In the ENU frame, why is it described as a local tangent plane?

• A. Because it is a local tangent plane anchored at a reference point with East, North, Up axes.
• B. It is a global frame with East, North, Up axes.
• C. It is oriented to magnetic north.
• D. Up axis points toward the center of Earth.

Answer: (A)

The ENU frame is defined at a specific point on the Earth and uses a plane tangent to the surface at that point as its reference. In this setup, East and North lie in that tangent plane, pointing along directions on the surface, while Up is perpendicular to the plane, pointing outward from the surface. That combination makes ENU a local coordinate system tied to one location, valid for small displacements near that point and approximating surface distances well there. Because it relies on the tangent plane at a single reference point, it isn’t a global frame, and distances farther away would be distorted by Earth's curvature. Also, Up represents the local vertical (outward from the surface), not toward the Earth’s center, and North aligns with true north rather than magnetic north.