Height above mean sea level

Height above mean sea level is the vertical distance from a point to a reference surface approximating the average sea-surface level. Its value depends on the chosen vertical datum and how that datum is realized.

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Mean sea level: The average height of the sea surface over a specified period, location, and measurement method. It supplies the reference level named in this height measure.
Geodetic leveling: A surveying method that transfers height differences between points using precise level observations. It carries datum-based elevations from control points across the landscape.
Topographic map: A map showing landforms and surface features, often with contour lines for elevation. Its contours express terrain height relative to a vertical reference.
Height above ground level: The vertical distance from a point to the local ground surface beneath it. It changes with local terrain rather than using a sea-level reference.
Vertical datum: A reference system defining elevations or depths from a specified surface or level. It specifies the reference surface against which the height is reported.
Global Navigation Satellite System: A satellite-based system for determining position and time on Earth. GNSS positions are often converted from ellipsoidal height to a sea-level-based height.
Digital elevation model: A digital representation of terrain elevations arranged across a geographic area. Its elevation values depend on the vertical datum used.
Dynamic height: A gravity-based vertical coordinate proportional to geopotential difference from a reference surface. It is a distinct height convention used in physical oceanography and geodesy.
Geoid: An irregular gravitational equipotential surface that approximates global mean sea level. It provides a gravity-based approximation to mean sea level on land.
Geoid model: A mathematical representation of geoid height relative to a reference ellipsoid. It enables conversion between satellite-derived heights and geoid-based elevations.