The State Plane Coordinate System is a set of map zones covering the United States. Each state is divided into one or more zones, and each zone has its own projection, chosen so that distances on the map stay close to distances on the ground inside that zone. Surveyors and engineers use the zone coordinates instead of latitude and longitude.
Each zone is a Projected Coordinate Reference System (PCS). The state-plane family is a particular collection of those systems, with fixed zones and fixed projections. A projected system built for some other region is not a state-plane zone.
Why the Country Is Split into Zones
- Distortion. A single projection of a whole state can stretch distances. A smaller zone stretches them less.
- A named zone. A coordinate is meaningless until the zone is named beside it.
- Two common projections. Zones that run east-west and zones that run north-south use different projections.
A State-Plane Zone and a One-Off Projection
| Compared | A State-Plane Zone | Some Other Projection |
| Extent | A fixed zone inside one state | Whatever region the author chose |
| Who defined it | The national geodetic survey | Whoever built that dataset |
| A coordinate without a zone name | Ambiguous | Ambiguous in the same way |