A Surveyor's Norman Conquest (Tech Trends Feature)2 Feb, 2009 By: Kenneth Wong
High-definition scanning helps document historic Nenagh Castle.
In 1860, Bishop Michael Flannery launched a plan to upgrade Nenagh Castle, a fine specimen of Norman architecture located on the patch of land he'd just purchased in Tipperary, Northern Ireland. For funding, he relied on donors from North America, among other sources. Naturally, when the U.S. Civil War broke out in 1861, the flow of funds dried up, and the bishop was forced to halt construction. Flannery did manage to leave his mark on the castle in the form of a series of battlements with windows. Little did the bishop know his crowning legacy would help solve a surveyor's dilemma one-and-a-half centuries later.
HDS Comes to Northern Ireland
Theobald Fitzwalter, a hereditary baron and the steward of the region at the time, decided to make Nenagh his seat of power in 1200. During his reign, he started to build Nenagh Castle as his residence, but it wasn't completed until his son's time.
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The castle now is one of the major tourist attractions in County Tipperary. If the baron's builders used any construction drawings, they haven't been unearthed. The Norman carpenters certainly didn't leave behind DWG files to chronicle their progress. So, in 2008, when the Office of Public Works (OPW) decided to undertake restoration works, they commissioned Gridpoint Solutions, a Belfast-based survey firm equipped with high-definition surveying (HDS) technology, to scan the site and help produce as-built drawings of the castle.
A few years ago, Gridpoint Solutions invested roughly $125,000 to acquire a Leica Geosystems HDS3000 laser scanner. HDS laser scanners remotely capture the detailed shape and geometry of target objects — usually existing architecture, facilities, or landscapes — in significantly reduced time. The HDS3000, for example, is reportedly capable of 6-mm accuracy for each scan point at a 50-m distance from the scanner. Leica's HDS hardware is complemented by the company's Cyclone and CloudWorx series of software modules and plug-ins that let surveyors and CAD professionals process the rich point-cloud data from the scanners and convert them into CAD-compatible formats (figure 1).
Figure 1. In Ireland, a market for HDS has emerged from the need to digitally archive heritage sites such as Nenagh Castle (top photo). HDS devices allow surveyors to remotely capture the shape of the target object in point clouds (middle and bottom images).
"We were, in fact, the first survey firm to offer HDS services in Northern Ireland," said Conor Graham, director of Gridpoint Solutions. "We've found a good market for it in the heritage preservation projects."
In recent years, North and South Ireland administrations have adopted digital preservation and archiving as the solution to document archeological sites destined to be replaced by new construction. On its web site, the Public Record Office of Northern Ireland (PRONI) declares its "statutory responsibility for selecting, acquiring, preserving, and maintaining access to records that form part of Northern Ireland's unique cultural heritage." The task traditionally was done on paper, but PRONI points out, "Northern Ireland Civil Service and wider public-sector business needs have changed and, in order to support that change, PRONI has to be in a position to provide a digital archiving service."
How to Flatten a Castle
One summer day, when the highland weather was ideal for scanning, Graham climbed with the 37-lb HDS scanner to the walled site where Nenagh Castle is located. (Graham and other HDS users would be glad to know that next-generation equipment is lighter. Geoff Jacobs, senior vice-president of strategic marketing for Leica Geosystems HDS, pointed out that the HDS6000 weighs 30 lb.)
To remain consistent with its own established archival protocols, the OPW requested that one of the deliverables be an unwrapped (or flattened) 2D elevation view of the castle. "No problem," Graham told the OPW officials. He thought he could find a special button or a command in the software to convert the point-cloud data of the semi-cylindrical shape into a flat surface.
Eventually, he discovered that the function he wanted didn't exist on the scanner's software menu. He would have to manually slice the point-cloud data into vertical strips and realign them into one continuous flat surface. But the structure had very little architectural variation. It was stonework all around, making it incredibly difficult to discern where each strip should begin and end. That's where Bishop Flannery — or, rather, the windows he installed in the 1860s — came to Graham's rescue.
"We did about six or seven scanning sessions from different vantage points," explained Graham. "We registered our [scanner] fields of view to one continuous point cloud, then used features on the tower [which were, in fact, the windows in Flannery's 1860 battlements] to identify and align the different strips of point clouds to be passed to the CAD technician."
Graham explained that to do the same project using photogrammetry (an option the OPW explored before settling on HDS) would require placing targets on the castle wall, a process that could lead to drilling and attaching semi-permanent fixtures in the castle surface. Because the castle is a heritage site, getting the necessary permits and approvals to do anything drastic on its surface would take time. Traditional surveying methods — taking physical measurements of the structure to create as-built drawings of the castle — would have required the use of scaffolding, a more time-consuming process that comes with safety concerns. By contrast, Graham spent only a day at the site, capturing the point clouds that make up the complete, detailed shape of the castle from a distance.
Onsite and Offsite Work
Unwrapping the point clouds was just one part of the client's requirements. The next step called for reproducing a stone-by-stone accounting of the castle's tower, or façade, to be printed on special archival paper for posterity (figure 2). So Graham partnered with a local archeology consultancy, Farrimond MacManus, to trace the stonework — one piece at a time — discernible in the point-cloud density. It's a process that took about four months to complete. But, with the data accessible from a local hard drive, the CAD drafter was able to work in the comfort of an office, with a warm cup of coffee, Graham pointed out.
Figure 2. Realigning the point-cloud data into one continuous surface, Gridpoint Solutions' partners produced this 2D elevation view of the castle. The stonework is not textured but meticulously traced — one piece at a time — to give an accurate accounting of the makeup of the façade.
"In archeology, traditionally, [the clients] want 2D detailed drawings because they can archive these on paper," explained Graham. "No matter what people say, paper archives are still the most reliable." With digital files, some formats could fall out of favor, becoming a casualty of the tug-of-war among software vendors' competition. Digital files could also become corrupt, unreadable after a period of time. A media that survives a decade might be deemed sufficiently durable in the fast-evolving technology sector, but in archeology, time is measured in hundreds and thousands of years.
When he has to do onsite work, Graham carries a Panasonic Toughbook, a rugged laptop computer designed for outdoor environments. With Leica's Cyclone software installed on his machine, he is able to register point clouds right at the site, creating a panoramic view of the target and the nearby landscape that's georeferenced. "This way, if the client drops our deliverables into a GIS [geographic information system] program, it'll show up at the right place," he noted.
From Point Clouds to Physical Parts
Leica's CloudWorx modules allow surveyors such as Graham to work with the large point-cloud data sets directly in the CAD environment preferred by their clients. Currently, CloudWorx plug-ins are available for AutoCAD, MicroStation, Aveva PDMS plant design software, and Intergraph SmartPlant Review.
In addition, Leica provides Leica Cyclone PUBLISHER software for publishing point clouds in an Internet-friendly format for intuitive viewing, measuring, and markup. The output can be viewed using Leica TruView, a free download. With PUBLISHER and TruView, government entities in charge of heritage sites such as Nenagh Castle can publish rich, panoramic views of the sites on their public web sites to promote interest.
The next time someone asks Graham to scan and unwrap a castle — or anything shaped like a castle — he might not have to use the manual alignment method he used for Nenagh Castle. In October, while attending Leica Geosystems HDS and Airborne Sensor Worldwide User Conference in San Ramon, California, Graham met another presenter who's written executable code to automatically accomplish the same task. But he hasn't yet come across any plug-ins to help him draw detailed stonework in AutoCAD based on point clouds, so he'll just have to do it the old-fashioned way — one stone at a time.
Cadalyst executive editor Kenneth Wong explores the innovative use of technology and its implications. Read his blog at www.cadalyst.com/kw