Geodetic transformation performs a cartographic transformation from a coordinate reference system to produce a geographic coordinate system (that is, latitude and longitude) that is referenced to the same datum as the source coordinate system. AutoCAD Map 3D automatically converts drawings from one geographic coordinate system to another using a library of transformations. You can define your own transformations based either on a grid file or on parameters you specify. For example, if you use data specifications from Infrastructure for Spatial Information in Europe (INSPIRE), you can configure ETRS89 transformations based on INSPIRE-provided grid files.
You can specify an interpolation method (using grid files) or an analytic method (using formulas). For analytic formulas, you can choose from among the following:
| Method | Notes | Scale | Geocentric Translation (in meters) | Axis Rotation of Arc (in seconds) | Axis Translation (in meters) | ||||||
| Delta-X | Delta-Y | Delta-Z | X-Axis | Y-Axis | Z-Axis | X-Axis | Y-Axis | Z-Axis | |||
| Geocentric Translation | X | X | X | ||||||||
| Molodensky | X | X | X | ||||||||
| Abridged Molodensky | X | X | X | ||||||||
| Four Parameter | X | X | X | X | |||||||
| Six Parameter | X | X | X | X | X | X | |||||
| Seven Parameter | X | X | X | X | X | X | X | ||||
| Bursa Wolfe – Position Vector | Rotation values are positive in a counterclockwise direction (as you look toward the origin of the X,Y,Z systems). | X | X | X | X | X | X | X | |||
| Bursa Wolfe – Coordinate Frame | Rotation values are positive in a clockwise direction (as you look toward the origin of the X,Y,Z systems). | X | X | X | X | X | X | X | |||
| Molodensky-Badekas | Given the X, Y, Z origin of rotation point, one can calculate an equivalent Coordinate Frame transformation. The Delta-X, Delta-Y, and Delta-Z values change but the rotation and scale values remain the same. | X | X | X | X | X | X | X | X | X | X |
Interpolation methods require grid files. If you use grid files in the geodetic transformations you create, you must copy them into the the folder where AutoCAD Map 3D expects to find them. That folder contains folders for individual countries where you can store your grid files. The location varies, depending on your operating system version.
| Windows XP (32-bit and 64-bit) | C:\Documents and Settings\All Users\Application Data\Autodesk\Geospatial Coordinate Systems |
| Windows Vista (32-bit and 64 bit) or Windows 7 (32-bit and 64-bit) |
C:\ProgramData\Autodesk\Geospatial Coordinate Systems |
In addition to standard geodetic transformations, you can create transformations that change only the units or projection used in a coordinate system (without changing any of the datum or ellipsoid specifications). This type of transformation is called an “arbitrary” transformation.
You can also define a “null” transformation. For example, the GDA94 (Geodetic Datum of Australia 1994) and WGS84 are technically two different datums. However, the differences between them is so small that no one has devised a way to convert from one to the other. In the coordinate system library, the two datums are listed separately, but the transformation between them is null.
Geodetic transformation paths determine the order of the transformations used to achieve coordinate system transformation. For information on geodetic transformation paths, see To Define a Geodetic Transformation Path.