In Didger, you can calibrate a project, warp an image, or convert coordinates with one of several spatial transformation methods, called Georeference Methods and Warp Methods in Didger. Calibration, image warping, and converting coordinates involve the process of mapping source coordinates to destination coordinates. These three processes require that several points with known coordinates are located in the original project. These points are known as calibration points or control points. With these known points and the selected transformation method, Didger maps the known points to the desired points. The points cannot fall into a straight line.
The spatial transformation methods correct for translation, rotation, and differential scaling. Spatial transformation is analogous to stretching and pinning a rubber sheet. The sheet is pinned down in various locations (control points) and is consequently stretched and contracted between these points. Spatial transformations can stretch the project in several different directions at one time. Therefore, it is beneficial to define more control points where distortion is greatest.
Spatial Transformation Methods
A generalized discussion of the spatial transformation methods is included in this help file. For mathematical details, refer to one of the references. A graphical illustration of each method is included. Keep in mind that the results are dependant upon the mapping from the source control points to the destination control points and the spatial transformation selected. In this case, the graphics are exaggerated for detail.
If a selected spatial transformation method is incompatible with the number of control points, Didger replaces the method with the best method available. If you are unsure of which method to use, select Affine Polynomial.
See Also