Conversion Models and Scope of Application

SuperMap Desktop products support five types of coordinate transformation models: Position Vector (7-para) (Position Vector Transformation), Coordinate Frame (7-para) (Coordinate Frame Rotation Transformation), China_3D_7P (7-para) (3D Seven-Parameter Transformation Model), China_2D_7P (7-para) (2D Seven-Parameter Transformation Model), and China_2D_4P (4-para) (2D Four-Parameter Transformation Model).

Conversion Models

- Position Vector (7-para), Coordinate Frame (7-para): These two methods are commonly known as seven-parameter transformations, or Bursa models, including three translation transformation parameters, three rotation transformation parameters, and one scale change parameter. They can be considered as the same model, differing only in the sign convention for rotation defined by different countries.
  - Position Vector (Position Vector Transformation), defined in Europe, where counterclockwise rotation is negative.
  - Coordinate Frame (Coordinate Frame Rotation Transformation), defined in the United States and Australia, where counterclockwise rotation is positive.
- China_3D_7P (7-para): The 3D seven-parameter transformation model is used for point coordinate transformation between geodetic coordinate systems under different Earth ellipsoid datums. It involves three translation parameters, three rotation parameters, one scale change parameter, and accounts for differences in the semi-major axis and flattening of the two Earth ellipsoids corresponding to the coordinate systems.
- China_2D_7P (7-para): The 2D seven-parameter transformation model is used for point coordinate transformation from geocentric coordinate systems to geodetic coordinate systems under different Earth ellipsoid datums. It involves three translation parameters, three rotation parameters, and one scale change parameter.
- China_2D_4P (4-para): The 2D four-parameter transformation model is used for coordinate transformation between different Gaussian projection planes within local regions. It involves two translation parameters, one rotation parameter, and one scale parameter. For 3D coordinates, they must first be projected to plane coordinates via Gaussian projection before calculating transformation parameters.

Scope of Application

Since the selection of a transformation model is influenced by the coordinate system of control points and the transformation area, users can refer to the scope of application provided in the "Technical Specifications for Geodetic Control Point Coordinate Transformation" and choose the model based on the source data's control point coordinate system and applicable regional range.

- Position Vector (7-para), Coordinate Frame (7-para): Suitable for space rectangular coordinate conversion of control points at provincial and national levels.
- China_3D_7P (7-para): Suitable for coordinate transformation of control points at provincial and national levels with ellipsoidal coverage of 3° or more.
- China_2D_7P (7-para): Suitable for coordinate transformation of control points at provincial and national levels with ellipsoidal coverage of 3° or more.
- China_2D_4P (4-para): Suitable for planar coordinate transformation of control points in small areas, and for establishing connections between relatively independent planar coordinate systems and the 2000 National Geodetic Coordinate System.

The table below shows the selection of coordinate transformation models and scope of application for transforming control points from different coordinate systems to the 2000 National Geodetic Coordinate System:

Application Scenarios

Through SuperMap Desktop, users can convert local coordinate system data to the 2000 National Geodetic Coordinate System without additional plugins, achieving transformation results across different coordinate systems.

Step 1: Transform from the local coordinate system to the common coordinate system: Use the "Reproject Dataset function", select the 2D four-parameter (China_2D_4P) transformation model to convert the local coordinate system to the actual common coordinate system.

Step 2: Transform from the common coordinate system to the 2000 National Geodetic Coordinate System: Use "Calculate Transformation Model Parameters" to compute transformation parameters, then use "Reproject Dataset function", select transformation models such as 2D seven-parameter (China_2D_7P), 3D seven-parameter (China_3D_7P), or Coordinate Frame to achieve the conversion.

If the local coordinate system is derived from a deviation of a common coordinate system (e.g., Xian1980, Beijing1954, etc.), converting it to the 2000 National Geodetic Coordinate System requires first transforming to the actual common coordinate system, then to the 2000 system. In SuperMap Desktop, this is achieved as follows:
If the local coordinate system is based on a true coordinate system within a projection zone, it can be directly converted to the 2000 National Geodetic Coordinate System using Step 2 above.

Model Expressions

- Position Vector (7-para), Coordinate Frame (7-para)
- China_3D_7P (7-para)
- China_2D_7P (7-para)
- China_2D_4P (4-para)

Notes：

When user data is in a Gauss-Kruger projected coordinate system, note that the horizontal and vertical coordinate values are reversed compared to XY coordinates in applications, i.e., Gauss-Kruger coordinates are (Y,X), while applications display them as (X,Y).