How can we determine the exact location of a point on the earth? This is a question to be answered by the geographic coordinate system. Because the earth is a sphere, converting a point on the earth to a two-dimensional plane map depends on the projection coordinate system selected by the user. Therefore**There are two phases for converting a point on the earth to a flat map.**That is, to determine the unique identification coordinate of a point on the earth and to perform projection planization of the unique identification coordinate. This requires the two coordinate systems that are frequently used in ArcGIS --**Geographic coordinate system and projection Coordinate System**-- Understanding.

Geographic coordinate system (geographic coordinate system), also known as the real-world coordinate system, is a coordinate system that determines the location of a ground object on the earth. It is based on the longitude and latitude as the storage unit of the map, and the longitude and latitude are angles. To determine the coordinates of the earth, a digital simulation must be performed on the earth to abstract an elliptical body similar to the earth. The elliptical body has the following features: it can be quantitatively calculated, features long half axis, short half axis, and eccentric heart rate. Because the time, method, and region of the elliptical body are different, the results are often inconsistent. Therefore, there are many types of Earth elliptical body parameter values, such as Hayford), craasovsky, I. u. g.g. China used the Hayford elliptical body before 1952, from 1953 to 1980. With the launch of man-made earth satellites, we have more precise conditions for measuring the Earth's shape. In 1975, the 16th International Survey and geophysical Federation adopted the earth elliptical body published in resolution 1 of the international survey Association, known as GRs (1975 ),**Since 1980, GRs (1975) has been adopted in China.**Because the difference between the long radius and the short radius of the earth's elliptical is very small, when creating a small scale map, it is often treated as a sphere with a radius of 6371 kilometers. The following rows are grs_1980 elliptical objects and their corresponding parameters:

**Spheroid: grs_1980**

**Semimajor axis: 6378137.000000000000000000**

**Semiminor axis: 6356752.314140356100000000**

**Inverse flattening: 298.257222101000020000**

However, it is not enough to have this elliptical body. The geographic coordinate system also needs a to locate the elliptical body, which will locate the reference system of the point on the earth and define the joint direction of the starting point of the geographic line. You need to select an elliptical shape for the establishment of the datum level, and then select a point on the earth as the "Origin". All other vertices on the elliptic are defined relative to this origin point:

In addition to the global WGS84 and wgs72, you can also use your local data in different regions, such as Beijing 1954, Xi'an 80, and European ED50, these baselines can be converted to each other. In the coordinate system description, you can often see such a line:

Datum: d_beijing_1954

The ing_1954 d_d_d_d_d_d_d_ d_d_d. With the basic conditions of spheroid and datum, the geographic coordinate system can be used.

The complete parameters of a geographic coordinate system are as follows:

Alias:

Abbreviation:

Remarks:

Angular unit: Degree (0.017453292519943299)

Prime meridian: Greenwich (0.000000000000000000)

Datum: d_beijing_1954

Spheroid: craovsky_1940

Semimajor axis: 6378245.000000000000000000

Semiminor axis: 6356863.018773047300000000

Inverse flattening: 298.300000000000010000

Geographic coordinate system is the most common coordinate system object, so it is used to describe the location of the ground. Longitude is usually expressed by the letter λ. According to international regulations, the meridian of the Greenwich Mean Observatory is the prime meridian. as the starting point for calculating the longitude, the longitude of the line is 0 degrees, and the longitude between 0 and 180 degrees is called the eastern longitude, 0-180 degrees west longitude. The latitude is usually expressed by a letter (PHI. The latitude is calculated from the equator, and the upper latitude of the equator is 0 degrees.

The longer the latitude is, the more latitude it is to 90 degrees to the pole. North of the equator is north latitude and south latitude. The location of any point on the ground is usually determined by longitude and latitude. Longitude and latitude curves are the curves of the two orthogonal groups on the earth surface (with an intersection of 90 degrees). The coordinates of these two groups of orthogonal curves are called Geographical coordinate systems. The longitude difference between two points on the ground surface is called longitude difference, and the latitude difference between two points is called latitude difference. For example, the location of Beijing on the earth can be

Determined by Latitude 39 ° 56 'and longitude 116 ° 24. Longitude and latitude are not uniform in terms of measurement. For example, the longitude at the equator is 111 km, the latitude at 60 degrees is 55.8 km, and the latitude at the north and south poles is 0 km.

Projection coordinate system is a coordinate system used to project a three-dimensional geographic coordinate system to a two-dimensional flat map. Therefore, geographic information systems must consider map projection. The use of map projection ensures the connection and integrity of spatial information in the region. During the establishment of various geographic information systems, selecting an appropriate map projection system is the first consideration. Because the earth's elliptical surface is a curved surface, while a map is usually drawn on a plan paper, the first thing to do is to develop the surface into a plane, but the spherical surface is an invisible surface, that is, when it is directly converted into a plane, it is impossible not to crack or fold. In order to prevent this situation, cartography use a variety of special methods to expand the sphere. These methods sacrifice other attributes while keeping a certain feature unchanged, for example, equisigned projection, equiproduct projection, and positive projection.

In the selection of a large scale map,**China generally uses Gaussian-kerlugs**In Europe and America, this projection method is called the projection horizontal axis mocato projection. The following are some parameters in a Gaussian-gram projection Coordinate System: Projection: gauss_kruger

Parameters:

False_easting: 500000.000000

False_normal: 0.000000

Central_meridian: 117.000000

Scale_factor: 1.000000

Latitude_of_origin: 0.000000

Linear Unit: meter (1.000000)

Geographic coordinate system:

Name: gcs_beijing_1954

Alias:

Abbreviation:

Remarks:

Angular unit: Degree (0.017453292519943299)

Prime meridian: Greenwich (0.000000000000000000)

Datum: d_beijing_1954

Spheroid: craovsky_1940

Semimajor axis: 6378245.000000000000000000

Semiminor axis: 6356863.018773047300000000

Inverse flattening: 298.300000000000010000

We can see from the parameters that each projection coordinate system must have a geographic coordinate system. The projection coordinate system is essentially a plane coordinate system, and its map unit is usually meters. The projection coordinate system uses the X and Y coordinates to describe the location on the ground. It uses the Earth's elliptical sphere spheroid to simulate the earth. It uses projection to represent the projection calculation method and unit to represent the unit, geocoordsys is used to represent the source of the projection coordinate system. That is to say, to obtain the projection coordinate, there must be a spherical coordinate used for the projection before the projection algorithm can be used for the projection. That is, each projection coordinate system must have a geographic coordinate system parameter.