What are NAD27 and NAD83, and how do they differ?

• A. NAD27 uses Clarke 1866 ellipsoid with a fixed ground reference in North America.
• B. NAD27 and NAD83 use the same ellipsoid but different datums.
• C. NAD27 uses Clarke 1866; NAD83 is geocentric and ties to a global reference frame with a different ellipsoid.
• D. NAD27 is a modern GNSS datum; NAD83 is used for local surveying.

Answer: (C)

The key idea is how a geodetic datum defines your reference surface and its origin. NAD27 (North American Datum 1927) is built on Clarke 1866 and ties to a fixed ground reference in North America, so its coordinates describe positions relative to that local frame rather than a global center. NAD83 (North American Datum 1983) is geocentric, meaning its origin is effectively at the Earth's center of mass and it aligns with a global reference frame using a different ellipsoid (GRS80, closely related to modern frames like WGS84). Because the ellipsoid and the origin differ, the same point has different coordinates in NAD27 and NAD83, and converting between them requires a datum transformation.

The other statements miss this combination: data say the two use the same ellipsoid, which isn’t true (NAD83 uses GRS80, not Clarke 1866); NAD27 isn’t a modern GNSS datum and NAD83 isn’t just for local surveying, it’s the global, geocentric frame used with GNSS data.