Mobile LiDAR Systems in Comparison: Tasks and Efficiency of Flexible and Permanently Installed Measuring Systems in Route Monitoring
LiDAR (Light Detection and Ranging) surveying systems are characterised by specific technological advantages, including speed of data acquisition, independence from ambient light conditions and the ability to detect multiple targets in one direction.
These characteristics explain why LiDAR is one of the favoured surveying technologies in the railway sector – not only by means of systems installed on a train, as in Mobile Laser Scanning (MLS). Relevant surveying data is also acquired using Airborne Laser Scanning (ALS), Terrestrial Laser Scanning (TLS) or UAV-based (Unmanned) Laser Scanning (ULS), depending on the task.
Airborne Laser Scanning is predestined for topographic surveys of very extensive areas. This comes in as a plus in inventory or for planning purposes, where a terrain model (Digital Terrain Model DTM) is needed. Airborne LiDAR systems are also used from lower flight altitudes, both from manned helicopters and by integrating miniature systems on unmanned aerial vehicles in Unmanned Laser Scanning (ULS). Typical tasks – apart from the recording of the immediate railway infrastructure and route monitoring – include documentation of the surrounding topography to secure rail routes and to evaluate the extent of damage or potential risk due to environmental influences. For highly accurate measurement of very complex environments, static Terrestrial Laser Scanning (TLS) is used. The methodological advantages of TLS devices are particularly evident in tasks where indoor spaces, roofed areas or even underground rooms are to be surveyed. TLS is chosen when the highest measuring accuracy in the millimetre range is required. Ideally, TLS data from locations that require special attention and detailed accuracy are entered into a primary information data archive. Finally, there is the operation of the measurement system from the train itself. Mobile systems have been used in this set-up for several years. Aiming for utmost point cloud quality the RIEGL mobile mapping systems are equipped with up to three VUX-1HA high-accuracy laser scanners rigidly coupled with a high grade INS/GNSS sub system. The system architecture, which has to ensure a fix cohesion of all single parts and the perfect synchronisation of the integrated sensors even in the event of – as is to be expected on the train – strong vibrations, is of importance. The higher the pulse repetition rate of the individual scanners, the closer the point spacing within a scan line. This enables the exact measurement of even thin structures such as rail heads and overhead lines. In addition to the pulse repetition rate, the frequency of the scan lines covering 360 degrees is a decisive key figure. The higher the number of scan lines per second, the higher the speed of the train that can be selected without reducing line spacing density during the measuring run. RIEGL may refer to many years of experience with the mobile LiDAR systems RIEGL VMX-450 and its successor system RIEGL VMX-2HA.
ULS data acquired with the RIEGL VUX-SYS airborne laser scanning system at the railway station Gare St. Charles, Marseille.
These systems comprise two 2D laser scanners oriented obliquely to each other. This orientation generates a simultaneous forward and backward view, enlarging the field of view of the measuring system and reducing shadowed areas. The two measuring ranges overlap to a large extent at surfaces in the centre of the area of interest. This creates an increase in redundancy, detection reliability and generates a regular crossed scan pattern on the track body. The mobile mapping system’s measuring head is installed on the train. The control components (PC, data storage device, power supply) are located at the operator’s station inside the rail car. The installation height of the measuring head determines the field of view and the measuring angle of the laser beam on the rail environment. The main task of LiDAR systems on trains is to monitor the clearance area along the rail track. RIEGL “Smart Waveform” technology allows detecting multiple targets per laser shot, to achieve an optimal distribution of measurement points, and the output of calibrated amplitudes and reflectivity values. Due to these features, the technology is particularly suitable for surveying and penetrating vegetation. In the railway sector, a meaningful vegetation survey means that safety and maintenance measures can be planned according to the specific local requirements, thus saving on labour and materials. This includes, for example, cutting back obstructive vegetation, applying herbicides along the rail track or maintaining and repairing the railway embankment, whose profile is clearly detected by LiDAR even under vegetation.
The compact design of the VMX450 and the VMX-2HA systems facilitates project-specific measurements, for which the flexible use of the system on different platforms is advantageous. Additionally, there is currently an increasing demand for permanently installed mobile mapping systems for integration into specifically equipped measuring trains. The RIEGL VMX-RAIL is a such a system designed for permanent installation and operation.
Three RIEGL VUX-1HA laser scanners provide a unique crossed point cloud pattern along 360 degrees of the clearance profile.
The technical concept of the VMXRAIL is based on the combination of three high-accuracy laser scanners compactly mounted on one system platform. The integration of three scanners in one system achieves a uniquely dense scan pattern with crossed scan lines. The special arrangement and alignment of the scanners results in a scan shadow-minimised and well-structured point cloud. The point pattern of the VMX-RAIL thus offers an optimal basis for object extraction and 3D modelling.
The measuring head meets the highest standards regarding material stress and protection against environmental influences. The laser scanners are encased in special protective housings which can be dismantled separately if necessary. The same applies to the INS/GNSS unit and the synchronisation electronics. In order to enable integration with different models of measuring trains, the mounting device, which also contains the vibration absorbers, is implemented separately and can be flexibly adapted to the respective conditions.
The number of applications in which digital 3D point clouds or digital 3D models are used in rail-specific applications is constantly increasing. While clearance checking and asset management are still the core applications for kinematically acquired 3D data, today LiDAR is also used for recording highly accurate data for infrastructure maintenance planning. Strong innovation trends in response to the new demands of, for example, increased traffic volumes and automation, promise further challenging tasks for LiDAR surveying in the near future.