1. Geographic coordinate system
Geographic coordinate system is literally translated as a geographic coordinate system, which uses latitude and longitude as the storage unit of the map. Obviously, the Geographic coordinate system is a spherical coordinate system. We want to store the digital information on the earth on the spherical coordinate system, how to operate it? The earth is an irregular ellipsoid. How to store data and information on the ellipsoid in a scientific way? This necessarily requires us to find such an ellipsoid. Such an ellipsoid has the characteristics that it can be calculated quantitatively. With long semi-axis, short semi-axis, eccentricity. The following lines are the Krasovsky_1940 ellipsoid and its corresponding parameters.
Spheroid: Krasovsky_1940
Semimajor Axis: 6378245.000000000000000000
Semiminor Axis: 6356863.018773047300000000
Inverse Flattening: 298.3000000000000010000
However, with this ellipsoid, it is not enough, and a large ellipsoid is needed. In the coordinate system description, you can see that there is such a line:
Datum: D_Beijing_1954
indicates that the geodetic datum is D_Beijing_1954.
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With the two basic conditions of Spheroid and Datum, the geographic coordinate system can be used.
Full parameters:
Alias:
Abbreviation:
Remarks:
Unit the Angular: Degree (0.017453292519943299)
Prime the Meridian (starting longitude): Greenwich (0.000000000000000000)
Datum (geodetic datum): D_Beijing_1954
Spheroid (reference ellipsoid): Krasovsky_1940
Semimajor the Axis: 6378245.000000000000000000
Semiminor the Axis: 6356863.018773047300000000
Inverse Flattening: 298.300000000000010000
2. Projection coordinate system (projection coordinate system) , first look at some parameters in the projection coordinate system.
Projection: Gauss_Kruger
the Parameters:
False_Easting: 500,000.000000
False_Northing: 0.000000
Central_Meridian: 117.000000
scale_factor: 1.000000
Latitude_Of_Origin: 0.000000
Linear Unit: Meter (1.000000)
Geographic Coordinate System:
the Name: GCS_Beijing_1954
Alias:
both the abbreviation:
Remarks:
Angular Unit: Degree (.017453292519943299)
Prime Meridian: Greenwich (0.000000000000000000)
Datum: D_Beijing_1954
Spheroid: Krasovsky_1940
Semimajor Axis: 6378245.0000000000000000000
Semiminor Axis: 6356863.018773047300000000
Inverse Flattening: 298.300000000000010000
As can be seen from the parameters, each projected coordinate system must have a Geographic Coordinate System. The projected coordinate system is essentially a plane coordinate system, and its map unit is usually meters. So why do the parameters of the coordinate system exist in the projected coordinate system?
At this time, the meaning of projection should be explained again: the process of converting spherical coordinates into plane coordinates is called projection. Well, the conditions for the projection come out:
a, spherical coordinates
b, the transformation process (that is, the algorithm)
That is to say, to get the projected coordinates, you must have a spherical coordinate "bringing" the projection, and then you can use the algorithm to projection! That is, each projected coordinate system must require the Geographic Coordinate System parameter.
3. Beijing 54 and Xi'an 80 are the coordinate systems we use the most. The
large and medium scale maps in China all use Gauss-Krüger projection, which is usually divided into 6 degrees and 3 degrees, 1:25,000-1:500,000 The scale topographic map is divided by 6 degrees of longitude difference, and the topographic map of 1:10,000 scale is divided by 3 degrees of longitude difference. The specific zoning method is: the 6-degree zoning starts from the prime meridian, and is divided from west to east according to the longitude difference of 6 degrees as a projection zone. There are 60 projection zones in the world, with zone numbers 1-60; 3-degree projection zone It starts from the east longitude of 1 degree and 30 seconds longitude, and divides it from west to east according to the longitude difference of 3 degrees as a projection zone. There are 120 projection zones in the world. In order to facilitate the measurement of topographic maps, a plane Cartesian coordinate system is arranged in the Gauss-Krüger projection zone. The specific method is to define the central meridian as the X-axis, the equator as the Y-axis, and the intersection of the central meridian and the equator as the coordinate origin, x Values are positive in the northern hemisphere and negative in the southern hemisphere, and y values are positive east of the central meridian and negative to the west of the central meridian. Since the territory of our country is in the northern hemisphere, the x values are all positive. In order to avoid negative y values, it is stipulated that the vertical axis of the coordinates of each projection zone is moved westward by 500km, and the original abscissa value on the central meridian changes from 0 to 500km. In order to facilitate the distinction of points between bands, the band number can be added before the 100-kilometer digit of the abscissa y value of each point. For example, the coordinates of point A in the 20 band can be expressed as YA=20 745 921.8m.
In the Coordinate Systems/Projected Coordinate Systems/Gauss Kruger/Beijing 1954 catalog we can see four different nomenclatures:
Beijing 1954 3 Degree GK CM 75E.prj
Beijing 1954 3 Degree GK Zone 25.prj
Beijing 1954 GK Zone 13.prj
Beijing 1954 GK Zone 13N.prj
Their descriptions are as follows:
The Beijing 54 coordinate system of the three-degree zoning method, the zoning coordinates of the central meridian at 75 degrees east, the Beijing 54 coordinate system of the three-degree zoning method without the number before the abscissa, the zoning
coordinates of the central meridian at 75 degrees east ,
the Beijing 54 coordinate system of the six-degree banding method before the abscissa, the band number is 13, and
the Beijing 54 coordinate system of the six-degree banding method with the band number before the abscissa, the band number is 13, and the abscissa without number
In the Coordinate Systems/Projected Coordinate Systems/Gauss Kruger/Xian 1980 directory, the file naming has changed:
Xian 1980 3 Degree GK CM 75E.prj The zone coordinates of the central meridian at 75 degrees east, without the band number before the abscissa
Xian 1980 3 Degree GK Zone 25.prj The zone number is 25, without the band number before the abscissa
Xian 1980 GK CM 75E.prj The zone coordinates of the central meridian at 75 degrees east, with a zone number before the abscissa
Xian 1980 GK Zone 13.prj The zone number is 13, and no zone number is added before the abscissa
The naming method and meaning of the Xi'an 80 coordinate file are the same as the first two coordinates of Beijing 54. If there is a letter "N" after the number or it is indicated by the central meridian, the number will not be added before the coordinate. The other three degrees, six degrees
The distinction of the belt is more obvious, and the 3-degree belt will be marked.
Convert Xi'an 80 coordinate system to Beijing 54 coordinate system in ArcGIS
1. Data Description
The source data of this projecting transformation coordinate adopts the geographic coordinate system of Xi'an in 1980 and the DLG data of 1 : 50000 of the national elevation datum in 1985 .
Second, the basic knowledge of projection transformation preparation
The conversion between the Beijing 54 coordinate system and the Xi'an 80 coordinate system is actually the conversion between two different ellipsoid parameters.
Two sets of coordinate systems are defined in ArcGIS : Geographic coordinate system and Projected coordinate system .
1. The geographic coordinate system is a spherical coordinate system with latitude and longitude as the storage unit of the map. The earth is an irregular ellipsoid. In order to put the data information on the ellipsoid in a scientific way, it is necessary to have an ellipsoid that can be calculated quantitatively. With long semi-axis, short semi-axis, eccentricity. The next few lines are the GCS_Xian_1980 ellipsoid and its corresponding parameters.
Geographic Coordinate System: GCS_Xian_1980
Datum: D_Xian_1980
Prime Meridian: Greenwich
Angular Unit: Degree
Each ellipsoid needs a geodetic datum to locate the ellipsoid, so you can see that there is a description of Datum: D_Xian_1980 in the coordinate system , indicating that the geodetic datum is D_Xian_1980 .
2. With the two basic conditions of ellipsoid and datum, the geographic coordinate system can define the projected coordinate system. The following are the parameters of the projected coordinate system with Beijing_1954 coordinates defined:
Projected Coordinate System: Beijing_1954_GK_Zone_19
Projection: Gauss_Kruger
False_Easting: 19500000.00000000
False_Northing: 0.00000000
Central_Meridian: 111.00000000
Scale_Factor: 1.00000000
Latitude_Of_Origin: 0.00000000
Linear Unit: Meter
Geographic Coordinate System: GCS_Beijing_1954
Date: D_Beijing_1954
Prime Meridian: Greenwich
Angular Unit: Degree
A projected coordinate system, essentially a planar coordinate system, whose map units are meters. The process of converting spherical coordinates into plane coordinates is called projection, that is, the conditions for projection are that one has spherical coordinates, and the other is a conversion algorithm. Therefore, as can be seen from the parameters, each projected coordinate system must have a Geographic Coordinate System .
3. The problem of coordinate offset
( 1 ) Origin of offset
Different countries use different reference ellipsoids and positioning methods, so the geodetic coordinate values of the same ground point in different coordinate systems are also different. The origin of the Beijing 1954 coordinate system is Pulkovo in the western part of the former Soviet Union, and the Krasovsky ellipsoid is used; the Xi'an 1980 coordinate system is the reference ellipsoid recommended by the International Geodetic Association in 1975 , and its coordinate origin is set in China It is located in Jingyang County near Xi'an City in the central part.
Therefore, under normal circumstances, there will be certain errors in the directly converted data, so in order to ensure the accuracy of the data, the converted coordinate value is corrected by setting the offset of the abscissa and the ordinate during the conversion process.
Converted from the Xi'an 1980 coordinate system to the Beijing 1954 coordinate system, their offsets are the offset of the Beijing 1954 coordinate system relative to the WGS84 ellipsoid minus the offset of the Xi'an 1980 coordinate system relative to WGS84 .
( 2 ) Calculation method of offset
Select a national known point ( X1 , Y1 ) near the survey area , use GPS to measure the longitude and latitude of WGS84 coordinates on the known point, regard this coordinate as the wrong Xi'an 80 coordinate system, and convert it to Xi'an 80 coordinate system The plane rectangular coordinates X , Y , and then compared with the known coordinates to calculate the offset.
That is, △ X1=X- X1
△ Y1= Y- Y1
In the same way, the offset of Beijing 54 coordinate system relative to WGS84 coordinate can be obtained △ X2 , △ Y2 , so the offset from Xi’an 80 coordinate to Beijing 54 coordinate is: △ X= △ X2- △ X1 , △ Y= △ Y2- △ Y1
3. Operation steps for converting " Xi'an 80 coordinate system " to " Beijing 54 coordinate system "
1. Start ArcMAP , load the coverage data layer, load the arctoolbox toolbox, select Data Management Tools—>projections and transformations—>feature—>project , open the project dialog box, a . Select the desired transformation in Input Dataset or Feature Class b . Select the output path and output file name in Output Dataset or Feature Class , c . Enter the geographic coordinate type Xian 1980.prj that needs to be defined in Output Coordinate System .
The geographic coordinate system is defined in Geographic Coordinate Systems , and the projected coordinate system is selected in Projected Coordinate Systems .
2. After defining the geographic coordinates of Xi'an 1980 , the above coverage data is converted into a file in .shp format. Open the project dialog box as above, select this file in .shp format for projection, and select Xian 1980 GK Zone as the projection type. 19.prj .
In order to distinguish points between different zones, add the zone number before the abscissa of each point, such as Xian 1980 GK Zone 19.prj , which is the Xi'an 80 coordinate system representing the six-degree zone method. The number is 19 , and a band number is added before the abscissa.
3. In order to correctly convert the data into the Beijing 54 coordinate system, it is necessary to perform translation correction on the data that defines the Xi'an 80 coordinate system.
Load the result data of the second step, load the Editor toolbar, open Editor—>start editing , and make the data in the editing state. Load the Spatial Adjustment toolbar, select the new displacement link icon, select four Link points on the image, then open the Link Tabel table to correct the coordinates, the correction formula is X.destination = X.source + 66 , Y.destination = Y.source + 53 , 66 and 53 are the calculated coordinate offsets. If the calculation is correct, you can see that the residual error has all become zero after the calculation. Close the Link Tabel table.
4. The data that has been translated and corrected can be directly converted into Beijing 54 coordinates. Also , select Data Management Tools—>projections and transformations—>Define Projection in the arctoolbox toolbox . When the amount of data is relatively large, batches can be used. Processing operations will be much faster . Select Data Management—>Projections—>Batch Define Coordinate System in the Samples tool .
Reprinted in: https://www.cnblogs.com/nilesry/p/3153059.html