A geographic coordinate system (GCS) defines locations on the Earth's surface using angular coordinates — latitude and longitude — measured in degrees relative to a reference datum and its underlying ellipsoid. Unlike a projected coordinate system, a GCS does not flatten the Earth onto a plane; coordinates remain spherical angles.
What it is
A GCS has three components: an angular unit (usually decimal degrees), a prime meridian (typically Greenwich, 0°), and a datum that ties the coordinate frame to a specific ellipsoid model of the Earth. The most common GCS is WGS 84 (EPSG:4326), the datum used by GPS. Others include NAD83 (EPSG:4269) for North America and ETRS89 (EPSG:4258) for Europe.
Why it matters
GCS is the lingua franca for storing and exchanging position data: GeoJSON mandates WGS 84 lon/lat, and most GPS, drone, and field datasets arrive in degrees. It is excellent for archiving and global referencing. But because a degree of longitude shrinks from about 111 km at the equator to zero at the poles, you cannot reliably measure distance, area, or angles directly in a GCS.
Concrete example
In EPSG:4326, a point is written as latitude, longitude in degrees — for example 48.8566, 2.3522 (Paris). Note the axis-order trap: EPSG:4326 formally defines latitude first, but GeoJSON, many APIs, and most software expect longitude first. Mismatched order plots points in the wrong hemisphere or in the ocean.
Common pitfall
Running buffers or area calculations on a GCS produces nonsense, since the software treats degrees as if they were planar units. Reproject to an appropriate projected CRS (a local UTM zone or an equal-area projection) before any metric analysis, then convert back to a GCS for storage or web display.