Coordinate system "slang" in GIS world

Introduction There are many in the field of maps “ slang ” I cannot understand you , Those who feel the same way can call 1 Ha When I first stepped into the field of maps , For those who do not have a major , Cartography 、GIS theory , Coordinate system , Coordinate conversion, etc (e) fan (xin) This article focuses on the common in the coordinate system “ slang ”, I try to be as straightforward as possible , Welcome to make corrections , Communicate and grow together .

This article mainly talks about the following points ：

1、 Coordinate systems are common “ slang ”

2、 Common coordinate system codes

3、 Special coordinate system

One 、 Coordinate systems are common “ slang ”

（ One ） What is? WGS

World geodetic system （World geodetic system, abbreviation WGS） Refer to 1960 Since then , By the Defense Mapping Agency （DMA） Four world geodetic systems established （WGS60、WGS66、WGS72 and WGS84） Is called

（ Two ） What is the third order approximation of the earth

Our earth is not a ball , It's like an ellipsoid . There are mountains and depressions on the natural surface of the earth , It's rugged , In order to express it, we must find a relatively regular face --- Geoid （ First order approximation ）

Geoid is the first approximation of the earth's surface . Suppose that when the sea water is in a state of complete static equilibrium , From sea level to all land and underground , And a continuous line orthogonal to the direction of the earth's gravity 、 Closed surfaces , This is the geoid .

An approximate sphere formed by a geoid , But it is not a rule , Its shape is close to a regular ellipsoid formed by an ellipse with minimal oblateness rotating around the short axis , This ellipsoid ---- Ellipsoid of the earth （ Second order approximation ） As shown in red in the figure below

With the ellipsoid of the earth , It doesn't exactly coincide with the earth's surface , Some places are close to each other （ The reference ellipsoid is close to the earth's surface ）, Some places are not very close , Then we introduce the geodetic datum （ Third order approximation ） To control the position of the ellipsoid and the earth

The geodetic datum contains two types of datum ：

1、 Geocentric datums

From satellite data , Using the center of mass of the earth as the origin , The most widely used is WGS 1984

2、 Regional datum

In certain areas, it coincides with the surface of the earth , The origin of the earth is the point where the reference ellipsoid is tangent to the geoid , for example Beijing54、Xian80

The difference between the geocentric coordinate system and the parametrical coordinate system that we will mention later is that

（ 3、 ... and ） What is? CRS

Coordinate reference system （Coordinate Reference Systems  CRS）, contain GCS and PCS Two coordinate systems , Three standard formats proj.4、EPSG（European Petroleum Survey Group）、WKT（Well-known Text）

proj.4 expression

+proj=longlat +datum=WGS84 +no_defs

 EPSG expression

EPSG:4326

WKT expression

GEOGCRS["WGS 84",
    DATUM["World Geodetic System 1984",
        ELLIPSOID["WGS 84",6378137,298.257223563,
            LENGTHUNIT["metre",1]]],
    PRIMEM["Greenwich",0,
        ANGLEUNIT["degree",0.0174532925199433]],
    CS[ellipsoidal,2],
        AXIS["geodetic latitude (Lat)",north,
            ORDER[1],
            ANGLEUNIT["degree",0.0174532925199433]],
        AXIS["geodetic longitude (Lon)",east,
            ORDER[2],
            ANGLEUNIT["degree",0.0174532925199433]],
    USAGE[
        SCOPE["unknown"],
        AREA["World"],
        BBOX[-90,-180,90,180]],
    ID["EPSG",4326]]

（ 3、 ... and ） What is? GCS

1、 Geographic coordinate system

Is spherical coordinates . The reference plane is an ellipsoid , The coordinate unit is latitude and longitude

（1） The geocentric coordinate system , After positioning and orientation , The center of the earth's ellipsoid coincides with the earth's center of mass

（2） Reference center coordinate system , After positioning and orientation , The center of the earth's ellipsoid does not coincide with the earth's center of mass, but is close to the earth's center of mass

Geographical coordinates , Is to use longitude （ Meridian ）、 Weft 、 longitude 、 Latitude represents the spherical coordinates of the ground points , Can express anywhere in the world .

2、 Latitude and longitude of the earth

Usually used on maps .（ Here's the picture ）

The longitude of the earth （L）： The two-sided angle between the great meridional plane and the primitive meridional plane at a point on the reference ellipsoid . To the East is positive , To the west is negative ;

Latitude of the earth （B）： The angle between the normal of a point on the reference ellipsoid and the equatorial plane . The north is positive , South is negative ;

The earth is high （H）： It refers to the distance from a point along the normal direction to the reference ellipsoid

（ Four ） What is? PCS

Projection coordinate system , It's a plane coordinate system , The reference plane is the horizontal plane , The coordinate unit is meter

Geographical coordinates can be converted to projection coordinates , a pair N The relationship between , The conversion process is called “ Projection ”（ Projection ： Convert an irregular earth surface to a plane ）

A mathematical method for establishing the functional relationship between points between the ellipsoid and the plane of the earth , It's called map projection

Projection coordinate system =  Projection method ( Geographic coordinate system )

1、 Projection family

a) Cylindrical projection ,b) Conic projection , c) Plane projection

2、 Projection combination

（1） Classification according to deformation properties ：

Isometric projection ： The angular deformation is zero

Isometric projection ： The area deformation is zero

Arbitrary projection ： length 、 There are deformations in both angle and area

（ among , Various deformations are interrelated and affect each other , Equal product and equal angle are mutually exclusive , Large deformation of equal product projection angle , Large deformation of isometric projection area ）

（2） From the type of projection surface ：

Transverse cylindrical projection ： The projection plane is a horizontal cylinder

Conic projection ： The projection plane is conical

Azimuth projection ： The projection plane is a plane

（3） From the relationship between the projection plane and the earth's position, it can be divided into ：

Orthaxial projection ： The central axis of the projection plane coincides with the earth axis

Oblique projection ： The central axis of the projection plane intersects obliquely with the earth axis

Transverse projection ： The central axis of the projection plane is perpendicular to the earth axis

Tangent projection ： The projection plane is tangent to the ellipsoid

Secant projection ： The projection plane cuts the ellipsoid

3、 Projection combines a representative projection coordinate system

Projection combination name	Projection coordinate system
Positive axis equiangular tangent cylinder	Mercator projection （Mercator）
Positive axis equiangular cutting cone	Lambert （Lambert）
Positive axis equal area cutting cone	Arber （Albers）
Horizontal axis equiangular tangent cylinder	Gauss Kruger （Gauss Kruger）
Horizontal axis equiangular cutting cylinder	Universal horizontal axis Mercator （UTM）

4、 The most suitable projection coordinate system in China

Gauss projection and normal axis equal angle conic projection are used in our country's geographic information system , Here's why ：

（1） Basic scale topographic map of China (1∶5 thousand ,1∶1 ten thousand ,1∶2.5 ten thousand ,1∶5 ten thousand ,1∶10 ten thousand ,1∶25 ten thousand ,1∶50 and 1∶100 ten thousand ) Medium greater than or equal to 1∶50 The graphs of Wan all adopt Gauss — Kruger projection is based on geography

（2） In our country 1∶100 The ten thousand topographic map adopts the positive axis equiangular cutting conic projection , Its division is consistent with that adopted by the international millionth

（3） Most of the provincial maps in China mostly adopt the normal axis equiangular cutting conic projection and the normal axis equiarea cutting conic projection belonging to the same projection system

（4） Positive axis isometric conic projection , The shortest distance between two points on the earth's surface ( The great circle route ) It appears to be close to a straight line , This is conducive to the correct implementation of spatial analysis and information measurement in GIS

however ！ however ！ however ！ Gao De of our country 、 Baidu 、 Tencent map , Google Maps （ Both at home and abroad ）、osm All the maps are displayed in a flat way by using the Internet Mercator , Due to the information security requirements of our country , Domestic cartographers should use a special coordinate system to express （ Later, we will talk about the special coordinate system ）

Two 、 Common coordinate system codes

（ One ） Geographic coordinate system

Beijing 54---EPSG:4214

Xian 80---EPSG:4610

CGCS 2000---EPSG:4490

WGS 84---EPSG:4326

（ Two ） Projection coordinate system

WGS 84 / Pseudo-Mercator---EPSG:3857

WGS_1984_Web_Mercator_Auxiliary_Sphere---ESRI:102100（Esri For internal use ID, And EPSG:3857 The corresponding ）

The following two are in 2008 Year to year Web Mercator Set up WKID, But the datum plane of this coordinate system is a sphere , No WGS 1984. It's been around for a while and then it's abandoned

Popular Visualisation CRS / Mercator---EPSG:3785

WGS_1984_Web_Mercator---ESRI:102113（Esri For internal use ID, And EPSG:3785 The corresponding ）

3、 ... and 、 Special coordinate system

1、GCJ02（ National Survey Bureau 02 or Mars coordinates ）

This is an offset algorithm , Instead of a single coordinate system . We are used to calling the coordinates with offset algorithm gcj02 coordinate . The deviation of map data processed by migration algorithm is generally 300~500 rice

2、BD09（ Baidu 09 coordinate ）

Baidu map is in gcj02 Based on this, the second addition migration is carried out , The parameters will be optimized and adjusted at regular intervals , Prevent cracking

Four 、 Summary

Above blabla quite a lot , I hope you are interested in maps TX We can understand the generation of coordinate system 、 Classification and the coordinate systems commonly used by major graphic manufacturers , You can sort out the concept in your daily work , handle very skillfully .