You can create geodetic transformations from scratch, or by duplicating and editing one of the transformations in the Coordinate System Library.
To define a geodetic transformation
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Click
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- In the Geodetic Transformation Definition wizard, Create A Geodetic Transformation is selected. Click Next.
- Specify whether to create a geodetic transformation from scratch, or modify an existing one. Then click Next.
To modify an existing transformation, click Select and click the transformation in the list that displays.
- Enter a code and description for this transformation.
These entries are for your convenience. The code will be used in lists of geodetic transformations, and the description can help you remember any salient details of the transformation.
- Specify the source and target datum for this geodetic transformation and click Next.
To use an existing datum, click Select and select the datum in the list that displays.
- Specify whether to use an analytic formula, an interpolation, or a null transformation method, and then click Next.
Interpolation transformations require grid files for both the source and target datums.
Analytic formulas do not require grid files. If you select this method, indicate which conversion method to use. See the Concept tab for this topic for information on the methods available.
Null transformations transform data between two datums that have no significant differences.
- If you selected an analytic formula method, enter the values for the parameters and click Next.
Parameters can include any of the following:
Geocentric Translation
- Delta X (meters): -5,000 to +5,000
- Delta Y (meters): -5,000 to +5,000
- Delta Z (meters): -5,000 to +5,000
Axis Rotation of Arc
- X-axis (seconds) : -100 to +100
- Y-axis (seconds) : -100 to +100
- Z-axis (seconds) : -100 to +100
Axis Translation
- X-axis (meters) : -100,000 to +100,000
- Y-axis (meters) : -100,000 to +100,000
- Z-axis (meters) : -100,000 to +100,000
Scale
- -100 to + 100
- If you selected the interpolation method, follow these steps:
- Select the grid format.
The grid files that use this format appear in the left-hand list.
- Select the grid files required for this transformation.
Click + to browse to grid files on your system. AutoCAD Map 3D does not check whether your grid file is in the selected format. You must ensure that the format and the grid file match.
You can add one or more grid files. Grid files are added in their default order, but you can change their position with the Up and Down buttons. If the coverage of the files does not overlap, the order of the files does not matter. However, when converting a point in an overlapping area of grid files, the first grid file listed is used. To remove a grid file you added, select it and click -.
- To automatically generate a formula to get back to the first coordinate system from given coordinates in the second, using the specified grid files, select Inverse Transformation.
- To deal with the possible failure of your method (for example, if the grid files are not found at the time of transformation), select Specify A Fallback Transformation and click Select. Click an existing analytical geodetic transformation in the Coordinate System Library.
- Click Next.
- Select the grid format.
- Specify the minimum and maximum latitude and longitude values that define the useful range for this transformation.
Values are expressed in degrees. Longitude parameters are relative to Greenwich.
- Click Next to see a summary of the geodetic transformation.
To edit any displayed value, click Edit for that section. Make your changes and click Next to cycle through the remaining screens and return to the summary.
- To make sure the transformation works properly, click Test in the Geodetic Transformation Test dialog box and enter appropriate geographic coordinates.
The conversion is performed using the new transformation parameters. AutoCAD Map 3D displays and verifies the results.
- When you are satisfied with the transformation definition, click Finish.