Method
Strip adjustment, also known as point cloud registration, is a topic that has been investigated in the areas of research where dynamic measurement systems are used. The goal is to remove inconsistencies in the point cloud. Most first studies of strip adjustment are concentrated on the estimation of the three boresight angles. Indeed, the boresight angles have considerable impacts on the point cloud and could cause large displacements. In addition, generated effects are proportional to the measurement range.
Inconsistencies on the point cloud can also be induced by trajectories errors. Trajectory errors can be time-varying and therefore an estimate of constant parameters, the boresight angles, would not be efficient to remove inconsistencies in the point cloud generated by trajectory errors. Indeed, the correction model is not appropriate. To overcome this, work has been conducted on the strip adjustment which includes corrections to the trajectory, and some are based on the ICP (Iterative Closest Point) approach.
The method developed by Geocue is also based on ICP and the goal is to estimate errors on the trajectory. This developed method is rigorous since real points are considered, and the model also considers the geolocation of points and thus raw lidar data are used instead of the georeferenced point cloud. In doing so we do not have undesirable effects that can be induced by interpolation, as is the case with most existing methods. The uncertainty associated with each point is also computed from uncertainties of raw data: this enables a reduction of the impact of uncertainties related to the system on the estimated trajectory correction.
The adjustment model of strip adjustment is such a way that it is possible to define some reference lines. The associated trajectory will not be adjusted for the reference lines and the other lines are supposed to stick on them. These references lines can be supplied as raw data or directly as a georeferenced point cloud.
Strip adjustment is fully automatic and requires a limited number of input parameters. Advanced parameters are designed and provided for advanced users.
With the Geocue strip adjustment, only points belonging to strips in overlapping/side-lapping areas are used to refine the trajectory. From a multi-strip survey data set, the Geocue strip adjustment module:
- Extracts overlapping areas.
- Preprocesses data and selects a set of Tie points for adjustment of the trajectories.
- Adjusts, by an advanced Least Square method, a time-dependent trajectory.
Optimal Context for Strip Adjustment
The strip adjustment is a tool that should be used to refine the consistency of a point cloud. So, it should be noted that: strip adjustment assumes to have a good initial orientation of strips and a refinement will be performed.
In general, airborne laser systems data are collected strip-wise with some overlap for full coverage of
a survey area. Even if the lidar system is well calibrated (for boresight, lever-arms, timing) and free of systematic errors from integration parameters, there may be time-varying navigation errors generated by the GNSS and/or IMU uncertainties and weak estimation of the trajectory may occur. Indeed, due to the limited accuracy of navigation data, as given by the GNSS and IMU, data gaps may be found within the overlaps between strips. In addition, trajectory measurements are affected by external influences (number of satellites, multipath, loss of carrier phase…), thus their accuracy could dynamically change. This usually causes time-dependent navigation (attitude and position) errors in the lidar strips.
To overcome this problem, strip adjustment can be used to improve the data consistency within overlapping areas.
Before using the strip adjustment, we should make sure to have a good initial orientation of strips, this means a good calibration of the survey system should been done. Then, proper boresight angles, lever arms, and potential latency should be known and it is advised to use the strip adjustment only when needed to improve the point cloud precision.
To summarize, if after geocoding data with proper integration parameters (boresight angles, levers arms, and latency) there are still undesirable inconsistencies in the point cloud, the strip adjustment should be used to improve the consistency of the point cloud.
See Strip Calibration.
See How to use Strip Adjustment.
Or, for some recommendations on using Strip Adjustment for Processing Multiple flights.
