Instructions for use

SuperMap supports the calculation of common four-parameter and seven-parameter parameter values through operations such as coincidence point selection, Coordinate Transformation model, and accuracy evaluation. Users can use the parameter values to convert the coordinate system of data results.

Preparations

• Example description: The user has a copy of 1980 Xi'an Coordinate System Vector Data of a certain area, and now he wants to convert it to 2000 National Geodetic Coordinate System. The user has surveyed some control points in the survey area. These control points have 1980 Xi'an coordinates and 2000 national geodetic coordinates. The following is a detailed description of the operation steps with the Data Conversion as an example.
• Preparation: Select the control points of 1980 Xi'an coordinate system and 2000 national geodetic coordinate system as the coincidence point file.
• The selection of coincidence point is very important for the calculation of Transformation Parameters. The user needs to select the control point with coordinate results under both coordinate systems as the coincidence point.
• Basic principles for selection of coincidence points: high grade, high precision, uniform distribution, covering the whole conversion area, and small local deformation.
• Number of coincident points: In general, the number of coincident points for conversion to 2000 series shall not be less than 6, the number of external check points shall not be less than 6, and the points shall be evenly distributed to cover the whole conversion area. Considering that there may be rough points, it is necessary to prepare more coincidence points as backup. In general, as many coincident points as possible should be selected.

Figure: Data coincidence point

Function entrance

• Go to Start tab-> Data Processing group-> Projection Transformation Drop-down Button-> Model Transformation Parameter Calculation.

Operating instructions

Figure: Model Transformation Parameter Calculation "Dialog Box

1. Model Settings:: Select the model method for conversion, and click the Drop-down Button on the right side of the "Model Method" tab. The drop-down menu list that pops up shows the five conversion models provided by the program. Contains Position Vector (7-para), Coordinate Frame (7-para), China _ 3D _ 7P (7-para), China _ 2D _ 7P (7-para) and China _ 2D _ 4P (7-para).

   Since the selection of the conversion model is affected by the coordinate system of the control points and the conversion area, the selection of the model method depends on the specific situation. In that "technical specification for geodetic Measurement Control PointCoordinate Transformation", the Scope of application of the conversion model is given, and the user can select the conversion model according to the coordinate system of the control point of the source data, Select the conversion model according to the applicable area of the conversion model. For the transformation model and Scope of application , refer to Transformation model 。

2. Set the coordinate system types to be converted to each other, i.e., Source Coordinate System and Target Coordinate System. The program supports Geographic Coordinate System, Projected Coordinate System, and the transformation between the two:
   • Source Coordinate System Settings: You can set the coordinate system of the source data at the Source Coordinate System. There are two ways for Coordinate System Settings:
     • Projection Settings: Select the Projection Settings radio box and click the Settings.. button. Select one of the Geographic Coordinate System or Projected Coordinate System provided by the program that is the same as the source point data and set it as the Current Coordinate System. Please refer to Projection Settings for specific operation of setting projection.
     • Import Projection File: Select the Import Projection File radio box, click the Select button, select the Projection Info File and import it.
   • Target Coordinate System Settings: Set the converted data coordinate system type.
3. Superposition Point Settings: Enter a coincident point with Source Coordinate System coordinates and Target Coordinate System coordinates. The coordinate format of the coincidence point shall be consistent with the Source Coordinate System and Target Coordinate System set in the previous step, otherwise the program will prompt that the coordinate format of the control point is wrong.

   You can add a matching dot by Create a matching dot string, Import a matching dot file, and Manually enter:

   • Create a matching dot string: When the matching dot exists in Datasource in the form of Dataset, Select Fields can be specified separately for the source data and the matching dot in Target Dataset. To establish a connection relationship between the source data and the Target Dataset through the Specify Field value, click the Create a matching dot string button in the Superposition Point Settings toolbar. Perform operations in the Create a matching dot string dialog.
   • Import coincident points: Click the Import button on the Superposition Point Settings toolbar to import a coincident point file (.txt). The format of the coincidence point file (.txt) is shown in the following figure:

     Figure: Coincidence Point (.txt) File

   • Manual input: Click the "Add Data" "button in the Superposition Point Settings toolbar, add a record in the coincidence point list box, and manually input the coordinates corresponding to the coincidence point.

After the coincident points are
4. imported, the Details of the coincident points can be viewed in the list box, and you can set the points to participate in the Coordinate Transformation Parameters calculation by checking the check box at Apply. Control point pairs should not be less than 6 groups.
5. Calculate Coordinate Transformation Parameters: Click the "Calculate" button to execute the Coordinate Transformation Parameters calculation, and the calculation results are as follows:

   ① The program substitutes the coincidence points into the transformation model, and uses the least square method to calculate the Transformation Parameters, including offset X, offset Y, offset Z, rotation X, rotation Y, rotation Z and scale difference. Where Offset X, Offset Y, Offset Z are in Meters; Rotation X, Rotation Y, Rotation Z are in Angular Seconds; and the proportional difference is in parts per million (ppm).

   ② Calculate and obtain coordinate residual error of coincidence point (Dx, Dy, DH) and mean square error of point position. The program determines that the coincidence point with coordinate residual error greater than 3 times of mean square error of point position is an unavailable point, that is, it is displayed as No in the column of Available; otherwise, it meets the accuracy requirements.

   Note: To improve the accuracy of the Transformation Parameters, you can reject or set to not apply the RecalculateTransformation Parameters for the coincident points that are not available.

Correlation error calculation formula
Residual error of
• coincident point (Dx, Dy, DH) = transformed coordinate value of coincident point-known coordinate value
of coincident point Calculation formula of mean square error of
• point position:
Mean square error of
• plane point position:


6. Calibrate Precision: Click the Calibrate Precision button to pop up the Calibrate Precision dialog box. Import the coincident points that do not participate in the calculation of Transformation Parameters as external check points, with the number of points not less than 6 and evenly distributed; The accuracy of Coordinate Transformation is evaluated according to the mean square error of the residual error of the coincidence point of the calculated Transformation Parameters, and the accuracy of the point where the residual error is less than 3 times the mean square error of the point meets the requirements.
7. Export Transformation Parameters: After Calibrate Precision is qualified, click the Export button in the Coordinate Transformation Parameters combo box. The calculated Transformation Parameters are exported as Projection TransformationParameter File (*.ctp).
8. The Exported Projection TransformationParameter File (*.ctp) performs Projection Transformation on the entire Dataset of the control point area. For Dataset Projection Transformation, see the Dataset Projection Transformation. Import the calculated Transformation Parameters in the Parameter Settings dialog box. Thereby realizing the Projection Transformation of the whole Dataset.
9. The result of superimposing the converted data and the data before conversion is shown in the following figure:

Figure: Data comparison before and after conversion

Related topics

Dataset Projection Transformation

Batch Projection Transformation

Coordinate Point Conversion

Model Transformation Parameter Calculation