Writing in the front

Geographic location is the soul of GIS is not too much, like weather forecast, rocket launch, including earthquakes, volcanoes and other accidents, the news media will say XX degrees east longitude, YY degrees north latitude what happened, as well as Autonavi Baidu map navigation, positioning and so on all need to use the coordinate system, Because there is no accurate location information can not express the location relationship of ground objects, map query analysis and so on is out of the way

Coordinate system can be divided into geographic coordinate system and projection coordinate system

Today, we will use coordinate transformation in Cesium as a citation, to popularize the knowledge of coordinate system for non-professional partners, more conceptual, do not need to remember all, understand the concept can be, let’s talk about geographic coordinate system first

Geographic Coordinate System, or Geographic Coordinate System, or GCS for short, is a location reference System generated by spatial elements on the surface of the Earth, which is very official. So let’s move on

PS: Because the map is too difficult to draw, all the pictures in this article are from the Internet, if there is any infringement, please inform us and delete it immediately

Cognitive earth

Historical evolution

If we want to know the location of a place or object, we definitely need a reference. However, for modern people living on the Earth, we can only use the Earth as a reference for precise global positioning, because we clearly understand the shape and size of the Earth and the relationship between human beings and the earth

But people in ancient times, because of their small range of activities, only saw a small area of their own region, so the simple intuition of all kinds of round heaven and earth

For example, in the Zhou Dynasty, more than two thousand years ago, there was a theory that the sky was as round as Zhang Gai and the place was like a chess game

The ancient Egyptians knew that the sky was like a vault and the earth like a box

To the Russian, the earth is like a shield, carried on the backs of three great whales, floating in the vast ocean

The Indians have similar tales of Russia, but they believe that the land is carried not by whales, but by three elephants on the backs of turtles, whose movements cause earthquakes

Aristotle, the ancient Greek scientist, proposed that the earth was spherical, and it was not until Magellan’s successful circumnavigation of the earth in 1519-1521 that the name earth became official

With the continuous progress of measurement technology, especially the use of man-made earth satellites, we have a clear understanding of the Earth. The Earth is not a regular sphere, but an irregular sphere with slightly flat poles and slightly bulged equator, which can be called an ellipsoid. The average radius of the Earth is 6371 km, and the polar radius is 6356 km. The difference is 21 kilometers, the maximum circumference of what we call the equator is about 40,000 kilometers, and the surface area is about 510 million square kilometers, as shown below

Longitude and latitude & radian

The geographic coordinate system is defined by latitude and longitude, so for those of you who don’t know longitude and latitude, so I’ll give you the latitude and longitude as well

Longitude measures angles east or west from the prime meridian, positive in the Eastern hemisphere and negative in the western hemisphere, while latitude measures angles north or south from the equator, positive north of the equator and negative south of the equator

Oh, by the way, prime meridian refers to the meridian that connects the north and south poles on the Earth. It has many names, such as zero meridian, prime meridian, zero meridian, etc. The equator is the longest circumference of the earth’s surface along with the earth’s rotation. It divides the north and south hemispheres

The prime meridian and the equator are the baselines of the geographic coordinate system. They are tangent to each other and can be understood as the X/Y coordinate axis. Then we refer to longitude as the X value and latitude as the Y value, as shown in the figure below

The units of latitude and longitude can be expressed in terms of minutes (DMS), decimal degrees (DD), or radians (rad), as we might say in calculus

In degree-minute-and-second notation, 1 degree is equal to 60 minutes and 1 minute is equal to 60 seconds (1° = 60 ‘1’ = 60 “). For example, latitude and longitude 45°52 ’30 “, which we convert into decimal notation is 45.875° (45+52/60+30/3600). Besides, radians commonly used by us, Let’s just remember the two values. Get to the point

1rad	= 57.2958°
1°  = 0.01745rad
Copy the code

Geoid

The earth’s surface is rugged with mountains and depressions, so when we want to use mathematical laws to describe it, we have to find a relatively regular mathematical surface

The earth, as we all know, may also be called a water sphere, since most of it is an ocean, and since the waters of the oceans are assumed to be in perfect equilibrium, extending from sea level to the underside of all continents, a continuous, closed surface orthogonal to the direction of the earth’s gravity is the geoid, as shown below

Earth ellipsoid & ellipsoid

Above, we say that the geoid ignores the rule of convex and concave unevenness on the ground. However, due to the uneven distribution of materials in the earth, the geoid is still undulating. Although it is very close to a regular ellipsoid, it is not completely regular and cannot be expressed mathematically

We know with elliptical orbit around the short axis rotation can generate a spheroid, so in order to quantitatively describe the shape of the earth and not affected by the ups and downs, measured on the geoid and conform to the ideal rotation ellipsoid is called the earth ellipsoid is also called the reference ellipsoid, and select the ellipsoid can use mathematical equations of decent as datum plane of projection, This datum is called the reference ellipsoid, or ellipsoid for short, as shown in the figure below

Geodatum

We say above the geoid, the ellipsoid earth ellipsoid, such as concept, then we come to the datum, in short, we compare the earth to a potato 🥔, because it is bumpy surface, so we speak of above reference ellipsoid can use duck eggs 🥚 said, because it is more rules, We choose a azimuth Angle of the duck eggs on potato, rotation or zoom in duck eggs to make it as far as possible to fit the bearing of potato noodles, after adjusting the azimuth Angle of duck egg noodles is the datum (potato said of duck eggs from came across an article on the net, feel very apt, can’t find the address..)

Because we are free to the rotation of the potato to transform azimuth, duck eggs (reference ellipsoid) will change potato (earth) with the Angle of orientation, datum will have some produce multiple azimuth measurement, as the earth’s different countries or regions will choose their relative position measurement of joint to produce their own datum, This leads to the problem of conversion between different coordinate systems, which we are now very worried about, because of their different datum, as shown in the following figure

The Beijing 54 coordinate system and Xi ‘an 80 coordinate system, which we often hear, actually refer to the two geodetic datum planes in China

The position of potato (earth) in each country is different, and the duck egg (reference ellipsoid) produced by rotation amplification or reduction is also different, so many reference ellipsoids are produced. China established the Beijing 54 coordinate system by referring to the Krassovsky ellipsoid adopted by the former Soviet Union since 1953. In 1978, a new geodetic coordinate system, Xi ‘an 80, was established by using the 1975 Earth ellipsoid (IAG75) recommended by the International Geodetic Association

Parameter-centered coordinate system

Ball measurement must be determined, its the duck eggs we have said above is the reference ellipsoid, the earth’s core, not sure, but the reference ellipsoid is rules, then to the geometric center of the reference ellipsoid for the origin of geodetic coordinate system is the heart coordinate system, and the involved heart means the reference ellipsoid of our plan for its origin O

The parameter-centered coordinate system usually refers to the parameter-centered space cartesian coordinate system (with x, y, z as the coordinate elements) and parameter-centered geodetic coordinate system (with B, L, H as the coordinate elements).

The reference center coordinate system is an O-XYZ coordinate system established within the reference ellipsoid. The origin O is the geometric center of the reference ellipsoid. The X-axis coincides with the intersection of the equatorial plane and the prime meridional plane, and is positive to the east. The z-axis overlaps with the short axis of the ellipsoid of rotation and is positive to the north. The Y-axis is perpendicular to the XZ plane to form the right hand system, as shown in the figure below

In the measurement, in order to deal with observation results and calculate the coordinates of the surface control network, usually must select a reference ellipsoid as the basic reference, pick a reference point as geodetic survey to count points (this point is the origin of the earth), amount of astronomical observations using geodetic origin determines the location of the reference ellipsoid in the interior of the earth and direction

Geocentric coordinate system

Before the 1950s, a country or a region established its local geodetic coordinate system according to radian measurement method under the condition that the selected reference ellipsoid best fit the geoid of the region

At that time, the geodetic work could only be carried out on each continent except for the sparse gravity survey on the ocean, and there was almost no connection between the local geodetic coordinate system in each continent. However, at the level of scientific development at that time, the local geodetic coordinate system could basically meet the requirements of geodetic and cartographic work in all countries

But later in order to study the shape of the earth as a whole and its external gravity field and geodynamic phenomenon, especially in the late ’50 s, the artificial earth satellite and long-range ballistic weapons, in order to describe them in space position and movement, and said its ground station and the location of the ground, must use geocentric coordinate system, therefore, The establishment of the global geocentric coordinate system (also known as the world coordinate system) has become an urgent task faced by geodesy, and later the establishment of the earth’s center of mass (that is, the sole center of the earth in the unity of the world)

Under this background, the space cartesian coordinate system based on the earth centroid or the geodetic coordinate system based on the earth ellipsoid coincides with the earth centroid is born

The geodetic coordinate system with the earth centroid as its origin is usually divided into geocentric space rectangular coordinate system (with X, Y, z as its coordinate elements) and geocentric geodetic coordinate system (with B, L, H as its coordinate elements).

Geocentric coordinate system is set up in the earth body O – XYZ coordinate system, the origin is located in the center of mass of earth body, O with perpendicular to the X, Y, Z three axis, the X axis and meridian plane and below the equator line, in the east is positive, the Z axis is coincide with earth’s axis of rotation, to the north is positive, the Y axis and XZ plane perpendicular to the right hand, and the heart figure, I’m not going to show you the picture

Now that you have a new understanding of our earth, let’s introduce some commonly used coordinate systems

Common coordinate system

Here we briefly introduce four common coordinate systems, which are Beijing 54 and Xi ‘an 80, the two coordinate systems used in early China, CGCS2000, the mainstream coordinate system in China, and WGS84, the geographical coordinate system commonly used in the world

Beijing 54

After the founding of the People’s Republic of China, China adopted the Krasovsky ellipsoid parameter of the former Soviet Union (that is to say, it is a parameter-centered coordinate system), carried out joint measurement with the 1942 coordinate system of the former Soviet Union, and established China’s geodetic coordinate system through calculation, which was named Beijing coordinate system in 1954, or Beijing 54 coordinate system for short

The 1954 Beijing coordinate system can be considered as an extension of the 1942 Soviet coordinate system, because its origin is not in Beijing but in Pulkovo, the Former Soviet Union

At the beginning of the founding of the People’s Republic of China, when the technical conditions were not high at that time, the geographic coordinate system of the Soviet Union was introduced to China. The geodetic origin was not in China, so Beijing 54 had a great deviation in China and was gradually abandoned

Xi ‘an 80

In April 1978, the national Astronomical and geodetic Network adjustment conference was held in Xi ‘an, which determined the repositioning and established a new coordinate system in China. For this purpose, the National geodetic coordinate system was established in 1980. The 1980 National geodetic coordinate system adopts the earth ellipsoid basic parameters as the data recommended by the sixteenth Congress of the International Union of Geodesy and Geophysics in 1975 (also the reference center coordinate system).

The geodetic origin of this coordinate system is located in Yongle Town, Jingyang County, Shaanxi Province, central China, about 60 kilometers northwest of Xi ‘an city, so it is called xi ‘an coordinate system 1980, or xi ‘an 80 coordinate system for short

Its datum is the yellow Sea average sea level (1985 national elevation datum) determined by Qingdao Dagang Tide Survey Station from 1952 to 1979, which is gradually abandoned now

Xi ‘an 80 and Beijing 54

Xi ‘an 80 coordinates with Beijing 54 coordinate system is actually a kind of ellipsoid parameter transformation as the conversion in the same ellipsoid transformation are rigorous, and conversion between different ellipsoid is not tight, so there is no a set of transformation parameters can be generic across the country, will be different in each place, and because they are two different ellipsoid benchmark

Beijing 54 and Xi ‘an 80 are two different geodetic datum and reference ellipsoid. Therefore, under the two maps, the coordinates of the same point are different, whether it is the coordinates of the third degree belt or the coordinates of the sixth degree belt or the coordinates of the latitude and longitude. The same thing is that both of them have been gradually abandoned

CGCS2000

From Beijing 54 to Xi ‘an 80, with the change of the situation and the passage of time, the above two local geodetic coordinate systems based on classical measurement technology have been unable to adapt to the development of science and technology, especially space technology, and to meet the needs of China’s economic construction and national defense construction. It is the objective need of economic construction, national defense construction, social development and scientific and technological development

The geocentric geodetic coordinate system with the center of mass as the origin is the universal basic geodetic coordinate system in the space age of the 21st century, so the geocentric geodetic coordinate system based on space technology is a suitable choice for the new generation of Geodetic coordinate system in China

CGCS2000 is the 2000 National geodetic coordinate system, which belongs to the geocentric geodetic coordinate system. This coordinate system is established by the Continuous operation reference station of China GPS, space geodetic control network and the joint adjustment of astronomical geodetic network and space geodetic network. National Geodetic Coordinate System 2000 (China) is based on ITRF 97 reference frame with epoch of 2000.0, full name is China Geodetic Coordinate System 2000, its English name is China Geodetic Coordinate System 2000

The sub-coordinate system is the mainstream coordinate system in our country at present. You may not know this coordinate system, so to speak, China’s GPS system – Beidou navigation system and the map of heaven and Earth issued by the state, use this set of geographic coordinate system, do you know

WGS84

WGS84 is a global geographic coordinate system used by GPS in the United States. OSM Maps, Google Maps (foreign version), Landsat series of satellite images are all used in THE GEOGRAPHIC coordinate system. Many apis for developing maps use WGS84 by default.

Its full name is World Geodetic System 1984, and most of the data we communicate on the Internet is WGS84, so it’s probably the most widely used coordinate System in the World right now

China’s gold, baidu also is WGS84 used the map, but they are all based on WGS84 encryption, again in folk called Mars coordinates, USES is the well-known GCJ02 encryption algorithm, it has to do with the real WGS84 coordinates is error, the error value is not fixed, usually a few meters to a few hundred meters (baidu map more malicious, On the basis of GCJ02, we also made another offset, namely BD09, we usually call baidu coordinate system), as for why encryption, need not I say, you understand

CGCS2000 and WGS84

As mentioned above, both CGCS2000 and WGS84 coordinate systems are geocentric coordinate systems, so the definition of CGCS2000 is essentially the same as that of WGS84, and the reference ellipsoid adopted is very close

But due to the ellipsoid surface flattening differences cause maximum of 0.1 mm, latitude and altitude change within the scope of the current measurement accuracy can be ignore this difference, say both compatible to the cm level, but if the coordinates of a point accuracy can not reach the cm level, just don’t think CGCS2000 WGS84 coordinates is compatible, and can be both

Summary of common coordinate systems

So just to summarize these common coordinate systems, because they’re pretty official, we’re going to keep it down to earth

Beijing 54 reference center coordinate system was introduced from the Soviet Union’s geographic coordinate system to our country when the technical conditions were not good at the beginning of the founding of the People’s Republic of China. The geodetic origin is not in our country, so there is a large deviation in our country and it has been gradually abandoned

After the reform and opening up, xi ‘an 80 reference center coordinate system is a kind of geographical coordinate system suitable for China in order to solve the problem of Beijing 54 deviation. It has been gradually abandoned now

CGCS2000, which is used in the Beidou Navigation System and Map heaven and Earth, is the mainstream geocentric coordinate system in China

WGS84 is a global geographic coordinate system (GEocentric coordinate system) originated from the United States and widely circulated in the world

This is a good picture

First explain, ArcGIS WKID, we should all know ArcGIS, American EnvironmentalSystems ResearchInstitute ERSl (EnvironmentalSystems ResearchInstitute) do, they can be said to be the first GIS industry

WKID refers to the well-known ID of space reference. To put it simply, there are too many coordinate systems in the world. We compare each coordinate system to a person. And can be renamed what, but the ID number is unique and unchanged, this ID number is referring to the WKID, it in the use of spatial data, conversion, sharing, etc., play a key role, so remember these several commonly used ID has advantages

To check out WKID, go to 👉 JavaScript

The last

Due to space problems, the projection coordinate system is discussed below

Thank you for reading, welcome to pay attention to the public account “not serious front end”, remember to like oh!!

The resources

Introduction to Geographic Information Systems – 9th edition

Talk about GIS in the coordinate system | reprint