London, UK: Westminster Abbey, October 2020: Westminster Abbey has been standing on the north bank of the Thames for almost 1,000 years. Its spires, pinnacles, flying buttresses, gargoyles, finials, crockets, corbels and carved bosses are recognisable throughout the world as the pinnacle of English architecture, and its walls are steeped in the history of Coronations, Royal Weddings and St George’s Day celebrations. Its flagstones have felt the tread of generations of worshippers and visitors.
The Abbey exists also, however, as a ghostly presence in a separate dimension. The most digitally accurate BIM model of the building, generated using laser-scanning technology, exists in three dimensions in virtual space as a cloud of many millions of points, each defined by 3D co-ordinates to within millimetres of the Abbey’s actual external and internal surfaces. Every window, door, buttress, butt-joint, crook and cranny is digitally present in this model: a three-dimensional map of the building in which all parts of its external and internal elevations are represented, albeit in a different medium. For BIM specialists, this digital twin can be read, interrogated, understood and compared.
The Twinned Building
With laser scanning and BIM increasingly part of the standard tools in the tool-bag of many heritage professionals in the UK, specialists in conservation and listed building architects are faced with new and emerging ways of understanding and safeguarding structures for the future. There is a still, as yet, uncharted confluence of the practicalities of on-the-ground traditional conservation practice, and the promise of emerging technology. These structures are old; some of them are in need of urgent attention and preservation; and they are also facing the accelerating and potentially catastrophic impacts of the climate emergency, development pressure and general wear and tear. Combining the power of BIM, and the latest developments in laser-scanning technology could help enable traditional architects, architects, quantity surveyors, and building services engineers working in historic buildings and on historic structures to overcome those challenges.
Mapping the past
Terrestrial laser-scanning, also known as LiDAR, has rapidly become part of the toolkit of every heritage professional over the last 20 years or so. Although the idea of creating a map using reflected laser-light is relatively straightforward, the actual operation of a LiDAR device, and the operation of these scanners has developed and improved with time. Millions of laser pulses are emitted from a scanner each second, and the duration of each pulse’s round trip (emission to target and back) is measured. This data can be used to build up a point-cloud – a three-dimensional grid of co-ordinates that maps the external and internal surfaces of everything in the scanner’s line of sight with the precision of a few millimetres. Most scanners can build up one million points per second, and many contemporary commercial scanners offer an accuracy of ±2mm at distances of up to 100 metres, generating data which would have been unimaginable to most professionals 20 years ago.
UK-based Historic England’s Geospatial Imaging team have undertaken laser-scanning of some of the UK’s most important historic monuments in recent years. In 2011, a complete digital record of Stonehenge was undertaken by the team in order to create a baseline for future understanding and conservation of the prehistoric monument; and in 2012 laser-scanning of the world’s first iron bridge, in Shropshire, was undertaken to create an accurate, millimetre-precise digital model. The model has been used to plan conservation and repair works, as well as to monitor the structure’s ongoing movement. These are not static academic exercises but dynamic, living digital archives, which will outlast photographs, films, videos and drawings in terms of their accuracy and longevity.
In addition, by using the technology in ways that go further than pure digital record-making, fascinating new insights into historic buildings can be found. Historic England also laser-scanned Carlisle Castle and its results exposed previously unseen distortions in the building’s walls, which are crucial for future analysis of structural movement and how to best repair it. Westminster Abbey, as described above, has been laser-scanned several times, creating a full digital model of the entire complex. This allows **conservation architects** to model the Abbey in three-dimensions, understand its geometry and behaviour and to effectively visualise both the building’s features and ongoing changes to it, making it a major advantage when planning and carrying out interventions in the building.
Conservation in Practice: The Example of Laser Scanning
Perhaps the most impressive examples of the use of laser scanning in practice are in the documentation of cathedrals. Cologne Cathedral in Germany, whose 157-metre-high Gothic spires are some of the tallest in Europe, was entirely scanned, inside and out, to create a complete digital model of the entire building. The BIM model records every pinnacle, buttress and carved detail to millimetre precision and, along with an extensive archive of documents, has become the basis for the cathedral’s ongoing conservation programme. The building has, since its completion in 1880, never stopped being worked on by a team of conservators, who have been able to make more detailed and extensive records of stone decay, conservation and repair than ever before possible.
In the UK, Exeter, Ely and Wells cathedrals have all been the subject of extensive laser-scanning surveys. Such surveys have revealed new information about medieval construction techniques, showing the way in which builders used local materials and adapted their methods according to site conditions and local geology. At Exeter, it was found that the world-famous vaulted ceiling – the longest unbroken Gothic vault in existence – has a very slight but significant sag. The point-clouds also contain records of past repairs and alterations to the buildings which had been lost from memory and are now effectively digitally recorded.
Possibly the most tragic, and hence important, example of the use of laser-scanning is Andrew Tallon’s work at Notre Dame Cathedral, Paris. Between 2010 and 2012, Tallon – an art and architectural historian at Vassar College in New York – laser-scanned the entire cathedral in exhaustive detail to produce a digital record of over one billion data points and an all-encompassing map of every surface of the medieval masterpiece. In April 2019, the cathedral was devastated by fire, and the roof and spire were destroyed. In the restoration project that followed, Tallon’s scans, for which he had died in 2018, became an irreplaceable record of the destroyed components of the building, allowing architects to understand with accuracy exactly how those components related to the rest of the building. The scans themselves are an insurance policy for the building, as well as a historical and architectural record.
HBIM: from point-cloud to BIM
Building Information Modelling has become the norm for the design and management of new buildings, but what place can it have in the understanding of historic buildings and structures? The standard model of BIM has a narrow idea of what a building is and is assumed to be built with standardised, repeatable elements. Window openings are regular; construction grids are expected to be regular; construction details are standard. Medieval masons, by contrast, had no standard measurement, and their work varied with site conditions, available materials and required tolerances. They adjusted their work, often with no apparent pre-planning, to achieve an end result. To adapt BIM to historic structures requires a new way of thinking about BIM. Enter Heritage Building Information Modelling, or HBIM.
HBIM starts with point-cloud data, generated using laser-scanning, but turns it into something much more valuable than a mere 3D photograph of the building. Heritage architects and building conservators use that data to interrogate, look into and around every surface and space. Instead of a single surface, walls, floors and windows become individual objects in the HBIM model, with their own properties, such as material, construction date, condition and history. Windows are modelled with specific information about the glazing, frame construction and paint analysis and historic repairs and alterations. This task of turning point-clouds into BIM models is time-consuming and labour-intensive, but the result is a digital twin which acts as a permanent repository for all that information.
HBIM is just as useful during a building’s conservation life-cycle as it is as a record of what a building once was. During the assessment phase, the model provides a structure around which to organise all other information – survey data, materials analysis and historical research and recording can be linked to the three-dimensional model in a structured way. At the intervention-planning stage, different approaches can be trialled in the model in order to better understand how the proposed changes will affect the fabric and overall appearance and performance of the building. At the construction stage, the model can provide a detailed, exact record of what is required and help ensure that contractors and sub-contractors understand what is required of them. Once construction is complete, the updated HBIM model becomes the basis for an ongoing, living digital record of the building’s history.
A number of pioneering HBIM projects have shown the potential for the technology. Westminster Abbey, as described earlier, has been the subject of laser scanning by Historic England, and that data has been developed into a complete BIM model, with the structural information, materials data and conservation records all collated within it. The model allows facilities managers to plan access for events, monitor maintenance activity and understand how the various building services systems in the Abbey interact. When it comes to planning conservation or other work in the building, architects and conservators can access the model to understand the existing conditions, understand the history of previous interventions and ensure that any new work respects the building’s significance. The model is a living document, which is constantly updated as new information becomes available or new work is carried out.
Applications: Monitoring, Planning and Preserving
One of the most practical applications for laser scanning is structural monitoring. Buildings move. Always have. Timber shrinks and swells with humidity, masonry settles under its own weight, and foundations heave and subside in response to groundwater levels. But if movement is gradual and consistent, it may be benign; if rapid or unexpected, a problem. How can we tell the difference? We need to be able to measure accurately to establish how much movement is occurring. Repeated laser scanning surveys, carried out at intervals of months or years, can reveal movements of just a few millimetres, allowing an early warning of potential structural issues before they develop into something more serious. Laser scanning is revolutionising conservation, shifting it from reactive repair to proactive monitoring and management.
Monitoring the Iron Bridge has been one of the primary goals of Historic England’s scanning programme. The first scan, carried out in 2012, has provided a baseline record of the bridge’s geometry to which all subsequent scans can be compared. Repeated scans carried out at regular intervals are compared to the baseline to monitor changes over time. The analysis has shown that the bridge is still moving, the arch spreading slightly as the abutments shift. This is not a surprise: the bridge has been moving since it was built. But by precisely quantifying that movement, engineers can distinguish between historic movement that has stabilised and new movement that may need to be addressed. The digital monitoring programme provides objective data to inform decisions rather than rely on guesswork.
Accurate survey data in combination with HBIM modelling provides conservation architects with a powerful planning tool. When carrying out repairs or alterations to a listed building, architects must balance a complex set of often competing requirements. These include the need to preserve historic fabric, meet modern performance standards, comply with regulatory requirements, and work within budget constraints. Digital models allow different approaches to be tested in the virtual world to understand their implications before committing to a specific solution. The laser scan of Oxford Town Hall has enabled the architects to plan the insertion of modern building services – heating, ventilation, electrical systems – in a way that minimises impact on historic fabric. By understanding the building’s geometry with such precision, architects can identify service runs that avoid key architectural features and structural elements.
The archival value of digital documentation may turn out to be its most enduring legacy. Buildings change through use, weathering and intervention, often gradually and imperceptibly. Traditional methods of documentation – photographs, drawings, written descriptions — can capture only a limited set of the building’s condition at a particular point in time. Laser scanning provides an accurate record that captures everything that’s visible, including information the surveyor may not have recognised as important at the time. When future conservators look back on these buildings, they will have access to extraordinarily detailed records of earlier states and be able to understand how the building has changed and make better-informed decisions about its future care. In an era of rapid climate change, when heritage buildings are increasingly subject to extreme weather events and other environmental stresses, these digital archives may become critical tools for understanding and responding to building pathology.
Challenges and the Path Forward
Applying digital technology to heritage conservation is not without its challenges. One of the most basic is simply the data itself. A comprehensive laser scan of a large building generates billions of data points, resulting in files that can be several hundred gigabytes in size. Processing, storing and managing this data requires significant computing resources and specialist expertise. Turning point clouds into practical HBIM models remains a labour-intensive process requiring skilled professionals who understand both digital technology and historic construction. Many conservation practices, especially smaller firms, lack the resources to invest in the necessary hardware, software and training.
Standardisation is another issue. BIM for modern construction has developed common standards and protocols that enable different professionals to share data and collaborate effectively. HBIM, by contrast, remains a more open field, with different projects adopting different approaches to modelling historic elements. This makes it harder to share data between projects or compare results. Historic England and other heritage organisations are working to develop guidance and standards in this area, but the inherent variability of historic buildings makes standardisation a more complex problem than in modern construction. Each historic building is unique, and modelling approaches need to be flexible enough to accommodate this while also maintaining sufficient consistency to enable data sharing.
Careful consideration is needed on what to model and at what level of detail. Documenting every surface irregularity and historic mark might be an ideal goal, but it would create models so complex they would be difficult to use. **Listed building architects** need to make pragmatic decisions about appropriate levels of detail. Balancing accuracy against usability. A model for structural analysis will require different information to one created for facilities management or public interpretation. Having a clear understanding of the project objectives and how the model will be used helps to guide these decisions, but they also require judgment and experience that comes only with practice.
The cost of digital technology remains a significant barrier to wider adoption. Laser scanning equipment represents a significant capital outlay, and the specialist skills required to operate the scanners and process data command a premium rate. For major projects involving significant heritage assets, these costs can be justified. But for more modest interventions on locally listed buildings, the expense may seem hard to justify. As the technology matures and becomes more widely available, costs are gradually coming down. Handheld scanners and photogrammetry techniques now offer lower-cost alternatives to traditional terrestrial laser scanning, although often with reduced accuracy. Finding the right balance between cost and capability for different project types remains a challenge.
Looking to the future, several promising developments are likely to enhance the role of digital technology in heritage conservation. Artificial intelligence and machine learning algorithms are being developed to automate aspects of point cloud processing and model creation, potentially reducing the time and cost involved in creating HBIM models. Integration with other technologies — thermal imaging, ground-penetrating radar, moisture mapping and so on — will create even more comprehensive digital twins that model not just geometry but also building performance and condition. Cloud-based platforms are making it easier to share and collaborate on large datasets, enabling distributed teams to work together more effectively.
Democratisation of digital heritage documentation represents both an opportunity and a challenge. Consumer-grade scanning technology and photogrammetry software now mean that enthusiasts with a few hundred pounds and a decent camera can create three-dimensional models of heritage buildings. This grassroots documentation effort is creating an enormous archive of digital heritage records, but quality and accuracy can vary widely. Professional organisations and heritage bodies are working to provide guidance and training in this area to ensure that volunteer efforts result in useful data while also maintaining appropriate quality standards.
Guardians of the Digital and the Physical
As we stand at the confluence of ancient craft and cutting-edge technology, the role of conservation architects and heritage professionals are changing in interesting and important ways. The skills and qualities that have always marked out the best conservation practice remain as important as ever. Deep knowledge of historic construction, understanding of materials and their behaviour, sensitivity to architectural significance, the ability to read and understand historic buildings in all their complexity – these remain the foundation of conservation excellence. But these traditional competencies are now being augmented by a range of digital capabilities that would have been unimaginable to previous generations of conservators. The ability to capture a building’s geometry with millimetre precision, to create comprehensive digital twins that integrate information from a wide range of sources, to monitor structural behaviour over time are tools that fundamentally enhance our capacity to understand and protect architectural heritage.
The digital doppelgängers that laser scanning and HBIM create are more than mere facsimiles: they are active tools of conservation that enable better decisions, more effective interventions and more comprehensive documentation. They allow us to see buildings in new ways, revealing patterns and relationships that were always present but invisible. They create permanent records that will outlast any physical archive, providing information that future generations will have access to even if disaster strikes. And they’re changing the economics of conservation, enabling buildings to be surveyed and documented more quickly and comprehensively than ever before and potentially bringing the benefits of detailed documentation to a wider range of heritage assets.
Technology is a tool, however, not a replacement for human expertise and judgment. The laser scanner is brilliant at capturing surfaces but not at interpreting significance. The BIM model is good at organising information but not at determining conservation philosophy. These decisions still need the knowledge, experience, and sensitivity that listed building architects and heritage professionals bring to their work. The most successful applications of digital technology in conservation will be those that recognise this and use digital tools to augment rather than supplant traditional skills and approaches.
As climate change accelerates, as urban development pressures intensify, and as the buildings that we’ve inherited from previous generations age into their third, fourth or tenth centuries, the need for effective conservation has never been greater. Digital technology offers powerful new capabilities to meet this challenge, but only if it is deployed thoughtfully, directed by clear conservation principles and a deep understanding and respect for the buildings in our care. The digital doppelgängers we’re creating today will not just serve our generation but those to come, equipping them with the information they need to continue the never-ending work of preservation. In this sense, laser scanning and HBIM represent not a revolution that overturns traditional practice but an evolution that extends and enhances it, a new set of tools in the service of an ancient calling, helping us to fulfil our role as temporary custodians of a permanent heritage.
