geoidais

Geoidais is a term that appears in some non-English geodesy literature as the plural form of geoid; in English, the standard plural is geoids. The geoid itself is a model of Earth's gravity field represented as an equipotential surface that best fits mean sea level extended through the continents. It serves as a natural reference surface for measuring elevations and heights.

The geoid reflects variations in Earth's gravitational potential caused by irregular mass distribution within the planet and by surface topography. The deviation of the geoid from a reference reference ellipsoid is called the geoid undulation (N), and these undulations can reach roughly ±100 meters on global scales, with larger or smaller values in local regions depending on gravity anomalies.

Geoid models are derived from gravity measurements on land and at sea, supplemented by satellite data from missions such as GOCE and GRACE. Methods combine gravimetric analysis, astro-geodetic observations, and GNSS-levelling to produce regional or global representations. These models enable the conversion between ellipsoidal heights, obtained from satellite positioning, and orthometric heights, which sit above mean sea level.

Applications of geoid models include establishing vertical datums for surveying, construction, and civil engineering; improving oceanographic and hydrological studies; and supporting precise navigation and geophysical research. Limitations involve data quality and coverage, time variability from mass redistribution (atmosphere, oceans, hydrosphere, and the solid Earth), and the resolution of local gravity anomalies. Understanding geoidais—as the plural form used in some languages—reiterates the broader concept of the geoid and its central role in height reference systems.