Choosing Calibration Method
The choice of calibration method is based on the nature of image distortion and the quantity and location of calibration pairs.
If the type of image distortion is unknown, then choose a calibration method automatically. In this case the program automatically chooses the optimal calibration method for the set of calibration pairs.
The following table describes possible distortions and calibration methods, used to correct them.
| Method | Distortion |
|---|---|
| Linear conformal | For linear transformation moving, rotating, and proportional scaling. |
| Affine | For linear transformation moving, rotating, and non-proportional scaling. Raster ellipses can be transformed to circles. |
| Bilinear | For parallelogram or trapezoid distortions on map or mechanical drawings. |
| Grid adaptive bi¬linear | Useful for relatively small raster grid distortions if the grid sells are convex. Can only be used if the calibration grid is defined. |
| Polynomial | Nonlinear deformations caused by either three-dimensional expansion of the original graphic document or by an image type itself. For example, aerial photos of the Earth’s surface. |
| Surface Splines | For distortions of all kinds, it is more accurate, and works on an arbitrary set of pairs. |
For every calibration method there is a minimum number of calibration pairs that enables the use of this model. If the number of calibration pairs exceeds a certain value, then all models, except Surface Splines, produce a non-zero deviation.
The following table describes the restrictions on the number of calibration pairs for each calibration method.
| Method name | Number of calibration pairs required | Number of calibration pairs producing non-zero error |
|---|---|---|
| Linear conformal | 2 | 3 |
| Affine | 3 | 4 |
| Bilinear | 4 | 5 |
| Grid adaptive bilinear |
the calibration grid must be defined. | the calibration grid must be defined. |
| Polynomial 2 degree | 6 | 7 |
| Polynomial 3 degree |
10 | 11 |
| Polynomial 4 degree |
15 | 16 |
| Polynomial 5 de¬gree |
21 | 22 |
| Polynomial 6 degree |
28 | 29 |
| Surface Splines |
3 | not applicable |
To choose a calibration method
Select a calibration method from the Calibration method list of the Calibration dialog box.
Feedback
Post your comment on this topic.