OR/15/057 Appendix 1 - Re-adjusting camera calibration
|Jones, L D. 2015. Ground-based geomatic surveys at the BGS - a manual for basic data collection & processing (2015). British Geological Survey Internal Report, OR/15/057.|
This guide will show you how to re-adjust the camera mounting calibration in RiSCAN PRO, following the removal and re-mounting of the camera. This is necessary when the scanner and camera must be separated for storage in their individual carry cases. It may also be necessary if the camera goes out of calibration — if, for example, the camera is knocked, or removed for maintenance.
1. Identify tie-points common to scan and photos
Open the RiSCAN PRO project, and locate a scan in which there are identifiable features (building corners, signposts, road markings, etc.)
Double-click the scan to open it, and use the parameters in Figure 1 to display it. Click the ‘Calculate minimum/maximum’ button, and wait for the scan to be processed; then click OK.
Now open the first image, by expanding the ‘SCANPOSIMAGES’ folder (Figure 2) and double-clicking an image.
Click the ‘Arrange windows vertically’ button to arrange the scan and image side-by-side.
Now identify features common to both scan and photo. You are looking for a good spread of point in both the vertical and horizontal directions. Avoid picking points in clusters — make sure there is a good spread throughout the scan.
When you have found one, left-click the pixel in the photo to place a marker (a small blue cross), then right-click the marker, and choose ‘Add point to TPL’.
In the dialogue box that appears, enter a name for the tie-point and click “OK”.
Now find the corresponding pixel in the scan, and right-click the pixel and choose “Create tie- point here”.
In the dialogue box that appears, enter the same name for the tie-point and click “OK”.
Repeat this procedure until you have at least four pairs of corresponding points.
Now, close the scan and photo views by clicking the small “X” in the top-right corner of the RiSCAN PRO window (not the red “X”, which would close RiSCAN PRO.
2. Link tie-points together
When you created tie-points, they were added to the ‘tie point lists’ (TPL) of both the scan and the photos. Now we need to link the pairs together, and to do this we need to open both TPLs.
The scan TPL is located just below the SOP in the Project Manager (each Scan Position has it’s own TPL). Double-click the item TPL (SOCS).
This will open the TPL in the main window.
Now open the TPL for the first image in which you have created tie-points. This is located in the SCANPOSIMAGES folder, under the image, and is called TPL (IMAGE):
Now, drag the tie-point in the image TPL onto the corresponding tie-point in the scan TPL- this will create a ‘link’ between the tie-points. Note the ‘link’ column in the image TPL has been filled in.
Repeat for the remaining tie-points. You may have to open the other image TPLs to find the rest of the image tie-points.
When you have finished, each tie-point in the scan TPL should have a “1” in the Ref... column This shows that it is referenced by another tie-point.
Close the TPLs.
3. Re-adjust the mounting calibration
RiSCAN PRO now has all the information it needs to adjust the calibration. To perform the adjustment, locate the mounting calibration in the folder CALIBRATIONS > MOUNTING. Right-click it, and choose ‘Attributes...’
Switch to the tab ‘Re-adjustment of camera mounting’. You will see listed all of the image tie-points that were created.
Check that the following options are set under SETTINGS:
Calculation mode: modify rotation only (default) Figure of merit: least square fitting
Click the button ‘Start re-adjustment’. The software will find the best match between the tie- point pairs, and the results will be displayed in the LOG/BEST RESULTS panel.
In this example, the distance between the tie-points has been reduced from 24.9 pixels to 1.7 pixels. This means the calibration has been greatly improved, and the images now match the scans to within 2 pixels.
If you are happy with the results, click OK to save the new calibration and move on to the next step. Otherwise, go back to the beginning and select the tie-points more carefully.
4. Assign new calibration to photos
Now that the calibration has been updated, it must be assigned to the images.
Right-click the mounting calibration again in the Project Manager, and select ‘Assign to images...’
In the dialogue box that appears, click the button ‘All’.
Click ‘OK’. Thee new calibration will be applied to all the images in the project.
5. Re-colour the scans
If the scans had been coloured before this re-adjustment procedure was completed, they will still have the previous colouring. Therefore you must now colour the scans again.
Right-click each scan in turn, and select ‘Color from images’. The colours will be re-assigned to the scan points, using the new camera calibration.
Before calibration adjustment.
After calibration adjustment.