First mobile laser scanning of Forth Bridges leads to £300k project greenlight
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. Postprocessing 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 testproject 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.”