What is a geodetic system? What is a geodetic reference frame?

Geodetic system stipulates the starting datum, scale standard and solid line mode of geodesy. Geodetic system includes coordinate system, height system, depth datum and gravity reference system. Corresponding to geodetic system, there are three geodetic reference systems: coordinate reference system, elevation reference system and gravity measurement reference system.

(1) geodetic coordinate system and geodetic coordinate system

Geodetic coordinate system is divided into geocentric coordinate system and parametric coordinate system according to its origin position; From the expression form, it can be divided into spatial rectangular coordinate system (X, Y, Z) and geodetic coordinate system (B, L, H). Note: the height h of the earth refers to the distance from the space point to the ellipsoid along the normal direction of the ellipsoid.

Paracentric coordinate frame: the coordinate origin is located in the center of reference ellipsoid, which is realized and maintained by astronomical geodetic network. It is a regional two-dimensional static coordinate frame. Such as China's 1954 Beijing coordinate system and1980xian coordinate system.

Geocentric coordinate frame: the coordinate origin is located in the center of the earth, which is realized and maintained by space geodesy technology. It is a global three-dimensional dynamic coordinate frame. For example, China's 2000 National Geodetic Coordinate System.

(2) Height system and height frame

Elevation datum: the elevation datum defines the starting point of elevation measurement on land, and generally the average position of the sea surface can be determined by tidal observation as the elevation datum.

China uses two elevation datums:

Yellow Sea 1956 height system: the tidal observation results in 7 years, and the elevation of the leveling origin is 72.289 m;

1985 national elevation datum: tidal observation results in recent 19 years, and the elevation of datum point is 72.2604m ..

Height system: China's height system adopts the normal height system, and the starting surface of normal height is quasi-geoid.

Realization of altitude system. China's elevation framework is realized by the second-stage national first-class leveling network and the results of re-survey. The national elevation framework is divided into four levels, which are defined as the first, second, third and fourth level control networks respectively. Another form of elevation frame is realized by quasi-geoid refinement.

(3) Gravity system and gravity measurement framework

Gravity measurement is to determine the acceleration of gravity at a certain point in space. Gravity datum is the standard to calibrate the absolute gravity value of a country or region. The gravity reference frame is an ellipsoid parameter and its corresponding normal gravity field. The gravity survey framework consists of several absolute gravity points and relative gravity points distributed in various places and several baselines.

(4) Depth benchmark

The selection of depth datum is related to the tidal situation in the sea area, and the local tidal harmonic constant is often used for calculation. The depth datum can be theoretical depth datum, average low tide surface, lowest low tide surface or average low tide surface of spring tide. Before 1956, China adopted the lowest low tide surface, the average low tide surface of spring tide and the measured lowest tide surface as the depth benchmark. 1957, take the theoretical depth datum as the depth datum.

Tidal harmonic constant decomposes the measured tidal level data into many sub-tides, and calculates the average amplitude and late angle of each sub-tide.

The starting surface of water depth in nautical charts and various nautical charts. Also known as "chart depth datum" and "water depth zero". It is the theoretical tidal height calculated by taking the harmonic constants of eight main tidal components, which is lower than the local average sea level.