ALS Project workflow
Even though the system has been calibrated, you may still find systematic errors in project data. TerraMatch can be used with actual project data to solve mismatches between laser data from different flightlines or between laser data and known points.
All parallel flightlines covering a project area should have a crossing flightline at both ends. For large project areas it is recommended to fly at least two or more crossing flightlines over sloped open terrain in order to provide good data for the matching task. Known points (ground control points) should be distributed close to the corners or edges of the project area.
As the project data volume can be huge, it is desirable to minimize the number of steps in the processing workflow. You probably want to run TerraMatch only if you notice that there are significant mismatches in the laser data.
Another difficulty with project data is that you do not know the nature of the errors beforehand. Mismatches may be a result of mistakes made during the setup of GPS reference stations, during computing trajectories or during operation of the airborne system. At some point within the correction workflow you must establish what parameters need to be corrected.
Processing steps
The general project workflow can be outlined with the following steps:
1. Solve GPS trajectories.
2. Compute xyz laser points with system specific software using the last known calibration values.
3. Import trajectories into TerraScan.
Split trajectories if necessary.
4. Import time stamped laser points into TerraScan.
Make sure that the flightline numbering of the laser points matches trajectory numbers (in TerraScan, set Line to Deduce using time in the Import points into project dialog for laser points or use Deduce using time command from Line pulldown menu or the corresponding Macro step).
5. Compare flightlines visually in cross sections. Try to locate sloped surfaces both along flight direction and perpendicular to flight direction.
6. If no significant mismatches are visible, you may skip the consecutive steps and continue with the normal processing workflow.
7. Classify low points.
8. Classify ground for each fligthline. If necessary, exclude water areas from the ground class.
9. (Optional) Locate areas which are best suited for matching (crossing flightlines, visible clean sloped surfaces). Create a TerraScan project with blocks only in those areas.
10. (Optional) Classify buildings for each flightline at the locations which are best suited for matching.
Continue according to the matching method.
Surface-to-surface matching:
11. Run Find Match and solve for heading, roll, pitch and mirror scale corrections for the whole data set.
Apply the correction if it is significant.
12. Run Find Match and solve for elevation or roll + elevation correction for individual flightlines.
Apply the correction if it is significant.
13. Run Find Fluctuations and solve for fluctuating elevation correction.
Apply the correction if it is significant.
Tie line-based matching:
11. Search for tie lines of type Surface lines based on ground and (optional) building class.
12. Run Find Tie Line Match and solve for heading, roll, pitch and mirror scale corrections for the whole data set.
Apply the correction to the tie lines and the laser data if it is significant.
13. Check the tie lines for worse observations.
14. Run Find Tie Line Match and solve for heading, roll, pitch and elevation corrections for flightline groups (if there are any). Depending on the system, solve for mirror scale corrections per flightline groups as well.
Apply the correction to the tie lines and the laser data if it is significant.
15. Check the tie lines for worse observations.
16. Run Find Tie Line Match and solve for heading, roll, pitch and elevation corrections for individual flightlines.
Apply the correction to the tie lines and the laser data if it is significant.
17. Check the tie lines for worse observations.
18. Run Find Tie Line Fluctuations and solve for fluctuating elevation corrections.
Apply the correction to the laser data if it is significant.
After each Apply corrections step you should check the flightlines visually in cross sections or using distance coloring to determine if the correction step improved the data. You have to decide if the correction was good or if you need to go back one step and try solving parameters with different settings.
Multi-day projects
Use GPS standard time to avoid conflicts between flight sessions with identical GPS seconds-of-week time stamps. There are tools in TerraScan for converting between GPS seconds-of-week and GPS standard time and vice versa. The tools are applicable to trajectories and to laser data.
Apply a group number to trajectories for each flight session (day 1 = trajectory group 1, day 2 = trajectory group 2, etc.). Optionally apply also a quality tag to trajectories of a flight session, e.g. if flight conditions are worse on one of the days.
Reduce the amount of data for processing as much as possible. This includes thinning of trajectories during the import, but also reducing the number of blocks for matching in a TerraScan project. Use only blocks that are suitable for the matching task and only classes that are necessary (e.g. ground, building classes).
For adding a new data set to an already matched data set, apply quality “bad” to the trajectories of the new data set and quality “normal” or “good” to the trajectories of the old data set. Use Find Match or Find Tie Line Match and solve for corrections for the “bad” flightlines only.
