not to be confused with
geode.
not to be confused with
geoid, a geocoding scheme.
geodesy 


fundamentals
 geodesy
 geodynamics
 geomatics
 history

concepts
 geographical distance
 geoid
 figure of the earth (earth radius and earth's circumference)
 geodetic datum
 geodesic
 geographic coordinate system
 horizontal position representation
 latitude / longitude
 map projection
 reference ellipsoid
 satellite geodesy
 spatial reference system
 spatial relations

technologies
 global nav. sat. systems (gnsss)
 global pos. system (gps)
 (russia)
 (china)
 (europe)
 (india)
 (japan)
 discrete global grid and geocoding

standards (history) ngvd 29  sea level datum 1929  osgb36  ordnance survey great britain 1936  sk42  systema koordinat 1942 goda  ed50  european datum 1950  sad69  south american datum 1969  grs 80  geodetic reference system 1980  iso 6709  geographic point coord. 1983  nad 83  north american datum 1983  wgs 84  world geodetic system 1984  navd 88  n. american vertical datum 1988  etrs89  european terrestrial ref. sys. 1989  gcj02  chinese obfuscated datum 2002  geo uri  internet link to a point 2010 
 international terrestrial reference system
 spatial reference system identifier (srid)
 universal transverse mercator (utm)


the geoid (/) is the shape that the ocean surface would take under the influence of the gravity and rotation of earth alone, if other influences such as winds and tides were absent. this surface is extended through the continents (such as with very narrow hypothetical canals). according to gauss, who first described it, it is the "mathematical figure of the earth", a smooth but irregular surface whose shape results from the uneven distribution of mass within and on the surface of earth. it can be known only through extensive gravitational measurements and calculations. despite being an important concept for almost 200 years in the history of geodesy and geophysics, it has been defined to high precision only since advances in satellite geodesy in the late 20th century.
all points on a geoid surface have the same effective potential (the sum of gravitational potential energy and centrifugal potential energy). the force of gravity acts everywhere perpendicular to the geoid, meaning that plumb lines point perpendicular and water levels parallel to the geoid if only gravity and rotational acceleration were at work. the surface of the geoid is higher than the reference ellipsoid wherever there is a positive gravity anomaly (mass excess) and lower than the reference ellipsoid wherever there is a negative gravity anomaly (mass deficit).^{[1]}
geoid undulation in false color, shaded relief and vertical exaggeration (10000 scale factor).
geoid undulation in false color, to scale.