In the
Bundle Adjustment Settings window, you can select from six available modes in the
Triangulation mode list to perform a bundle adjustment:
- Traditional block bundle: Uses the ground control points (GCP) and tie points (TP), and bases the adjustment on the weight assigned by the defined input accuracy of each point. The default GCP accuracy is one half of a pixel in space on disk and 1 meter in object space. The model bases the calculation on a simultaneous use of both GCPs and TPs. In this mode, OrthoEngine recalculates the math model several times to find the best possible solution. This option is recommended for most situations.
- Enforced fitting: Uses the GCPs and TPs; however, the GCPs and TPs do not carry the same weight. The weight of the TPs is assigned by the input accuracy whereas the weight of the GCPs is unlimited (contains no error).
You can use this mode when you are collecting GCPs for a high-resolution image from a lower-resolution source, provided the GCP source is accurate and reliable. In some cases, the residual error or root mean square (RMS) error produced by using this mode can be lower than the Traditional block bundle mode; however, the results may not be as accurate.
- Relative-absolute orientation: In this mode, both TPs and GCPs are used to perform the adjustment. The TPs are used first in the calculation followed by the GCPs to adjust the model.
An example of when to use this method is when the confidence in the accuracy of GCPs is not high or is unknown, but confidence in the TPs is higher.
The residual error or RMS error produced in this mode can sometimes be lower than that of Traditional block bundle; however, the results may not be as accurate.
When you select this mode, the
Absolute orientation options list becomes available, and you can then select an additional option to compute the absolute component in the bundle:
- 3-D conformal transformation: This is the default option and uses seven parameters: scale, translation (x, y, and z), and rotation (omega, phi, and kappa). It is the most conservative option to select when the GCP error is unknown or has lower accuracy. If the GCPs are inaccurate or the GCP error is not known, the stereo model will not be affected significantly by using this option.
- Linear transformation: This option can provide results that are closer to enforced fitting; however, if the GCP error is unknown or is large, then 3-D conformal transformation provides a more conservative approach.
- 2nd order polynomial transformation: If the GCP error is zero, or close to zero, this option provides results that are nearer to the enforced fitting method; however, choosing this option contains a higher risk if the GCPs contain and error.
- 3rd order polynomial transformation: If the GCP error is zero, or close to zero, this option provides results that are nearer to the enforced fitting method; however, choosing this option contains a higher risk if the GCPs contain and error.
- 3-D conformal polynomial: Select this option for aerial projects that contain only a single (potentially) long strip of data. If your block bundle contains more than one strip of data (more than one flight line), this option is not recommended.
The greater the number of options you specify, the greater the chance the result will be close to enforced fitting, because there is greater probability of affection by any GCP errors. This means the stereo model will be a greater risk of "twisting". To avoid this, make sure your GCPs are error-free.
- Relative orientation: In this mode you can separate the calculation of the model into two distinct steps.
Only TPs are used in the bundle-adjustment calculation. GCPs are required, but excluded from the calculation of the model. After computing the relative orientation, you can inspect the residuals, make adjustments, as necessary, and then recalculate.
This mode functions similarly to Relative-absolute orientation, except without the application of the GCPs into the model. You can select this mode when confidence in the GCPs is not as high as with using the Traditional block bundle or Enforced fitting modes.
- Absolute orientation: In this mode, both GCPs and TPs exist in the project. The model must have been computed previously by running a bundle adjustment in Relative orientation mode before using this mode. The GCPs are used to adjust the relative-orientation model computed previously.
This mode functions similarly to Relative-absolute orientation, except that the model is computed in two distinct steps instead of one. You can calculate both components (TPs and GCPs separately). You can select this mode when confidence in the GCPs is not as high as with using the Traditional block bundle or Enforced fitting modes.
- Free Network Adjustment: Select this mode when confidence in the Global Positioning System (GPS), inertial measurement unit (IMU) or both of the sensor is low.
In this mode, the accuracy of the camera position and angles will be discarded and replaced by default values. OrthoEngine will adjust the solution using a larger space.