Forming the
The geodetic surface is an irregular sealing surface formed by the still seawater surface and extends to the mainland. The earth level or similar to the earth is an elevation reference surface that acquires geospatial information. It is a gravity alias, that is, when the object moves along the surface, the gravity does not work (such as water does not flow on this surface). The earth level is an important physical reference plane describing the global shape, and is also a cross-cutting surface of the altitude elevation system. The determination of the geodes of the earth is to be achieved by determining its spacing of the reference ellipsoid - the geodetic surface gap (for similar geodes, it is called elevation abnormalities). Parameters and concepts such as earth level and altitude elevation are ubiquitous in the objective world, play an important role in national economic construction.
Significance
The geodetic surface is one of the geodetic basis, and it is determined that the earth level is an important project in the country's basic surveying and mapping. It combines geometric earth measurements and physically measured, so that people can obtain important information such as altitude and Earth gravity field relationship while determining the location of the space. The shape of the earth's level reflects information such as substance structure, density and distribution of the earth, and has an important role in research and application of oceanography, seismology, geophysics, geological exploration, petroleum exploration and other related geophysical fields.
With the development of earth measurement discipline, it is more than a multi-century, especially in nearly half a century, with the development of satellite and related geographical disciplines, this field. The research is increasingly active, determining a high-resolution high-precision global geodes have become a global strategic objective for the development of local measurement disciplines in this century.
The earth level is a calm ocean surface surrounding the world in the mapping effort, coincident with average annual average seawater surface in the world, and is a rotating ellipsoid, which is the ground elevation.
An imaginary, a gravity equipper with a coincident with a stationary seawater surface, and this extension facing the bottom of the mainland. It is the consolidation surface of the high system in an elevation measurement.
The distance between the earth's ovary or the reference ellipsoid or the reference ellipsoid surface is referred to as the large level of level gap. The former is absolute and unique; the latter is relatively, and the reference ellipsoid surface used is different from the reference ellipsoid.
Absolutely geographical surface gap The distance between the large earth level is between the average Ellipsoid (Figure 1). Its value is about ± 100 meters. The absolute geodorpiece gap can be used to use the global gravity abnormality in the Stokes integration formula (see the global shape), and the bit coefficient of the earth gravity field model can be used to calculate and calculate. In principle, any formula can be selected. Although the former is high, the operation is complicated; the latter cannot be calculated according to infinite grade, but the operation is convenient. Therefore, in practice, it is always based on different requirements, and one of the two or both of them is calculated using a mixed formula.
The absolute land level gap may be determined by the satellite hypotheors in addition to the above method, and the satellite stage measurement can be used.
relative to the geodes of the ground gap The distance between the earth is given to a reference ellipsoid. Because the size of the reference ellipsoid is not unique, the position of the relative major level of the ground is relatively significant relative to the size of the novetic size. The relative large ground gap of each point can be started by the main land origin, and the difference between the relative major level gap between the respective points is calculated from the method of astronomical or astronomical gravity level, and then it is pushed in paragraph.
Method
Astronomical level
A method of using only an astronomical earth measurement data to calculate a relative large level of level. Since the relative vertical line deviation component in the AB θ is a tilt in the AB direction indicating the large level level (Fig. 2). Obviously, as long as the relative vertical line deviation component is linearly changed between a , B , a , B The average value of the relative vertical line deviation θ multiplied by two points, the difference in the difference between the two points can be obtained. .
Because the relative vertical line deviation between the two points is linearly changed in a short distance, the astronomical level requires a very dense astronomical point in the mountains.
Astronomical gravity level
a method of calculating a relatively large-level level gap using an astronomical measurement and gravity measurement data. It is in the case of two known as many astronomical geodes a , (for example, tens of kilometers to more than 100 kilometers), using this two points The gravity abnormal data on the mainland level surface is corrected to correct the effect of relative vertical line deviation in astronomical levels. The formula is represented as:
,
δ Ng is a gravity correction term calculated using gravity abnormality. In this way, when calculating the difference in the difference between the relative large level, as long as the sparse astronomical point can be made, it can only use the astronomical point in the national land network, and the amount of astronomical measurement is reduced, and the generation is somewhat Heavy gravity measurement work.
1937, M.c. Moulinski once proposed the gravity correction of astronomical correction in the gravity correction of astronomical gravity level δ ng in accordance with the gravity abnormality of the elliptical double curvature template. In 1958, China's large-scale measuring scholar Fang Jun proposed a template for calculating this gravity change in the average gravity abnormality by average gravity. This work is calculated using an electronic computer.
Beginning in 1958, China has set astronomical standards and astronomical gravity levels along the first-class triangle, which consists of several closed loops. In order to avoid the accumulation of errors, it is divided into two levels (high precision) and second-class (low precision). In this way, from the origin of China, the elevation abnormal error of the astronomical level and astronomical gravity level will not exceed ± 3 meters, and this to satisfy the astronomical land network to start a long-term requirement.