Reference surface for measurements, calculations.

Reference line for measurements, calculations.

Reference point for measurements, calculations.

Perceived object of sensory experience.

Reference surface for elevation measurements.

In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built structures) and altitudes of satellite orbits and in aviation.

Reference level for nautical charts.

Reference level for measuring altitudes.

A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame, or terrestrial reference frame) is a global datum reference or reference frame for unambiguously representing the position of locations on Earth by means of either geodetic coordinates (and related vertical coordinates) or geocentric coordinates.

Reference level for measuring tides.