MLS Project workflow
For mobile scanning projects, the quality of GPS positioning and thus, the quality of the trajectory is usually not as good and constant as for airborne projects. This is caused by GPS failures due to large buildings, trees, rock formations, etc. along the roads or rails. The positioning accuracy of the trajectory can vary a lot during a drive session. Thus, the dominant error source for inaccurate data is the trajectory xyz positioning which shows up close to the scanner location.
Positional adjustment of mobile laser data in TerraMatch is based on tie line observations. These are typically collected on flat ground and/or on high intensity features such as paint markings.
A tie line observation can utilize ground control points or it can be for internal comparison only (multiple drive passes seeing the same location). It is recommended to measure ground control points along the road or other survey corridor at regular distances. The distances between control measurements can vary depending on the required accuracy level and the accuracy of the trajectory. Internal tie line observations should be added between the control measurements whenever the same location has been seen by multiple drive paths.
Ground control points can be located on corners of strong paint markings, at end points of thin strong paint markings or in the center of weaker paint markings. The latter type of control points is then used for partial xy control.
Instead of using paint markings for control measurements, signal markers of a specific pattern can be used as ground control points. This has the advantage that the software can find a bright signal marker on a dark surface automatically based on the intensity information of the laser points. See Signal markers category for more information about signal markers in TerraMatch.
More information about the different tie line types and the positional adjustment of mobile laser scanner project data can be found in Sections Tie line types and Fluctuating corrections with tie lines.
General project strategy
The general strategy of a mobile scanning project can be outlined as follows:
1. Collect some control measurements.
2. Execute LiDAR / image survey. Drive every place at least twice.
3. Check the system calibration.
4. Find locations of bad trajectory accuracy using color by RMS values for trajectories. Compare the drive paths visually.
5. Collect additional control measurements at locations where the trajectory positioning is bad.
6. Adjust xyz positioning of the laser data by matching drive paths to each other and to control measurements. This probably involves several processing steps.
7. Remove less accurate data, e.g. long range measurements where short range measurements exist, and cut off overlapping drive paths.
8. Continue processing the data according to the project requirements.
