11.0 CALIBRATION AND CORRECTION
11.1 System calibration
Section 10 of this document clearly shows that two possible sources of error for MLS point clouds are directly attributable to system calibration parameters, specifically the boresight angles that define the angular offsets between the laser(s) and the navigation system (IMU), and the lever arm, which is a measure of the physical 3D offset between the navigation system center and the laser(s) center. Both of these parameter sets are able to be recovered using calibration procedures. However, it is also important to note that depending upon the MLS system, and the methodology used to install the components, these parameters may not be of high temporal stability. Therefore, we recommend that the data procurement require the provider to submit a complete calibration report that documents:
a) The equipment used for data collection,
b) The calibration procedure used, along with the calibration parameters and their estimated accuracies,
c) Equipment installation schematics, and
d) Verification of temporal or long term stability of calibration parameters.
A sample report containing the typical information necessary is given in Appendix D.
Recommendation: Always request a calibration report from the data provider with the above information.
11.2 Geometric correction
Current processing procedures for most commercial providers of MLS data sets perform some sort of geometric correction to their point clouds post mission. In general, this geometric correction employs DGNSS or total station surveyed targets along the project corridor. These control points are identified in the laser data, and then the MLS point cloud is “adjusted” to the control point locations. This process is undertaken in an effort to improve the overall accuracy of the point cloud, and to also mitigate any problems with the computed navigation trajectory of the vehicle, usually caused by GNSS coverage outages due to obstructions such as vegetation, overpasses or tunnels. A geometric correction is applicable, as long as the observed control points are direct input as observations into the raw navigation trajectory estimation (i.e. the GNSS/INS post-processing software) and follow a mathematically and theoretically sound procedure. Any other ‘ad-hoc’ geometric corrections which are applied as transformations or shifts only to the final point cloud should be avoided or at a minimum limited to one 3D, Rigid Body Translation (X, Y, Z) of each MLS pass to fit the point cloud data to control per project. For larger projects, individual sections for geometric corrections should not be broken down into segments less than one mile. Full documentation should be provided (including the methodology, type, and magnitudes) for any applied geometric corrections should be provided.