Space reference in simple and simple--understanding of ArcGIS spatial reference

The space reference describes the real location of a feature on Earth. In order to describe the position correctly, we need to introduce a framework for measuring and calculating, so that the geodetic results can be described in this framework. And the earth is an irregularly shaped ellipsoid, so what is the method used to simulate the shape of the earth, and how to project the coordinates on the surface of the plane on the map? It is necessary to understand the concepts of geoid, reference ellipsoid, Datum plane, and the relationship between them. In addition, this paper analyzes the 802 coordinate systems of Beijing 54 and Xi ' an in detail.

1. GeoID (Geoid) and reference ellipsoid (spheroid)

The geoid provides a surface to be measured, which is basically consistent with the stationary sea level and perpendicular to the direction of gravity. The geoid is an irregular ellipsoid because of the different gravity directions at each point of the Earth's surface. In order to be able to use mathematical laws to describe the shape of the earth and to deal with the results of measurement, it is necessary to introduce a regular sphere, the concept of reference ellipsoid.

The reference ellipsoid is formed by the rotation of the ellipse on the two-dimensional plane around the short axis. The long half of the reference ellipsoid refers to the distance from the equator, and the short half of the reference ellipsoid refers to the distance from the Earth's poles from the core. The long and short axes of the different reference ellipsoid are different. As shown in the following table:

spheroid	semima Jor axis (m)	clarke 1866	6378206.4	6356583.8
GRS80 1980	6378137	6356752.31414
wgs84 1 984	6356752.31424518

Different geographical regions need to select different reference ellipsoid to describe, because different reference ellipsoid is used to simulate the Earth's geoid in different places. For example, in North America, the reference ellipsoid used in this geodetic coordinate system is the GRS 1980 ellipsoid (NAD83). For the same position, selecting a different reference ellipsoid and Datum plane will change the size of its coordinate value. The following example is the result of a different geodetic coordinate system used by the Bellingham in Washington state, and you can see that there is a big difference between the NAD1927 and the other two coordinate values.

Datum	Longitude	Latitude
NAD 1927	-122.46690368652	48.7440490722656
NAD 1983	-122.46818353793	48.7438798543649
WGS 1984	-122.46818353793	48.7438798534299

2. Datum plane (Datum)

The reference ellipsoid defines the shape of the Earth, and the datum plane describes the relationship between the center of the ellipsoid and the geocentric. The Datum plane is based on the selected reference ellipsoid and takes into account the complex surface conditions on the ground. Because the reference ellipsoid is not good enough to describe the concrete conditions of every place on earth, it can be understood that the datum plane is the result of the approximation of the geoid to a certain place, and it is a many-to-many relationship with the reference ellipsoid.

(1) Geocentric datum plane (geocentric datums)

In the past 15, the use of satellite data has provided a good model for the geodesy to simulate the Earth's ellipsoid, the geocentric coordinate system. The geocentric coordinate system uses the Earth's centroid as its center, and the most widely used is WGS 1984, the geocentric coordinate system.

(2) Local datum plane (datums)

The local datum plane moves the reference ellipsoid to a position closer to the local surface shape, and a point on the ellipsoid is bound to correspond to a position on the surface, which is called the origin of the Earth's starting point. The coordinate value of the origin of the Earth starting point is fixed, and the coordinate value of the other points can be computed from the point. The starting position of the local coordinate system is usually not in the center of the Earth, but the offset from the geocentric.