ul>\n\n\n\nQGIS can reproject layers on the fly<\/strong> so they display correctly together.<\/p>\n\n\n\n \n\n\n\n<\/span>6. FAQ<\/span><\/h1>\n\n\n\n<\/span>Data does not align with the basemap<\/span><\/h2>\n\n\n\nThis usually means that:<\/p>\n\n\n\n \nyour layer is in EPSG:4326<\/strong><\/li>\n\n\n\n your project is in EPSG:3857<\/strong>.<\/li>\n<\/ul>\n\n\n\nQGIS can handle this automatically if on-the-fly reprojection<\/strong> is enabled.<\/p>\n\n\n\n \n\n\n\n<\/span>The coordinates seem incorrect<\/span><\/h2>\n\n\n\nFor example:<\/p>\n\n\n\n X : -20\nY : 57\n<\/code><\/pre>\n\n\n\nThese values are likely latitude and longitude coordinates<\/strong>.<\/p>\n\n\n\n \n\n\n\n<\/span>Conclusion<\/span><\/h1>\n\n\n\nEPSG:4326 and EPSG:3857 are two coordinate systems widely used in GIS.<\/p>\n\n\n\n \nEPSG:4326<\/strong> is well suited for storing geographic coordinates.<\/li>\n\n\n\n EPSG:3857<\/strong> is mainly used for displaying web maps.<\/li>\n<\/ul>\n\n\n\nUnderstanding the difference between these two systems helps avoid many projection issues in QGIS and web mapping applications.<\/p>\n\n\n\n \n\n\n\n<\/p>\n","protected":false},"excerpt":{"rendered":" When working with geographic data in QGIS or in web mapping applications, two coordinate systems appear very frequently: EPSG:4326 and EPSG:3857. 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