Geographic coordinate system and projected coordinate

[Reprint] Geographic  Coordinate  System and projected  Coordinate

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Category: GISMAP

Original article address: Geographic  Coordinate  System and projected  Coordinate Author:Gruand2008

Two concepts should be clarified:Geographic Coordinate System and projected Coordinate System.
1. First understandGeographic Coordinate System,Geographic Coordinate SystemA geographical coordinate system is a MAP storage unit. Obviously,Geographic Coordinate SystemIs 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: 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

D_ing_1954 indicates that the is D_Beijing_1954.

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: Greenwich (0.000000000000000000)
Datum: D_Beijing_1954
Spheroid: craovsky_1940
Semimajor Axis: 6378245.000000000000000000
Semiminor Axis: 6356863.018773047300000000
Inverse Flattening: 298.300000000000010000

2. ProjectCoordinate System(Projection coordinate system), first 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

It can be seen from the parameters that every projection coordinate system must haveGeographic 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 projection coordinate system must haveGeographic Coordinate SystemParameters.

3. Many of the textbooks that we have seen have many names for the coordinate system, which can be attributed to the two projection types mentioned above. This includes our common "non-earth projection coordinate system ".) :

GeodeticCoordinate): Coordinates with reference to the elliptical sphere as the reference plane in the earth measurement. The position of the ground point P is represented by the Land longitude L, the land latitude B, and the Earth height H. When
When referring to the elliptical sphere, only large
Longitude and latitude. 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 ground height 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. The vertical line and horizontal line of coordinates are drawn every kilometer.
Simultaneous cabling
It is also a coordinate network cable parallel to the Cartesian axis, so 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. To encrypt
Network, there is also an encrypted ingress network's encrypted shard short-line (referred to as "index band" in the Scheme) between the internal and external charts. If necessary, the corresponding short-line connections can constitute an encrypted latitude and longitude network. On the 50 thousand topographic map, in addition to the Intranet encrypted partition on the internal chart, there is also a cross line 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 occurs in the coordinate system.
Quadrant. 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 square and latitude and longitude networks on a topographic map.
The latitude is the coordinate of the earth. At this time, the coordinate of the Earth is not the spherical coordinate. It is the same as the projection of the square net (usually Gaussian), and also the plane coordinate.