Random talk on GIS data (V) - geographic coordinate system

Geographic coordinate system （GCS）

Geographic coordinate system By angular measurement units （Unit）、 Prime meridian （PRIMEM） And datum （DATUM） To define . The coordinate value under the geographical coordinate system is Longitude and latitude , So sometimes it is also called Latitude and longitude coordinate system .CGCS2000,2000 National geodetic coordinate system . We often use it directly WGS84 Instead of CGCS2000 coordinate . because CGCS2000 Definition and WGS84 The essence is the same , The reference ellipsoid used is very close . The difference of oblateness causes the latitude and height on the ellipsoid to change up to 0.1mm. Within the current measurement accuracy , This difference can be ignored .

Now let's do two small experiments , Let's see CGCS2000 Longitude and latitude coordinates under the geographical coordinate system . Experiment 1 CGCS2000 Longitude and latitude obtained in the coordinate system WGS84 Is there a big difference between longitude and latitude obtained in the coordinate system ？ Suppose that Google Map The coordinates picked on the satellite base map are WGS-84 Of . Because the coordinate system of the sky map is CGCS2000 Of , Pick up the longitude and latitude of a point on the sky map to observe . Compare ：

hold CGCS2000 Longitude and latitude of 39.98180450091385 ,116.4546536945034 stay Google Map On “ Mark it ”, The comparison is as follows ：

The gap is not great , So in general applications （ The requirement of positioning accuracy is not high , Difference 1m It doesn't matter ） Negligible CGCS2000 and WGS-84 The difference between coordinate systems . But the map in Gaud is different , Because Gaode map uses GCJ-02 Coordinate system （ Tencent map is the same ）, Also known as the Mars coordinate system , It is an encryption algorithm for longitude and latitude data , That is, add random deviation . Experiment two From sky map （ National Geographic information system resource directory ） Download several areas 1:25w Vector data to observe . The layers downloaded from the original data of each sub frame are Shapefile file . Have a look first Shapefile In the document .prj file （.prj The file defines the coordinate system of the data ）.

Select the water surface data of four different places for comparison , First, superimpose these four surface data on the satellite base map of the sky map . It's used here QGIS, adopt XYZ Tiles Layer loaded sky map satellite base map , And then shp File overlay loading .

The third map is slightly offset , The offset of coordinate position may be the cause of the original data itself . For example, due to the particularity of the region , The published vector data itself is offset on the basis of the original surveying and mapping data , Reduce precision . Sometimes , Third party tile data at different levels （ scale ） I didn't handle it as “ Agreement ”.

summary ：

1. Two common geographical coordinate systems

China has fully implemented the use of 2000 National geodetic coordinate system . In the application of low positioning accuracy （ Difference between 1m It doesn't matter ）, Can think CGCS2000 And WGS-84 There is no difference between .

2. Internet maps , The coordinate systems used by them have been offset according to national laws and regulations

3. The longitude and latitude coordinates are superimposed on the base map and offset （1） Generally, it is caused by the mismatch between the coordinate system of longitude and latitude and the coordinate system of the map base . This situation , It can be processed by longitude and latitude coordinate conversion .（2） If there is an offset in the same coordinate system , It may be the problem of raw data .

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