What are datums primarily based on?

The correct answer is that datums are primarily based on a spheroid. In the context of geographic information systems and geodesy, a spheroid (or ellipsoid) is a mathematically defined surface that closely approximates the shape of the Earth. A spheroid accounts for the Earth's flattening at the poles and the bulging at the equator due to its rotation.

Datums utilize these spheroidal models to establish a reference frame from which geographic coordinates (latitude, longitude, and elevation) are defined. This reference is essential for accurately representing geographic locations on a map. Different datums can use different spheroids, depending on their specific application and the geographical focus of the mapping or positioning system.

In contrast, while a geoid represents the mean sea level and is important for understanding variations in Earth's gravity field, it does not serve as the basis for the mathematical modeling of coordinates. A sphere, while a simpler model than a spheroid, is less accurate for representing the Earth due to its uniform radius, which does not account for the planet's flattening. Coordinate systems are frameworks for defining point locations in space but rely on a datum (which in turn is based on a spheroid) for actual geospatial referencing.