Geographic coordinate system and earth Coordinate System of ArcGIS

For a long time, there are always many friends who cannot fully understand the concepts of geographic coordinate system and earth coordinate system. Recently, I saw an article about them on the Internet, which I like very much. So I want to forward it here, hoping it will be helpful to the drawing friends.

 

Geographic coordinates: spherical coordinates. The reference plane is an elliptical sphere, and the coordinate unit is longitude and latitude.
Coordinate: plane coordinate. The reference plane is a horizontal plane. The coordinate units are meters and kilometers.

The process of geographic coordinate transformation to the Earth coordinate can be understood as projection. (Projection: convert an irregular earth surface to a plane)
In ArcGIS, two sets of coordinate systems are pre-defined: geographic coordinate system (projected coordinate system)

 

1. First, understand the geographic coordinate system (geographic coordinate system. Apparently, geographic coordinate syst em is a spherical coordinate system. How can we store the digital information on the earth to the Spherical Coordinate System? The earth is an irregular elliptical shape. How can we store data information on the elliptical sphere in a scientific way? This requires us to find such an elliptical body. Such an elliptical body has the following features: it can be quantified. Features long half axis, short half axis, and eccentric heart rate. The following rows are the result of. Spheroid: craovsky_1940
Semimajor axis: 6378245.000000000000000000
Semiminor axis: 6356863.018773047300000000
Inverse flattening (flat rate): 298.300000000000010000

However, with this elliptical body, it is not enough. You still need a to locate the elliptical body. The following line is displayed in the coordinate system description:

Datum: d_beijing_1954 indicates that the ing plane is d_beijing_1954.

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With the basic conditions of spheroid and datum, the geographic coordinate system can be used.
Complete parameters:

Alias:
Abbreviation:
Remarks:
Angular unit: Degree (0.017453292519943299)
Prime meridian (start longitude): Greenwich (0.000000000000000000)
Datum (): d_beijing_1954
Spheroid (refer to the elliptical body): kraovsky_1940
Semimajor axis: 6378245.000000000000000000
Semiminor axis: 6356863.018773047300000000
Inverse flattening: 298.300000000000010000

 

2. The next step is projection coordinate system (projection Coordinate System). First, let's look at some parameters in the 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

From the parameters, we can see that every projection coordinate system must have geographic coordinate system.

The projection coordinate system is essentially a plane coordinate system, and its map unit is usually meters.

So why do the parameters of the coordinate system exist in the projection coordinate system?

At this time, we need to explain the significance of projection: the process of converting spherical coordinates into plane coordinates is called projection.

Now, the projection conditions are displayed:

A. Spherical Coordinates

B. conversion process (that is, algorithm)

That is to say, to obtain the projection coordinate, you must have a "used" spherical coordinate of the projection before using the algorithm for projection!

That is, each coordinate system must have the geographic coordinate system parameter.
About Beijing 54 and Xi'an 80, we use the most coordinate system.
First, we will briefly introduce the basic knowledge of Gaussian-kerluge projection, and skip it if you understand it. In China, large and medium scale maps all use Gaussian-kerluge projection, which usually is based on 6-degree and 3-degree band projection,. the terrain map of the 50 thousand-scale adopts a longitude difference of 6 degrees, and the terrain map of the scale adopts a longitude difference of 3 degrees. The specific zoning method is as follows: the 6th percentile starts from the prime meridian, and is divided into 60 projection bands from west to east according to the longitude difference of 6 degrees, the band numbers are 1-60 respectively. The 3-degree projection band starts from the longitude line of the east longitude 1 degree 30 seconds and is divided from the west to the east by the longitude difference 3 degrees, there are 120 projection bands in the world.

To facilitate Topographic Map measurement, a plane Cartesian coordinate system is arranged in the Gaussian-gram projection band. The specific method is to set the central longitude line to the X axis and the equator to the Y axis, the intersection of the central meridian line and the equator is the coordinate origin point. The X value is positive in the Northern Hemisphere, negative in the southern hemisphere, positive in the east of the central meridian line, and negative in the west of the central meridian line. Because our country's territory is in the Northern Hemisphere, X values are positive, in order to avoid negative values of y values, it is stipulated that the coordinate vertical axes of each projection belt are shifted to the West by 500 km, the original X-axis value on the central longitude line is changed from 0 to 500 km. In order to facilitate the differentiation of the Cross-band points, you can add the belt number before the bits of the Y value of each point, for example, the coordinates of Point A in the 20 bits can be expressed as YA = 20 745921.8 M.

In the coordinate systems \ projected coordinate systems \ Gauss Kruger \ Beijing 1954 directory, we can see four different naming methods:
Beijing 1954 3 degree GK cm 75e. prj
Beijing 1954 3 degree GK zone 25.prj
Beijing 1954 GK zone 13.prj
Beijing 1954 GK zone 13N. prj

Their descriptions are as follows:
The coordinate system of the Beijing 54 coordinate system using the three-degree belt method. The coordinate system of the central longitude line is located at the East 75 degrees.
The coordinate system of the Three-degree coordinate method BEIJING 54, the coordinate system of the central longitude line in the east 75 degrees, and the belt number before the horizontal coordinate
Coordinate system of beijing 54 using the Coordinate Method of six degrees, with the coordinate number 13.
The coordinate system of the Beijing 54 coordinate system, with a coordinate number of 13.

In the coordinate systems \ projected coordinate systems \ Gauss Kruger \ Xian 1980 directory, the file naming method has changed:
X Ian 1980 3 degree GK cm 75e. prj
Xian 1980 3 degree GK zone 25.prj
Xian 1980 GK cm 75e. prj
Xian 1980 GK zone 13.prj

The naming method and meaning of the Xi'an 80 COORDINATE file are the same as those of the first two coordinates of beijing 54, but there is no "with number + N". Why is there no uniform naming method? This is confusing.

Geodeticcoordinate. The position of the ground point P is represented by the Land longitude L, the land latitude B, and the Earth height H. When the point is on the reference elliptical sphere, it is expressed only by the longitude and latitude of the Earth. The longitude of the earth is the angle between the Earth's radial plane and the starting Earth's radial plane. the latitude of the earth is the angle between the normal and the equator of the point. The height of the earth is the distance from the ground point along the normal to the reference elliptical sphere.

Square grid: A square grid composed of two parallel lines parallel to the projection coordinate axis. Because the vertical line and the horizontal line of coordinates are drawn every kilometer, it is called the square internal network. Because the square internal line is also a coordinate network cable parallel to the Cartesian axis, it is also called the Cartesian coordinate network.

On a topographic map with a scale of-, the ing line is displayed only in the form of a graph contour and the corresponding degree is indicated at the graph angle. In order to encrypt a graph into a network when using it, there is also an encrypted shard line between the internal and external charts (the Scheme is called "index band "), if necessary, a short-line connection can constitute an encrypted latitude and longitude network. On a-topographic map, in addition to the Intranet-based encryption and partitioning on the internal chart, there are also cross lines for encryption.

China's-topographic map directly draws the latitude and longitude network on the drawing, and there is also a short line of encryption for the latitude and longitude network.

The coordinate system of the Cartesian coordinate network uses the straight line after the central longitude line projection as the X axis, the straight line after the equator projection as the Y axis, and their point of intersection as the coordinate origin. In this way, four quadrants appear in the coordinate system. The X-axis is positive to the north and negative to the south from the equator. The X-axis is positive to the east and negative to the west from the central Meridian.

Although we can think that the square net is the Cartesian coordinate, the earth coordinate is the spherical coordinate. However, we often see the square net and latitude and longitude net on a topographic map. We are used to calling the latitude and longitude net as the earth coordinate. At this time, the earth coordinate is not the spherical coordinate, she is the same as Fang Li's projection (generally Gaussian projection), which is also a plane coordinate.

 

Article transferred from: http://blog.csdn.net/arcgis_all/article/details/8216583

Geographic coordinate system and earth Coordinate System of ArcGIS