[Mori city] random talk on GIS data (IV) - coordinate system

Coordinate system

The surface of the earth is not a standard positive sphere , according to 2020 Measurement results in , The elevation of Mount Everest is 8848.86m, And the deepest trench on earth —— The depth of the Mariana Trench is 11034 m. The difference between the two is nearly 20 km！ Because the natural surface of the earth is uneven , The shape is complex , Obviously, it cannot be used as the reference plane for measurement . So people began to seek a regular surface close to the natural surface of the earth to replace the irregular surface of the earth . The sea area accounts for about 71%, The land area only accounts for 29%. So use the level to represent the physical surface of the earth , Simply put, suppose there is a static sea surface （ A wave free 、 No tide 、 No water 、 No atmospheric pressure change , Static sea level in fluid equilibrium ）, A closed surface extending to the land to represent the physical surface of the earth .

In reality, the sea surface will change constantly under the influence of tides , So people set up tide stations on the seashore , Through long-term observation , Find the average height of the sea surface . This level passing through the average height is called Geoid , also called Elevation datum . The elevation datum currently stipulated in China is 1985 Elevation datum  .

Reference resources ： Basic technical regulations of national geodesy GB 22021-2008

The geoid ignores the unevenness of the ground , But the mass distribution inside the earth is uneven , The geoid is still undulating . Although very close to a regular ellipsoid , But it's not a complete rule , There is no way to define and express with mathematical models .

So people further assume that the surface of a rotating ellipsoid is used to approach the geoid , Also known as the earth ellipsoid . A rotating ellipsoid is a body formed by the rotation of an ellipse around its minor axis , Its long half axis a（ Equatorial radius ） Larger than the short halfshaft b（ Polar radius ）. Oblateness f=(a-b)/a.

Different historical periods , Different countries and regions , According to the corresponding data, ellipsoids of different sizes are calculated to approximate the representative geoid .

The earth ellipsoid used in different periods in China is listed below ：

With an ellipsoid , The coordinate system can be established on the basis of ellipsoid to express the position of a point on the ground . But there is another problem at this time ： For different areas , Use an ellipsoid to “ Fit ” There must be some places when “ Fit ” Good. , In some places “ Fit ” Not good. , Therefore, a geodetic datum is also needed to determine the ellipsoid “ Location ” and “ Posture ”.

There are two types of geodetic datum bread ：

Geocentric datums ： From satellite data , Using the center of mass of the earth as the origin , Best fit globally .

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 .

The difference between the reference center coordinate system and the geocentric coordinate system ：

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 , It's regional （ Local scope ） The coordinate system of .

The geocentric coordinate system ： After positioning and orientation , The center of the earth's ellipsoid coincides with the earth's center of mass , Is a global coordinate system .

From the natural surface of the earth ——> Geoid ——> Ellipsoid of the earth ——> The process of geodetic datum can be seen as the “ Step by step approximation ”. With the development of Surveying and Mapping Science and Technology , The geodetic coordinate systems used in different periods in China are listed below ：

at present , China has fully implemented the use of 2000 National geodetic coordinate system 【China Geodetic Coordinate System 2000】（ namely CGCS2000 Coordinate system ）.

So if the longitude and latitude coordinates of a place are [116.56,39.97], It doesn't mean that the position he described can be accurately marked on the map , Because the data collected from the same place in different coordinate systems are different . therefore , To accurately mark the location of a place on the map , In addition to knowing the coordinate value , You also need to know what coordinate system this coordinate value is collected and produced in .

stay GIS Fields generally divide the coordinate system into Geographic coordinate system （Geographic Coordinate Systems） and Projection coordinate system （Projected Coordinate Systems）. If you also want to define the elevation of a point （ It can be simply understood as altitude ） Strictly speaking, it still needs Elevation coordinate system , Sometimes called Vertical coordinate system （Vertical Coordinate Systems）.

Tool recommendation

Mori City ： One click generation 3D City , Built in multiple templates , Free editing

• Mori city currently provides nationwide 110 Multiple cities Standard 3D scene construction service , Make the construction speed of urban 3D scene increase to minute level .

• Sen city supports the insertion and automatic fusion of different types of city data , And provide friendly and powerful scene editing ability and performance processing ability , Not required GIS、 Modeling and other professional skills , It's easy DIY Own cool 3D city scene .

• Mori city has open City scene output capability , There is no need to worry about the compatibility of subsequent use of the scene .

Low code ThingJS： Low code and flexible development of digital twin visualization applications

• Low code ThingJS Provides rich 3D Development API、 Perfect development documents and video tutorials , Familiar with basic JavaScript Front end knowledge can be used. Digital twins 3D Visual application development .

• Low code ThingJS Is pure H5 Of WebGL framework , Can be found in Run freely in all kinds of mainstream web browsers , Including direct access and operation at the mobile terminal .

• Low code ThingJS Provide full lifecycle development components , Greatly improve the development efficiency of digital twin visualization project .