Kinematic terrestrial laser scanning of the River Thames

A kinematic scanning project carried out on the River Thames in London, UK, has demonstrated the advantages of mobile mapping under real-world conditions within a limited time frame. Operating from a launch vessel, the survey covered a 12km stretch between Tower Bridge and Lambeth Bridge, capturing high-density laser data in radar mode. The project provided valuable insight into trajectory calculation, data quality and overall system performance in a dynamic river environment with constant vessel movement and variable GNSS conditions.

Launched in 2022, the RIEGL VZ-600i 3D terrestrial laser scanner – the instrument used in this survey – introduced a revised form factor combining a smaller and lighter build with improved performance. The scanner operates at a maximum laser pulse rate of 2.2MHz and features a polygon mirror design, enabling high-speed data acquisition alongside the measurement accuracy and reliability characteristic of the VZ-i series.

In terms of versatility, the laser scanner offers four laser pulse speeds (measurement modes) which cover up to 1,000m of ranging capabilities. "This unique combination delivers a 6mm-resolution-at-10m scan – or, in long-range scanning, 600mm at 1,000m – in 25 seconds of scan time. With time for initialization and pose calculations in real-world applications, this translates into the potential for a scan position per minute, or better," explains Dave Foster, managing director of RIEGL UK.

Onboard registration

Robust, near real-time onboard registration (not course alignment) supports field operations. Progress of registration can be viewed via the large touch screen, to monitor what has been captured. This can be set to slice at user-defined intervals in order to show only the relevant data on screen, or the whole registration file – displaying all scan positions registered – can be viewed if desired.

Because users can select from a number of point resolutions to suit the scan in question, and can also set user-defined options, they no longer have to scan all positions at maximum range and higher resolution to account for the range selection. Therefore, the scanner offers significant time savings. “Even whilst in static or ‘traditional’ mode, the VZ- 600i delivers huge productivity gains and therefore key return on investment,” adds Foster.

RIEGL was invited to demonstrate the kinematic capabilities of the terrestrial laser scanner in a real-world project close to GEO Business 2025. The team was able to source a launch vessel and operate on the River Thames, London, both for a demo and to extract data for analysis and testing.

Rapid project setup

Due to the relatively short notice, the setup of the project was really challenging, according to Foster. “The scanner was mounted simply on a tripod and held secure on the rear deck area of the launch. This proved stable for this particular one-off project, but would not be advised for regular survey operations,” he says.

The team was limited to one hour on the river. After careful calculation, the survey was commenced from near Tower Bridge and completed an out-and-back route to Lambeth Bridge, just upstream of the Palace of Westminster. This was a total of distance of 7.5 miles (12km).

The kinematic mode of the VZ-600i is enabled by the enhanced sensors: inertial measurement unit (IMU) and RTK GNSS. Higher laser pulse speeds allow the rotation of the scanner in radar mode to obtain a substantial amount of data. There are two key modes to operate from a moving platform: radar mode (continuous turning of the scanner) and line mode (scanner fixed to provide essential a profile scanning option as the platform moves).

“Preferably, two scanning operations should be completed – one in radar mode, then further in line mode. The data captured in radar mode forms a robust trajectory calculation from which data obtained in line mode can be processed. The data captured in line mode is by default a much richer dataset but is only applied in one direction,” states Foster. “The River Thames project was set up to be completed only in radar mode, but any future opportunity would allow for line-scan mode to enable a richer dataset.”

Reliable GNSS information and high-resolution images

VZ-i series scanners support simultaneous acquisition of RGB imagery and scan data. Where images are only required for point-cloud colourization, this is a positive impact, and does not slow the scanning operation.

The RIEGL RTK GNSS is the sole additional hardware requirement. This enables consistency of time stamps and related information to ensure the trajectory path recorded by the scanner is accurately calculated in the software. This can now be paired with the Sony ILX-1LR to provide imagery to colour the resultant point cloud. The use of the Sony offers a higher dynamic range and image resolution together with option to record RAW, jpg images and HDR image creation.

“In total, the survey was completed in 57 minutes, including obtaining a figure-of-eight path around Tower Bridge, where we were fortunate to have a brief quiet spell of river traffic,” adds Bernhard Groiss, application engineer at RIEGL Laser Measurement Systems, who was also on board for this project. “The weather at the time was dry with patchy cloud and breezy. Given the near-constant river traffic, there was significant wake from a number of vessels of varying sizes. This meant the onboard IMU had roll, pitch, yaw and some heave to contend with.”

Smooth processing

The software processing for the laser scanner is built upon a number of RIEGL principles, but tuned for the kinematic functionality. Overall processing was completed in around three hours. Processing required both initial and refined calculations of the trajectory path before the processing of the laser scan data. RTK GNSS solutions varied from fix through float to single. As a result, this required further processing with the laser scan data in order to refine the trajectory path, before a final refinement was applied to produce the registered point clouds.

High-accuracy measurement data from hard-to-reach spots

The kinematic survey resulted in a complete point cloud of approximately 380 million measurement points, which corresponds to a storage requirement of about 18GB. Depending on vessel speed and distance of the measured objects from the scanner, this resulted in an average point density on the bridges of approximately 500 to 10,000 points per square metre. The standard deviation of the distance, calculated relative to a fitted plane, was approximately 4-10mm.

“These facts demonstrate how flexibly, fast and precisely such a scenario can be captured with the RIEGL VZ-600i in kinematic operation by surveying bridge structures even in areas that would otherwise be inaccessible from any vantage point,” comments Groiss. “Combined with individually high-precision surveyed control points, external verification can also be carried out and thus, overall accuracy further improved.”

The rich dataset obtained, even within just radar mode, offers opportunities for structural monitoring, design, planning, construction as well as heritage documentation, suggests Foster. “The combination of scanner and useability would be attractive to Port and River Authorities who are required to both monitor and document waterside structures, especially where access is limited to a waterborne vessel,” he adds. “From the land, it can of course be used as a traditional laser scanner, but also be mounted to a vehicle or even backpack for a truly versatile static-to-mobile mapping system.”

Lessons learned

Foster looks back on the demo with satisfaction. “Data quality, despite only being captured in radar mode, was high. We can only surmise what it would have been had we been able to complete another pass in line scan mode,” he states.

“RTK fluctuations between fixed, float and single meant that processing took a little longer than might have been originally anticipated. The fact that the survey was able to be processed so well was a testament to the robust nature of the processes within RiSCAN PRO to bring the trajectory paths together,” he continues.

For similar applications, RIEGL refers to the option to record raw GNSS data and post-process that with base station information. That would enhance the absolute accuracy of the dataset and make trajectory calculations more robust. “Due to the tight schedule, that was not possible for this project. Nevertheless, the team proved the capabilities of the scanner for kinematic data acquisition and thus its real versatility and highest efficiency for a maximum of applications,” concludes Foster.