The coordinate system definition in GIS is published in the "GIS system development on" MapInfo "on the computer world online, there are several netizens to ask me questions about the coordinate system definition and conversion, the problem can be classified into (1) map MapInfo is shown very well, but it does not appear or displayed in Mapx; (2) How does the WGS84 coordinates obtained from GPS to go to Beijing 54 coordinate map. These problems have also been confused about my problem. I talked about some of my personal understanding and experience, for all reference, and I hope that the relevant professionals will give corrective and supplement.
1. The coordinate system definition in the ellipsoid, reference surface and map projection GIS is the basis of the GIS system, and the coordinate system of the GIS system is correct. The coordinate system in GIS is defined by the reference surface and the map projection two sets of parameters, and the definition of the reference plane is determined by a particular ellipsoid and its corresponding conversion parameters, so wants to correctly define the GIS system coordinate system, first must clarify the Earth Ellipse. Ellipsoid, Earth Bench (DATUM) and map projection The basic concept of the three people and the relationship between them. The reference surface is approximated using a particular ellipsoid on the surface of the Earth's surface, so each country or region has its own reference face. We usually call the Beijing 54 coordinate system. Xi'an 80 coordinate system actually refers to my country's two Earth baseline. my country's referred to the former Soviet Union has established my country's Beijing 54 coordinate system from Krassovsky ellipsoid since 1953. In 1978, China has established my country's new land coordinate system recommended by the International Earth Measurement Association. - Xi'an 80 coordinate system, currently the earth measurement is basically in Beijing 54 coordinate system as a reference, the conversion between Beijing 54 and Xi'an 80 coordinates can check the comparison form published by the State Surveying and Mapping Bureau. The WGS1984 reference surface adopts WGS84 ellipsoid. It is a central coordinate system, which is the center as an ellipsoid center. The GPS measurement data is more than WGS1984. The above three ellipsoid parameters are as follows:
Ellipsoid MapInfo Medium Agent Empty Availability Half-axis Short Semi-axis 1 / Extension
Krassovsky 3 1940 6378245 6356863 298.3 IAG 75 31 1975 6378140 6356755 298.25722101 WGS 84 28 1984 6378137.000 6356752.314 298.257223563
The relationship between the ellipsoid and the reference surface is a pair of relationships, that is, the reference surface is established based on the ellipsoid, but the ellipsoid cannot represent a reference surface, the same ellipsoid can define different reference faces, as before The Soviet Pulkovo 1942, the Afgoye reference papers in Somalia in Africa have adopted Krassovsky ellipsoid, but their benchmarks are obviously different.
The map projection is mathematical transformation that converts the map from the spherical surface to the plane. If someone said: This point Beijing 54 coordinate value is x = 4231898, y = 21655933, actually refers to the projection coordinates under Beijing 54 Bench, which is Beijing The projection result of the latitude latitude coordinates under the reference surface in the right angle planar coordinates.
2. Definition and conversion of reference plane in GIS
Although there is a predefined hundred reference papers for users in the existing GIS platform, there is no reference plane definition in our country. If the precision is not high, it can be used to replace Beijing 54 coordinate system for the Pulkovo 1942 reference face of the former Soviet Union. If the accuracy requires high, such as land use, sea area, urban infrastructure and other GIS systems, Define the reference plane. The reference plane in the GIS system is defined by the conversion of the WGS1984 to the WGS1984, and the conversion is implemented through similar transformation methods, and the specific algorithm can refer to Science Press 1999 Published "Standardization Guide" from the Urban Geographic Information System Standardization Guide "to page 76-86. Assuming XG, YG, ZG represents the three-axis axes, XT, YT, and ZT of the WGS84 cooked coordinate system represent the three-axis shaft of the local coordinate system, and the 7 parameters of the custom reference plane are: three translation parameters ΔX, Δy, ΔZ represents the translation value of the two-seat main point; εy, εz indicates that the local coordinate system is rotated in parallel to the ground coordinate system, respectively, respectively, respectively, respectively, respectively, respectively, respectively, respectively, the ratio correction factor, used Adjust the ellipsoid size. The method of the MapX is as follows: Datum.Set (EllipsoID, Shiftx, Shifty, Shiftz, Rotatex, Rotatey, Rotatez, Scaleadjust, PrimemerIDian)
Where parameters: EllipSoid is the ellipsoid adopted by the reference surface; Shiftx, Shifty, Shiftz is a translation parameter; Rotatex, Rotatey, Rotatez is a rotation parameter; scaleadjust is a proportional correction factor, with one million quarter; primemeridian In my country takes 0, it means that the longitude is at the end of Greenwich.
National Imagery and Mapping Agency announced the conversion of the local reference papers from most countries to the WGS1984 reference papers (translation parameters), which can be from http://164.214.2.59/gandg/wgs84dt/dtp. HTML downloads, including Hong Kong Hong Kong 1963 Bench, Taiwan's HU-TZU-SHAN reference surface converted 3 parameters, but there is no parameter in mainland China.
In the actual work, the conversion parameters are generally calculated according to the Beijing 54 coordinate control points known in the workspace. If there is enough in the work area, there is enough known Beijing 54 and WGS84 coordinate control points, and 7 parameters or 3 parameters of coordinate conversion can be directly calculated. When there are 3 known Beijing 54 and WGS84 coordinate control points in the work area, the conversion parameters of WGS84 to Beijing 54 coordinates can be used (A, B, C, D, E, F): X54 = AX84 BY84 C, Y54 = DX84 EY84 F, excessively used as a test; in the case of only one known control point (often this), the difference in the difference between Beijing 54 and WGS84 coordinate parameters, when work The area is not enough when the area is not large. From MapInfo China's URL (http://www.mapinfo.com.cn/download) to the mapinfow.prj file containing Beijing 54, Xi'an 80 coordinate system, where the defined Beijing 54 reference surface parameters are: (3 , 24, -123, -94, -0.02, 0), Xi'an 80 reference surface parameters are: (31, 24, -123, -94, -0.02, 0.25, 0.13, 1.1, 0) There is no source of its parameters in the file. I found that they are the same as the parameters listed in one of the MapInfo Reference Manual Appendix G "Define Custom Reference Face", so its reliability is worthy of suspicion, especially from Xi'an 80 and Beijing 54 In terms of the same 7 parameters, at least the reference parallel definition of Xi'an 80 is definitely wrong. Therefore, when the system accuracy is high, it must be detected, verified, and ensure the correctness of the coordinate system definition. 3. The map projected in GIS China's basic scale topographic map (1: 5,000, 1: 10,000, 1: 25,000, 1: 50,000, 1: 100,000, 1: 250,000, 1: 500,000, 1: 1 million), more than 500,000, Gauss-Kruger, also called the transverse mercator; less than 500,000 topographic maps are cut by positive axis The garden cone projection is also the lambert conformal conic; the sea is less than 500,000 topographic maps are projected by the positive axis and other corner gardens, also called Mercator, and China's GIS system should be adopted. The consistent map projection system of China's basic proportional topographic map series.
The coordinate system definition is composed of a reference surface, projection two partial parameters, and the method is as follows: Coordsys.set (Type, [Datum], [Units], [OriginLongIdom], [Originlatitude], [StandardParalLone], [StandardParalLelTwo] Azimuth], [ScaleFactor], [Falseeasting], [Falsenorthing], [Range], [Bounds], [AffineTransform])
Where parameters: Type indicates the type of projection, the map coordinates of Type are indicated by latitude, which is a must-have parameter. It is an optional parameter; Datum is a geodetic side object, if the non-earth coordinate (Nonearth) does not need Define this parameter; Units is the coordinate unit, such as the units of 7 indicates that the origin of the origin and latitude, StandardParalLone, StandardParalLone, STANDardParalLtelTwo, the first, second standard weft; Azimuth is azimuth Define this parameter; scalefactor is a scale factor; falseeasting, falsenorthing is a tangential offset, nortup-niece offset value; Range is a map visible latitude range; Bounds is a map coordinate range, a rectangular object, non-Earth coordinate (NoneAarth) must be defined This parameter; AffineTransform is the coordinate system transform object. The coordinate system parameter sequence of the corresponding Gaussian-Krogger projection, Lanbot is projected, and the Carto projection requires the defined coordinate system parameter sequence as follows:
Gaussian Croull: Projection Code (Type), Bench (DATUM), Unit (Unit), OriginlongIration, Originlatitude, Scalefactor, Falseeasting, North Lane Falsenorthing
Lanbot: Projection Code (TYPE), Bench (DATUM), Unit (Unit), Central Side (OriginLongration), Originlatitude, Standardparallelone, Standard 2 (StandardparallelTwo), Donggate Falseeasting, north latitude offset (Falsenorthing)
Carto: Projection Code (TYPE), Bench (Datum), Unit (Unit), OriginLongration, OrigInlatitude, StandardPallelone
The Gaussian-Krig projement of 6 degrees or 3 degrees is used in the city GIS system, as the general city coordinates use a 6 degree or 3 degree part of the Gaussian-Kluger projection coordinates. Gauss Kruger brought by 6 degrees or 3 degrees, each band constitutes a separate flat right coordinate network, the straight line of the projection belt after projection of the line is the X-axis (longitudinal axis, latitude direction), equatorial projection For the Y-axis (horizontal axis, longitude direction), in order to prevent the coordinates in the longitude direction, there is a negative value, which specifies that the central line is 500 kilometers per belt, which is 500 kilometers per zone, and due to Gauss Kruger The coordinates of a projection belt are the relative values of the original coordinate original, so the coordinates of each belt are exactly the same, so they are specified before the horizontal axis coordinates, such as (4231898, 21655933) wherein 21 is tie, the same The defined todped offset value also needs to be added, such as 21500,000 meters of the 21500,000 meter. If your work area is located in 21, it is 120 degrees to 126 degrees. The center of the belt is 123 degrees, which uses the Pulkovo 1942 reference surface, then define the 6 degree divider - Kluger projection coordinate system parameters To: (8, 1001, 7, 123, 0, 1, 21500000, 0).
Then when the accuracy is high, the measured data is WGS1984 coordinate data, want to convert to the Gaussian-Krig projection coordinates of Beijing 54 Bench, how to define the coordinate system? You can choose WGS 1984 (MapInfo Sino-Distilion 104) as a reference surface, when there is only one known control point (see Part 2), adjusting the east shift according to the translation parameter, the north latitude offset value realizes the conversion of WGS84 to Beijing 54 , Such as: (8, 104, 7, 123, 0, 1, 21500200, -200), can also utilize the AffineTransform coordinate system transform object, at which time the conversion coefficient (A, B, C, D, E, F) can be used A, B, D, E are 0, only the translation value C, F in the X, Y direction; when there are three known control points, the resulting conversion coefficient (A, B, C, D, E, F) can be utilized. Define the AffineTransform coordinate system transform object, to achieve the conversion of the coordinate system, such as: (8, 104, 7, 123, 8, 0, the map.AffineTransform), where affineTransform defines AffineTransform.Set (7, A, B, C, D, E, F) (7 represents unit meters); Of course, when there is enough known control points, the 7 parameter custom reference plane is directly determined.
