What are the two main datums that form the foundation for navigation and transportation in the United States?

• A. Ellipsoid datums and geoid datums
• B. Geodetic datums and horizontal datums
• C. Tidal datums and vertical datums
• D. Horizontal datums and vertical datums

Answer: (D)

Two kinds of reference surfaces are essential for navigation and transportation: horizontal datums and vertical datums. Horizontal datums define where a point is on the Earth's surface within a coordinate grid, enabling accurate positioning for maps, charts, and GPS navigation used by ships, aircraft, and road vehicles. Vertical datums provide a base for elevations by referencing heights above or below a standard surface (usually mean sea level), which is crucial for designing infrastructure, determining flood risk, and managing vertical clearances. In U.S. practice, the modern horizontal framework is NAD83 (and similar global systems like WGS84), while the vertical framework is NAVD88. Since navigation requires knowing both exact location and height relative to a reference surface, these two datum types form the foundation for navigation and transportation.