# Map Projection Series Introduction (i)----Earth space model

Source: Internet
Author: User

1, the real world and coordinate space relations

Any spatial feature is represented as a specific position on the Earth's surface, and the position depends on the established coordinate system to represent it.
By unifying the coordinate system and the elevation system, GIS data from different sources can be superimposed and displayed together, and spatial analysis is performed.

2. Description of the Earth space model

In order to study geospatial space, it is necessary to establish the geometrical model of the earth surface, which is the precondition of geodetic surveying. According to the results of geodesy, the geometrical model of the Earth's surface can be divided into three categories:

1) The first category is the natural surface of the Earth .

2) The second category is a relatively abstract surface, the geoid , which can be used to represent the physical shape of the earth. Where the geoid surrounds the sphere, called the Earth Sphere , is a level approximation of the Earth's form .

The earth has 71% of the ocean area, so it can be assumed that when the sea is in a completely stationary equilibrium, it extends from sea level to all the lower parts of the continent, and a continuous, closed surface perpendicular to the Earth's gravity direction is the geoid. It is a gravity-equal plane.

3) The third category is the Earth ellipsoid model based on the geoid.

Although the geoid is very complex, in general, the fluctuation is small, and the shape is close to a small flat rate ellipse around the short axis of the formation of the regular ellipsoid, the ellipsoid called the earth ellipsoid. Its surface is a regular mathematical surface, which can be expressed in mathematical formulae, so it is substituted for the Earth's natural surface in surveying and mapping. the two-magnitude approximation of the Earth's form .

The Earth ellipsoid has a long radius of a (equatorial radius) and a short radius b (polar radius), and F is the ellipse's flattening rate. A, B and F are the three elements that determine the shape and size of the Earth's ellipsoid.

Various Earth ellipsoid models (reference ellipsoid, described below) as shown.

We adopted the sea-Ford ellipsoid before 1952 and adopted the Krasovsky ellipsoid from 1953 onwards. In 1978, China decided to adopt the new ellipsoid Grs (1975), and established a new and independent geodetic coordinate system in our country, corresponding to the xian_1980 ellipsoid inside ArcGIS. The new ellipsoid GRS (1980) has been introduced since 1980, and the ellipsoid parameter is the same as the CGCS2000 ellipsoid in ArcGIS.

The Earth ellipsoid is considered a sphere : When making small-scale maps (less than 1:5 million), the earth can be regarded as a sphere, ignoring the Earth's flattening rate, due to its large reduction. The calculation is simpler with a radius of about 6,371-kilometer.

The Earth ellipsoid is considered an ellipsoid : When making large-scale maps (greater than 1:1 million), the earth must be considered an ellipsoid to ensure accuracy.

Map Projection Series Introduction (i)----Earth space model

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