Modern geodetic datums range from:

• A. Flat-earth models used for plane surveying to simple models for local use.
• B. To more complicated models limited to national boundaries.
• C. Flat-earth models used for plane surveying to complex models describing size, shape, orientation, gravity field, and angular velocity.
• D. Heuristic approximations without gravity field.

Answer: (C)

Datums in modern geodesy span a range from simple flat-earth plane-surveying models to complex, three-dimensional representations that describe the size and shape of the reference surface, how that surface is oriented in space, the Earth's gravity field, and the planet’s rotation (angular velocity). This full scope is essential for precise positioning, because you’re not just choosing a surface to measure against—you’re tying coordinates to a reference that accounts for the Earth's actual geometry, its orientation relative to space, the gravity potential that affects heights, and how the Earth rotates over time. Global examples like WGS84 use an Earth-centered ellipsoid aligned with the rotation and incorporate gravity-related information and rotation parameters, while local datums may use simpler, area-specific approximations. Choices that imply only plane or local approximations or omit gravity and rotation don’t reflect how modern datums are defined.