From TLS LiDAR to Videogrammetry: The Paradigm Shift Begins with 2fSLAM
Case Study: Tunnel Geometric Monitoring with 2fSLAM
This case study analyzes the application of the 2fSLAM 3D scanner by 2Freedom Imaging (an image-only-based scanner) for the daily geometric monitoring of a tunnel under construction.
The main objective was to capture 150 meters of tunnel daily with sub-centimeter precision, in the shortest possible time, and with processing that would allow the quality of the work to be certified quickly so as not to interfere with the progress of the work.
Previous Situation and Limitations
Initially, the client used a high-precision static laser scanner (TLS) that required positioning in multiple locations along the tunnel. This process, in addition to being costly and time-consuming, had inherent limitations due to the irregular morphology of the tunnel (lined with shotcrete), resulting in hidden, unscanned areas.
Despite having a common reference system using targets, the semi-automatic alignment required manual intervention to ensure proper merging of the different point clouds. This resulted in a slow capture and processing procedure with the potential for accumulated alignment errors that had to be manually corrected.
Implementation of the 2fSLAM Scanner System
Given these limitations, the 2fSLAM system was implemented, an advanced videogrammetric camera developed by the Spanish company 2Freedom Imaging.
This system integrates three sensors: a high-resolution camera with a global shutter for capturing precise images without distortion from camera movement, and a camera dedicated to Visual SLAM technology, in addition to an inertial measurement unit (IMU).
Historically, videogrammetry has not yielded good results primarily because image selection has not been optimal, usually selecting images only by time or distance, which causes cuts in the trajectory or accumulations of frames in certain places, which in turn generate inaccuracies.
However, on the other hand, the optimization of the photogrammetric process for sequential captures has not been adequately addressed by photogrammetric software either. Thanks to the algorithms developed by 2Freedom, the software optimizes the selection of images obtained in video sequences, overcoming the historical limitations of videogrammetry in infrastructure environments.
In this case study, the operator placed targets on both sides of the tunnel, spaced approximately every 10–15 meters, set up adequate lighting, and completed a full scan of 150-meter segments in approximately 10 minutes.
Our 2fSLAM is designed to easily integrate a lighting system and operate with both combined, allowing the scene to be illuminated while scanning.
The 2fSLAM system enables continuous and dynamic capture without the need for stops or repositioning the equipment. The data processing was then fully automatic, requiring no user intervention, and was completed in an average of 60–70 minutes.
Results and Comparisons
The accuracies obtained with the 2fSLAM scanner are on the order of 7-8 mm (RMS), having been verified by comparisons with static laser scanners and profilometers.
In all cases, deviations remained below one centimeter, meeting the client’s requirements. It is worth noting that by eliminating the need for multiple positionings and manual alignments, both data capture and processing times were significantly reduced.
Conclusion
The adoption of 2fSlam for daily geometric tunnel monitoring has resulted in a substantial improvement in efficiency, accuracy and operability compared to traditional methods with static laser scanners.
Currently, we are witnessing a paradigm shift where videogrammetric systems like 2fSlam are beginning to colonize areas where, until now, only laser scanners (static or dynamic) were capable of doing so.
Added to this is the ability to use AI to detect and segment objects or patterns with images that are functionally more optimal in images than in point clouds, so videogrammetry and 2fSlam scanners are presented as a fantastic alternative to current systems.
Thanks to the technology developed by 2Freedom Imaging, and to the trust of our client, they have been able to integrate a robust, fast and reliable system, allowing a smooth daily workflow without interruptions on the construction site.
Case study in collaboration with Carlson Software.
