You can georeference an image with one of several spatial transformation methods, called warp methods. Image warping involves the process of mapping source coordinates to destination coordinates. This process requires that several points with known coordinates are located in the image. These points are known as control points. With these known points and the selected warp method, Surfer maps the control points to the desired coordinates. 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 depend 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, Surfer replaces the method with the best method available. If you are unsure of which method to use, select Affine Polynomial.