What Is the Geoid?

The geoid is the level that merges with the average sea level and extends into the interior of the continent. It is a straight reference surface. In the survey work, all are based on the geoid. Due to the unevenness of the earth's surface and uneven distribution of the mass inside the earth, the geoid is an irregular surface with slight fluctuations. The shape enclosed by this surface is similar to a spheroid of ellipsoid, called "the terrane", which is often used to represent the physical shape of the earth.

A geoid is an irregular closed surface formed by a stationary sea surface and extending to the continent. A geoid or pseudo-geodesic is an elevation datum for obtaining geospatial information ^[1]

Geoid

Geoid astronomical level

A method of calculating the relative geoid gap using only astronomical geodetic data. The relative vertical deviation component in the AB direction indicates the inclination of the geoid in the AB direction (Figure 2). Obviously, as long as the relative vertical deviation component changes linearly between A and B , then multiplying the average value of the relative vertical deviation at two points A and B by the distance S between the two points, two The difference between the geodetic level of the point.

Because the relative vertical deviation between two points changes linearly only within a short distance, astronomical standards require very dense astronomical points, especially in mountainous areas.

Geoid astronomical gravity level

A method of calculating the relative geoid gap using a combination of astronomical and gravity measurements. It uses the gravity anomaly data on the geoid in a certain area around these two points to correct the astronomy when the two known astronomical locations A and B are far apart (for example, tens of kilometers to more than a hundred kilometers). The relative vertical deviation in the level is not affected linearly. Formulated as:

,

Where Ng is the gravity correction term calculated using gravity anomalies. In this way, when calculating the difference between the relative geoids, only sparse astronomical points can be used. Therefore, only the astronomical points existing in the national geodetic network can be used, which reduces the workload of astronomical surveys and replaces them with a certain range. Gravity measurement work inside.

In 1937, MC Molotynski proposed to use the elliptic hyperbolic coordinate system template to calculate the gravity correction term Ng in the astronomical gravity level according to the gravity anomaly of the point. In 1958, Chinese geodetic scholar Fang Jun proposed to use a rectangular coordinate system to calculate this gravity correction term according to the average gravity anomaly. At present, this work uses electronic computers for calculations.

Since 1958, China has set up astronomical levels and astronomical gravity levels along first-class triangle locks, forming several closed loops. In order to avoid the accumulation of errors, it is divided into first-class (high accuracy) and second-class (low accuracy) two levels. In this way, starting from the origin of the Chinese earth, the elevation anomaly error recursed to the farthest point along the astronomical level and astronomical gravity level line will not exceed ± 3 meters, so as to meet the requirements of the initial side length of the astronomical earth network.