A Coordinate Reference System (CRS) defines how the two-dimensional coordinates in a geospatial dataset relate to real locations on the Earth's surface. Because the Earth is an irregular three-dimensional shape, there is no single perfect way to map every point to a flat coordinate pair. Instead, geodesists have created hundreds of CRS definitions, each optimized for a particular region or purpose. Understanding which CRS your data uses — and when to reproject — is fundamental to avoiding misaligned layers and inaccurate distance or area calculations.
Geographic vs Projected Coordinate Systems
Geographic coordinate systems use angular units (degrees of latitude and longitude) measured from a reference ellipsoid. Projected coordinate systems transform those angular coordinates onto a flat plane using mathematical formulas, producing coordinates in linear units like meters or feet. The choice between geographic and projected systems depends on what you need to do with the data.
Common Coordinate Systems
EPSG:4326 (WGS 84)
The most common CRS in web mapping. It uses geographic coordinates expressed as longitude and latitude in decimal degrees and is the default for GPS receivers, GeoJSON files, and most web APIs.
EPSG:3857 (Web Mercator)
Projects the globe onto a flat square so that map tiles align seamlessly. Google Maps, OpenStreetMap, and Mapbox all render tiles in EPSG:3857. While Web Mercator preserves shapes at local scales, it dramatically distorts areas near the poles — Greenland appears as large as Africa despite being fourteen times smaller. For a detailed comparison, see our guide on EPSG:4326 vs EPSG:3857.
UTM Zones
Universal Transverse Mercator divides the globe into 60 narrow zones, each with its own projection optimized for accurate distance and area measurement within that strip. UTM is widely used for survey work, military mapping, and engineering projects.
Why CRS Matters
| Task | Recommended CRS | Why |
|---|---|---|
| Web map display | EPSG:3857 | Tile alignment |
| Data exchange | EPSG:4326 | Universal standard |
| Area calculation | Local UTM zone | Minimizes distortion |
| Distance measurement | Local UTM zone | Meters, not degrees |
Reprojection in GeoDataTools
When loading data into GeoDataTools, the application automatically detects the coordinate reference system and displays it in the sidebar. If you need to reproject between EPSG:4326 and EPSG:3857, or between common UTM zones, the built-in CRS transformation handles the conversion on the fly. This means you can combine datasets from different sources — say, a GPS track in WGS 84 and a land parcel dataset in a UTM zone — and see them aligned correctly on the same map without manual preprocessing.
FAQ
How do I find out what CRS my data uses?
GeoJSON files always use WGS 84 (EPSG:4326) by convention. Shapefiles include a .prj file that defines the CRS. In GeoDataTools, the detected CRS is shown in the sidebar when you load a file.
What happens if I mix CRS in the same project?
Layers in different CRS will appear misaligned unless reprojected to a common system. Always reproject to a single CRS before performing spatial analysis, overlays, or exports.
Is WGS 84 the same as GPS coordinates?
Yes. GPS receivers output coordinates in the WGS 84 datum, which is the reference frame for EPSG:4326. The coordinates are expressed as decimal degrees of latitude and longitude.