Features and functions: the geoid coincides with the average sea level of 7 1% of the earth's area and is very close to the natural surface of the earth; Geoid has the characteristics of geoid, which is orthogonal to the vertical line everywhere. The measuring instrument is leveled with a level and centered with a vertical ball. Therefore, the geoid is the benchmark of surveying operation.
The sea actually exists, and it is connected with all the coastal countries in the world. The average sea surface position can be obtained by taking the average of tidal observations.
Extended data:
The shape of geoid is determined by comprehensive utilization of ground and space geodesy technology. Ground geodesy technology includes gravity survey, astronomical geodesy, GPS/ leveling, etc. Space geodesy technology includes satellite altimetry, satellite laser ranging and satellite gravity measurement. There are mathematical models and digital models to describe the shape of the geoid.
The mathematical model uses spherical harmonic/ellipsoid harmonic function to express the difference between geoid and ellipsoid of the earth; The digital model discretizes the gap between the geoid and the ellipsoid of the earth in a certain range in the form of grid.
References:
Baidu encyclopedia-geoid