How would you differentiate a coordinate reference system from a map projection?

• A. A projection defines how coordinates relate to positions on the Earth; a CRS projects 3D to 2D.
• B. A coordinate reference system defines how coordinates relate to positions on the Earth (including datum and units); a map projection transforms 3D coordinates on the ellipsoid to a 2D plane.
• C. A CRS is only about map scale; a projection concerns coordinates.
• D. A projection includes datum definitions, while CRS only defines units.

Answer: (B)

The main idea is that a coordinate reference system defines how coordinates tie to real-world locations, including the earth model (datum) and the measurement units, while a map projection is the mathematical process that takes 3D positions on the Earth’s surface and lays them out on a 2D plane. A CRS tells you where a given latitude/longitude or easting/northing point actually sits on Earth and what units you’re using, and it may be geographic (lat/long) or projected (x/y in meters, for example). A map projection, by contrast, converts those 3D coordinates on the ellipsoid into 2D coordinates on a map, which inevitably introduces distortions in area, shape, distance, or direction depending on the projection chosen. In practice, a projected CRS combines a datum with a projection method to produce planar coordinates, whereas a geographic CRS uses angular coordinates tied to a specific ellipsoid and datum.