A framework that defines how geographic coordinates relate to specific locations on the Earth's surface, combining a coordinate system with a datum.
A Coordinate Reference System (CRS) has two components: a coordinate system (the mathematical framework for expressing positions, such as latitude/longitude or Easting/Northing) and a datum (the model of the Earth's shape and size that the coordinates are referenced to). Two datasets with different CRS values will not align correctly on a map unless one is reprojected to match the other.
Common CRS values used in Canada include:
- EPSG:4326 — WGS84 geographic coordinates (latitude/longitude), used by GPS
- EPSG:4269 — NAD83 geographic coordinates, the standard for Canadian government data
- EPSG:3347 — Statistics Canada Lambert projection, used for national-scale mapping
- UTM zones (e.g., EPSG:32617 for UTM Zone 17N) — used for local and regional mapping
Example: A developer downloads two datasets: road centrelines in NAD83 (EPSG:4269) and building footprints in WGS84 (EPSG:4326). When displayed together, the layers appear slightly misaligned. Reprojecting both to the same CRS resolves the alignment issue.