What spheroid is the State Plane coordinate system based on?

The State Plane Coordinate System is primarily based on the GRS 1980 spheroid, which is a reference ellipsoid used in geodesy. The GRS 1980 (Geodetic Reference System 1980) provides a mathematical model for the Earth's shape, allowing for accurate calculations of positions and distances on the surface of the Earth within specific zones defined by the State Plane system.

Choosing GRS 1980 over other options is supported by the historical and practical context in which the State Plane Coordinate System was developed. This system was designed in the United States to facilitate mapping and surveying, and GRS 1980 has been widely accepted for these applications due to its precision in representing the Earth’s shape for the areas it covers.

In contrast, while WGS84 is a global reference frame commonly used in GPS and other global applications, it is not the basis of the State Plane Coordinate System. EPSG:4326 is another way of referring to the WGS84 coordinate system and does not serve as the basis for State Plane. Lastly, NAD27, which is another spheroid used in the past for various mapping applications, has largely been replaced by NAD83, which in turn is tied to the GRS 1980