First Mobile Laser Scanning of Forth Bridges Leads to £300k Project Greenlight

27th January 2015 – Three bridges over the River Forth in the East of Scotland will soon represent three centuries of Scottish engineering, innovation and design. Between them, the Firth of Forth Rail Bridge, the Forth Road Bridge and the soon-to-be-complete Queensferry Crossing will demonstrate the pioneering use of a range of engineering solutions and offer a visual record of the development of British trade, engineering and construction from the 19th to 21st Centuries.

Renishaw was invited by the Centre for Digital Documentation and Visualisation (CDDV), a collaboration between Historic Scotland and The Glasgow School of Art’s Digital Design Studio to undertake the first mobile laser scanning of two of the three bridges – the Firth of Forth Rail Bridge and the Forth Road Bridge. Having already completed surveying for Historic Scotland, including inaccessible sites such as Kisimul Castle in the Outer Hebrides, Renishaw understood the benefits that long-range mobile laser scanning offered the collaboration, faced with the challenge of mapping sensitive sites in remote areas.

CDDV had identified mobile laser scanning as the most efficient and accurate method of surveying the bridges, since it allows for the recording of a huge volume of data, to a level of detail previously not possible. Renishaw was approached to conduct a survey of a section of each bridge, so that the ease of data acquisition and the suitability of the data for modelling could be determined. Since the digital data acquired through 3D laser scanning can be modelled into a vast range of deliverables – flythroughs, animations, point clouds and drawings – CDDV hoped to create a digital archive for future generations, providing virtual access to sites which are often difficult to access.

The proposed laser-scanning project posed unique challenges: the need to capture fine detail of such complex structures meant that vessel-based scanning was a necessity, yet high traffic on the riverway, and tidal patterns that limited access to the underside of the bridge to specific time windows, required careful planning. The area is renowned for poor Global Navigation Satellite System (GNSS) coverage, essential for the laser scanner positioning and navigation, and the safety risks involved in sailing too close to the bridge were additional factors that Renishaw was able to mitigate, using its two decades of experience in 3D laser scanning.

During the project planning stage, Renishaw’s spatial measurement team used tidal charts, navigation charts and studies of GNSS windows to plan the optimal time to carry out the data acquisition. Lead operator Jonathan Robinson advised use of a single-head unit, to minimise time spent on calibration on the day: essential given the possibility of downtime.

Jonathan Robinson said: “The Firth of Forth Rail Bridge is over 2.5 km long and 156 m high. We planned to survey both from the river to capture engineering details from the bottom of the bridge, and from surrounding roads to acquire data on the top of the structures and along the river banks. Renishaw develops and sells a range of Dynascan models, each one designed to give optimal performance, depending on the unique requirements of a given project. Our experience in many challenging environments, and above all our expertise in pre-planning, are as essential as the system itself in enabling us to complete projects to time, even when there are many variable factors, which are impossible to predict.”

The Renishaw team chose a Trimble MX2 single-head laser scanner to undertake the laser scanning, installing the system on the front of a coastguard rescue boat. The on-board GNSS-aided inertial navigation system provided the position and orientation of the scanner at any given moment to a high degree of accuracy, and as each model is factory calibrated, installation is quick. This meant that as much of the team’s time as possible could be spent gathering data; essential for this project with the risks of downtime posed by river traffic, tides and poor satellite signals.

The use of a single-head unit – which captures less data in a given time period than a dual head scanner, but is more suitable for marine applications due to the increased lateral range – meant that the boat needed to zig-zag up the river in order to ensure coverage of the full site. The boat and scanner were taken underneath both the road and rail bridges, and further scanning was done by sailing alongside the bridges.

Laser scanners such as the Trimble MX2 and the Renishaw Dynascan range, capture data by firing a laser beam from a scanner at a solid surface. The laser beam is reflected by the solid surface and bounces back to the scanner forming a data ‘point’. Because the position and orientation is known to a high degree of accuracy, the speed of light and ‘time of flight’ can be used to calculate a distance measurement. Laser scanners simply measure the distance between the scanner and a solid surface: the advantage is that they can quickly measure millions of survey points at hitherto unknown levels of accuracy. This collection of points is known as the ‘point cloud’, and it is the point cloud that CDDV’s own digital experts will use to create models, maps, flythroughs and other operational and educational resources.

Whereas traditional survey methods would have involved large teams and huge timescales, possibly disruptive to local river or road traffic, and exposing surveyors to risk, Renishaw’s team of two people took just one day to complete the scanning work. In fact, Renishaw scanned the entirety of two bridges, as well as capturing photographic imagery to aid with digital modelling and colourisation, achieving much more than CDDV had anticipated and contributing to the Scottish Government’s decision to commission CDDV to carry out a full-scale bridge surveying project. Post-processing by Renishaw’s data processing team took a further three hours, meaning CDDV had their data within 36 hours of the coastguard vessel setting sail.

CDDV’s project manager, Dr Lyn Wilson said: “Much more was achieved in the day than we had thought possible. The work that Renishaw undertook for us played a vital role in enabling us to assess the feasibility of fully laser scanning all the bridges.

“After considering various surveying technologies, we were confident that laser scanning offered the speed and accuracy we required, whilst minimising the disruption to road and river users. Our goal now is to carry out further laser scanning of the bridges and their environment. Ultimately, the digital models of the bridges that we will create will support a broad range of development and educational goals, including conservation and management, the development of engineering talent in Scotland and the promotion of the area and its rich industrial heritage across the world.

Mark Reid, Renishaw’s UK/RoW Sales Manager for Mapping Systems said: “The Forth Bridges test-project is one of several innovative and significant mapping projects we have undertaken with Historic Scotland in recent months. “Mobile laser scanning makes data easier and quicker than ever to acquire, but it is CDDV’s ambitious plans for the modelling of this data to achieve their very worthwhile conservational and educational goals that make this project stand out for us. Accurate data enables better planning and decision-making, and that is exactly what CDDV understand. We look forward to working with them again in the future.”

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