Understanding Coordinate Reference Systems

What is a CRS?

A Coordinate Reference System (CRS) defines how coordinates in your data map to locations on the Earth's surface. Without a CRS, coordinates like (500000, 4649776) are meaningless — they could refer to anywhere on the planet.

Geographic vs Projected CRS

• Geographic CRS: Uses latitude and longitude (degrees) on an ellipsoidal model of the Earth. Example: WGS 84 (EPSG:4326).
• Projected CRS: Projects the Earth's surface onto a flat plane using meters (or feet). Example: UTM Zone 18N (EPSG:32618). Better for measuring distances and areas.

What is an EPSG Code?

EPSG codes are standardized identifiers for coordinate reference systems, maintained by the International Association of Oil & Gas Producers. Each code maps to a specific CRS definition. Examples:

• EPSG:4326 — WGS 84 (the GPS standard, used by GeoJSON and KML)
• EPSG:3857 — Web Mercator (used by Google Maps, OpenStreetMap tiles)
• EPSG:32618 — UTM Zone 18N (eastern US, metric)
• EPSG:2154 — RGF93 / Lambert-93 (France)
• EPSG:27700 — OSGB 1936 / British National Grid (UK)

Why CRS Matters for Conversion

When converting between formats, the CRS determines whether reprojection is needed:

• GeoJSON and KML require WGS 84 (EPSG:4326). Data in other CRS will be automatically reprojected.
• Shapefile and GeoPackage support any CRS. You can keep the original or reproject.
• Mismatched CRS causes data to appear in the wrong location. If your data shows up in the middle of the ocean, the CRS is likely incorrect.

How ConvertGeoData Handles CRS

1. We auto-detect the source CRS from file metadata (.prj, GeoKeys, etc.).
2. If detection fails, you can search for the EPSG code or use the map-based CRS finder.
3. You can optionally reproject to any target EPSG code during conversion.
4. The CRS validation preview lets you visually verify the CRS is correct before converting.