In geodesy, datums are sets of data. Which of the following statements is true?

• A. Datums measure gravity.
• B. Datums are fixed heights above MSL.
• C. Datums are sets of data.
• D. Datums are methods of projection.

Answer: (C)

Datums in geodesy are sets of data that define the reference frame used to specify positions on the Earth. This means a datum includes the mathematical model of the Earth's shape (the reference ellipsoid), the origin and orientation of the coordinate system, and the way those coordinates tie to real-world locations. Because of this, coordinates you measure or compute depend on the chosen datum, and changing the datum changes the numerical values even if the point on the ground hasn’t moved. It’s not about gravity as a measurement, nor is it just a fixed height above mean sea level, and it isn’t a projection method. The idea that a datum is a defined collection of data that establishes how coordinates map to the Earth best captures what a datum is.