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Earth shape description
Dec 20, 2018

The actual surface of the earth is a rugged, complex-shaped physical surface that is difficult to accurately quantify. For the convenience of research, the imaginary ocean surface is stationary and extended to the land. The resulting closed surface is called the geoid. The shape surrounded by the geoid is called the geoid. The geodetic body is also an irregular geometry due to the uneven distribution of density within the Earth and the fluctuation of surface shape. In practice, it is desirable to use a simple mathematical equation to fit the geometry of the earth. There are three approximations from low to high according to the accuracy: 1 approximate to a sphere, the center is selected on the centroid of the earth, and the radius is about 6371km. The description is rough. It is suitable for occasions where the accuracy is not high; 2 is approximated as a spheroid, the earth's rotation axis (polar axis) coincides with an ellipse short half axis, and the elliptic ellipticity is about 1/300 (the length and the half axis differ by about 21km), ellipse Rotating around its short semi-axis constitutes the surface of the spheroid, in which the Earth's equator is circular; 3 is approximately a triaxial ellipsoid, where the equator is elliptical and the equatorial ellipticity is about 1/100,000 (the length of the equatorial axis is about 60m). Although the triaxial ellipsoid description is more accurate than the spheroid, the correlation calculation of the former is much more complicated than the latter. Considering that the equatorial ellipticity of the triaxial ellipsoid is not large, it can be circular, so the spheroid The most widely used.

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