What is a coordinate reference system?
A CRS combines a datum, coordinate axes, units, and—when the data is projected—a mathematical map projection. Geographic systems such as WGS 84 use angular longitude and latitude, while projected systems such as Web Mercator and UTM use flat x/y coordinates, usually in meters. The same pair of numbers can point to entirely different places when interpreted under different systems.
What is an EPSG code?
An EPSG code is a short, standardized identifier for a coordinate reference system. EPSG:4326 identifies WGS 84 longitude/latitude, EPSG:3857 identifies Web Mercator, and codes such as EPSG:32633 identify individual WGS 84 / UTM zones. Using the code avoids relying on ambiguous CRS names and lets conversion software select the exact transformation definition.
How reprojection works
Reprojection calculates new coordinate values so the features stay in the same real-world locations under the target CRS. It may change axis units, projection, and datum. For example, a WGS 84 point stored in decimal degrees receives different numeric coordinates when transformed to a UTM zone measured in meters, but it should still land on the same point on a map.
Assigning a CRS vs. reprojecting
Assigning a CRS only labels existing coordinate numbers; it does not transform them. Use assignment when a file is missing its CRS metadata but you know which system the numbers already use. Use reprojection when you need the coordinates converted into a different system. Assigning the target CRS to source coordinates is a common cause of layers appearing far from their true location.
EPSG:4326 vs EPSG:3857
EPSG:4326 and EPSG:3857 describe the same world with different coordinate grids. EPSG:4326 stores WGS 84 longitude and latitude in decimal degrees; EPSG:3857 projects the globe onto a flat Web Mercator plane in meters for web-map display.
| Difference | EPSG:4326 | EPSG:3857 |
|---|---|---|
| CRS type | Geographic coordinate system | Projected coordinate system |
| Coordinate units | Decimal degrees | Meters on the projected plane |
| Typical coordinate order | Longitude, latitude for GeoJSON | X (easting), Y (northing) |
| Best use | Data exchange, GPS and geographic storage | Displaying tiles and overlays on web maps |
| Measurement warning | Degrees are not constant ground distances | Scale and area distortion grow toward the poles |
Do not replace the CRS label to move data between these systems. Reproject the coordinates. EPSG:3857 is convenient for web display, but use a suitable local projected CRS rather than Web Mercator for accurate distance or area measurements.
Coordinate-system rules by file format
Some formats store a CRS, while others require WGS 84 or carry no spatial reference at all.
GeoJSON
Per RFC 7946, coordinates are longitude/latitude in decimal degrees on the WGS84 datum (EPSG:4326), and the specification removed the older "crs" member, so a compliant GeoJSON file is always assumed to be WGS84. Coordinate order is [longitude, latitude], optionally followed by elevation.
Shapefile
The coordinate reference system lives in the optional .prj sidecar as a WKT (well-known text) string. When the .prj is absent the CRS is genuinely unknown and consumers typically fall back to assuming WGS84, which can silently misplace data that was in another projection.
KML
KML coordinates are always geographic WGS84 longitude, latitude, and (optionally) altitude in that order; the format has no CRS or projection field, so any data written to KML is expressed in EPSG:4326 by definition.
KMZ
Because the payload is ordinary KML, a KMZ is inherently WGS84 (EPSG:4326) longitude/latitude/altitude, with no projection metadata and no possibility of reprojection inside the format.
GPX
The GPX specification fixes all coordinates to WGS84 latitude/longitude with elevation in meters, so like KML it carries no projection information and any exported data is expressed in EPSG:4326.
DXF
DXF is a CAD format and carries no coordinate reference system — coordinates are plain model units. When converting to or from GIS formats you often need to know (or assign) the drawing’s real-world CRS; GeoConvert assumes WGS84 (EPSG:4326) only if you ask to reproject.
A safe reprojection workflow
- 1Identify the source CRS from the file metadata, its provider, or the coordinate ranges; never guess when positional accuracy matters.
- 2Choose a target CRS suited to the destination software, region, and measurement units.
- 3Convert the dataset and request reprojection to the target EPSG code.
- 4Validate the result against a trusted basemap and check a known location before using it downstream.
Coordinate-system questions
- What is the difference between EPSG:4326 and EPSG:3857?
- EPSG:4326 is WGS 84 longitude/latitude in decimal degrees. EPSG:3857 is Web Mercator, a projected system in meters used by most web basemaps. They describe the same Earth through different coordinate grids.
- What happens if the source CRS is unknown?
- A reliable reprojection is not possible until the original CRS is identified. Coordinate ranges, nearby known locations, a missing Shapefile .prj, and information from the data provider can help determine it.
- Does converting a file format also change its coordinate system?
- Not necessarily. KML, KMZ, and GPX require WGS 84, and RFC 7946 GeoJSON does too. GeoConvert can preserve an approved legacy named EPSG CRS in GeoJSON, but warns that the result is not RFC 7946. Choose an explicit target EPSG code when you need a transformation.