What differentiates a geocentric datum from a local datum?

• A. Geocentric datum uses a region-specific origin; local datum uses the Earth's center.
• B. Geocentric datum uses the Earth's center as origin with global orientation; local datum is fixed to a region with its own origin, ellipsoid, and orientation.
• C. Geocentric datum spans only the Northern Hemisphere.
• D. Geocentric datum uses a different time standard.

Answer: (B)

A datum is the frame used to define positions on the Earth. A geocentric datum uses the Earth's center of mass as its origin and a single global orientation, so coordinates are defined in one worldwide 3D reference frame. A local datum, on the other hand, is fixed to a specific region and has its own origin on the surface, its own ellipsoid, and its own orientation to best fit that region. Because of that, local datums provide minimal distortion for that area but don’t align perfectly with global coordinates unless you apply a transformation between datums. For example, a global geocentric system like WGS84 covers the world, while regional datums such as NAD27 or ED50 are tailored to specific areas with region-specific parameters.