What defines a geodetic datum?

A geodetic datum is fundamentally a coordinate system that provides a framework for referencing geographical locations on the Earth's surface. It establishes the points on the Earth to which measurements are made, integrating both a defined reference point and a precise mathematical model of the Earth. This allows for the accurate representation of position across various mapping and surveying applications.

By providing fixed coordinates relative to a specific model of the Earth, a geodetic datum facilitates uniformity in navigating and representing geospatial information. For instance, different datums can yield different coordinates for the same physical location because they may refer to slightly different models of the Earth's shape and size, or they may be defined based on different reference points.

When considering the other options, while a theoretical model of the Earth, a fixed reference point in space, and a surveying instrument are related aspects of geodesy, they do not encapsulate the complete essence of what a geodetic datum is. A datum is not just a single fixed point or merely a model; it is an integrated system that combines these elements to establish a comprehensive coordinate system for mapping across various applications.