Adjust relative and absolute image positions
In general, the absolute accuracy of mobile images can be checked and improved in a similar way as for images of Airborne projects. However, most often the mismatch in mobile images is not a systematic shift but a drift that changes over time. The mismatch basically depends on the accuracy of the trajectory that was used to generate the image list. The trajectory accuracy usually varies a lot during a mobile survey due to differences in GPS signal availability.
Therefore, the GCPs are rather utilized to derive fluctuating drift values for individual drive paths. These values are then used by TerraMatch in order to compute fluctuating corrections.
For mobile projects, the relative and absolute image adjustment is usually done in one step. If no GCPs are available, images can be adjusted only internally. In this case, step 5 in the workflow below is not applicable.
1. Create an image list which contains only images of a camera that is oriented downward towards the ground. For example, use Delete / By camera command from the Images pulldown menu of the TPhoto window in order to reduce a complete image list.
2. Add known xyz tie points from TerraMatch tie points drawn in the CAD file. The points represent control point locations in the (unmatched) laser data. Enter the tie points only in images of the closest drive path.
3. Solve and apply misalignment angles with Output report command for the camera in order to match the images to the laser data.
4. Compute depth maps for all images, if not yet available. Use laser points that are Close in time, which means that depth maps are created based only on points of the same drive path as the images.
5. Add known depth tie points for all available GCPs. Use the best image for placing a tie point. The best image sees the GCP location best and thus, it is well suited for placing the tie point accurately at the GCP location.
6. Add depth tie points at places where no GCPs are available. The tie points should be placed at regular distances (25 - 50 m) in order to get a good control of the trajectory drift.
7. Start TerraMatch and search tie line fluctuations based on image tie points. Apply the fluctuating corrections to the image list using TerraMatch Apply corrections tool.
