Integration of laser and photogrammetric data for calibration purposes

Laser scanners are becoming popular since they provide fast and dense geometric surface information. However, sudden elevation changes along the surface are not clearly visible in the laser data due to the sparse distribution of captured points. In general, laser data provides high density surface information in homogenous areas and low density surface information elsewhere (i.e., object space break-lines). Photogrammetry, on the other hand, provides less dense surface information but with high quality, especially along object space discontinuities. Hence, a natural synergy of both systems can be inferred and consequently integration of the respective data would lead to higher quality surface information than that obtained through the use of a single sensor. However, prior to such integration, both systems should be precisely calibrated and aligned. The calibration is usually carried-out for each system independently using additional control information. In this paper, the calibration of the laser and photogrammetric systems is evaluated by checking the quality of fit between co-registered photogrammetric and laser surfaces. The paper starts by introducing a registration procedure where a set of linear features is extracted from both sets. First, planar surfaces from laser data are extracted and adjacent planes are intersected to determine three-dimensional straight line segments. Secondly, linear features from the photogrammetric dataset are obtained through aerial triangulation. A mathematical model for expressing the necessary constraints for the alignment of conjugate photogrammetric and laser straight lines is established. The model ensures that corresponding straight lines will be collinear after registering the two datasets relative to a common reference frame. The quality of fit between the registered surfaces is then used to evaluate and/or improve the calibration parameters of the photogrammetric and laser systems. In this paper, an experiment with real data is used to illustrate this concept. The registered surfaces in this example revealed the presence of systematic inconsistencies between the photogrammetric and laser systems. The pattern of these inconsistencies is found to resemble the effect of un-calibrated lens distortion. In this case, the laser data is used as control information for the determination of lens distortion, which when considered leads to a better fit between the registered surfaces. The estimated lens distortion using the laser was found to be very close to that determined through a rigorous camera calibration procedure.