Causes of Errors include positional erros with the GPS/IMU, range, scan angle, boresight angles, lever-arm offsets. These are systematic errors and corrections can be applied over the entire dataset.
Range Errors – Not corrected for, occur due to recording of the time between emission and return of laser – centimeter scale.
Beam Divergence – Common error in places with terrain effects which result in vertical errors.
Scale Error – caused by the rotating mirrors which occasionally have delay. Occurs in across-track profiles near edge of flightline.
Deceleration Error – related to mirror deceleration at the edge of a scan swath.
Encoder Latency – scan lag, timing delay between laser and scan angle measurement
Pitch- Slope Error – Laser head is not exactly perpendicular with mirror (likely needs factory calibration)
Lever-Arm Offsets – GPS, IMU and laser head not at same location. Measurements between these features need to be accurate.
Boresight Errors – Fixable Error within Microstation
Boresight Angles – very slight angular differences between IMU and scanning mirror. Visible in profile, looks like a shift in data between lines.
Boresight Roll Error – positions shift across-track. Due to angling of the laser. (parallel to flightline)
Boresight Pitch Error – As you move away from nadir you can increase the position shift (perpendicular to flight line)
Boresight Heading Error – positions rotate around the mirror
Calibration methods can be Rigorous (sensor specific, uses range data), Quasi-Rigorous (error specific, no range data) or Non-Rigorous (purely data driven (no trajectory etc).
Terramatch uses a Quasi-Rigorous technique and mostly corrects roll, pitch and heading errors.
All of these methods use geometry of points to minimize discrepancies (such as rooftops).