Vertical datum

A vertical datum is a reference surface and associated system for assigning elevations or depths. It may be based on mean sea level, a geoid model, or a defined ellipsoid.

Geoid: An equipotential surface of Earth's gravity field that approximates global mean sea level. Many vertical datums use a geoid-like surface to define zero elevation.
Geoid model: A mathematical representation of the geoid used to calculate its separation from a reference ellipsoid. It converts satellite-derived ellipsoidal heights into geoid-referenced elevations.
North American Vertical Datum of 1988: A geodetic vertical datum used to report elevations across much of North America. NAVD 88 is the principal mainland U.S. elevation reference for many mapping products.
Horizontal datum: A reference framework for specifying positions on Earth's surface using horizontal coordinates. It defines latitude and longitude, whereas a vertical datum defines elevation or depth.
Mean sea level: The average height of the sea surface over a specified period and location. Local mean sea level has historically supplied a practical zero for elevation datums.
Geoid height: The signed separation between the geoid and a reference ellipsoid at a location. This separation is needed to relate ellipsoidal and orthometric heights.
National Geodetic Vertical Datum of 1929: A former U.S. and Canadian vertical datum established from leveling and mean sea level observations. Its replacement by NAVD 88 illustrates why datum revisions matter.
Local vertical datum: A vertical reference established for a particular region, often tied to local sea-level observations. Local realizations can differ in origin and offset from national or global references.
Ellipsoid: A mathematically defined surface that approximates Earth's shape for geodetic calculations. Ellipsoidal heights use this geometric surface rather than a gravity-based level surface.
GNSS: A group of satellite navigation systems that determine positions using signals from orbiting satellites. GNSS commonly supplies ellipsoidal heights that require a vertical datum for elevation use.

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