Talking about GIS Data (5) - Geographic Coordinate System

Geographic Coordinate System (GCS)

Geographic coordinate systems are defined by units of angular measurement (Unit), prime meridian (PRIMEM), and datum (DATUM).The coordinate value in the geographic coordinate system is latitude and longitude, so it is sometimes called latitude and longitude coordinate system.CGCS2000, 2000 National Geodetic Coordinate System.We often use WGS84 coordinates directly instead of CGCS2000 coordinates.Because the definition of CGCS2000 is essentially the same as that of WGS84, the reference ellipsoid used is very close.The difference in flattening causes the latitude and height on the ellipsoid to vary by up to 0.1 mm.Within the current measurement accuracy range, this difference can be ignored.

Let's do two small experiments to observe the latitude and longitude coordinates in the CGCS2000 geographic coordinate system.Experiment 1 Is there a big difference between the longitude and latitude obtained in the CGCS2000 coordinate system and the longitude and latitude obtained in the WGS84 coordinate system?It is assumed that the coordinates picked up on Google Map with the satellite basemap as a reference are WGS-84.Since the coordinate system of the sky map is CGCS2000, pick the latitude and longitude of a point on the sky map to observe.Check it out:

"mark" the latitude and longitude of CGCS2000 39.98180450091385 , 116.4546536945034 on Google Map, and compare the two as follows:

The difference is not very large, so in general applications (the positioning accuracy is not high, it doesn't matter if the difference is about 1m) The difference between the CGCS2000 and WGS-84 coordinate systems can be ignored.However, it is different in AutoNavi, because AutoNavi uses the GCJ-02 coordinate system (similar to Tencent Maps), also known as the Mars coordinate system, which is an encryption algorithm for longitude and latitude data.That is, adding random bias.Experiment 2 Download the 1:25w vector data of several regions from Tiantu (National Geographic Information Resource Directory Service System) for observation.Each layer downloaded from the raw data of each tile is a Shapefile file.First look at the .prj file in the Shapefile (the coordinate system for this data is defined in the .prj file).

Select the water system surface data of four different places for comparison, and first superimpose the four surface data on the satellite base map of the sky map.QGIS is used here, the sky map satellite basemap loaded through the XYZ Tiles layer, and then the shp file is superimposed and loaded.

The third map is somewhat offset, and the offset of the coordinate position may be the cause of the original data itself.For example, due to the particularity of the region, the open vector data itself is offset on the basis of the original mapping data, reducing the accuracy.Sometimes, the third-party tile data itself is not processed as "consistent" at different levels (scales).

Summary:

1. Two common geographic coordinate systems

my country has fully implemented the use of the 2000 National Geodetic Coordinate System.In applications where the positioning accuracy is not high (the difference of 1m does not matter), it can be considered that there is no difference between CGCS2000 and WGS-84.

2. The coordinate systems used by each Internet map have been offset according to national laws and regulations

3. The longitude and latitude coordinates are superimposed on the map basemap and offset (1) Generally, it is caused by the mismatch between the longitude and latitude coordinate system and the coordinate system of the map basemap.In this case, it can be handled by converting the latitude and longitude coordinates.(2) If there is still an offset in the same coordinate system, it may be a problem with the original data.

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