Tracing the Evidence

August 31, 2008
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High-definition laser scanning aids disaster site investigations.

Stantec’s scan team recorded data at the New York City crane collapse site and dispatched it immediately. This type of fast response is crucial to forensic investigations.


Scan point cloud data were modeled into 3D images for highly detailed computer visualizations of accident scenes.

When catastrophe strikes--especially in a highly populated area like Manhattan, N.Y.--rescue workers, city officials, police officers, firefighters, onlookers and others throng to the spot, often knowingly and unknowingly altering the exact conditions of the site. Those conditions, however, are key evidence for a forensic engineer like Jim Wiethorn, who has faced this situation at numerous construction accident sites over the years.

“Invariably, that’s going to happen, and people will be moving evidence,” says Wiethorn, a failure and damage consultant with Haag Engineering in Houston. “And we’re sitting there gritting our teeth.”

But this situation was avoided after the recent crane collapses in New York City and Miami thanks to the speed and accuracy of high-definition 3D laser scanning. The technology allowed Wiethorn and his team to get onto the site quickly, collect the information they needed and begin piecing the puzzle together before the clean-up effort really got under way.

A Perfect Fit

Wiethorn was first introduced to 3D laser scanning by engineering firm Stantec during an investigation at a mining site near Philadelphia. “While we were on the site, it started raining,” says Wiethorn, “so Stantec suggested using the laser scanner instead of the usual [grid placement and measurement] method. As a result, we got a 3D image of the complex system of towers and conveyors, which allowed us to input our crane image from CAD drawings. We could sit in the operator’s seat and detail what he could and couldn’t see, which was a critical issue in the accident.”

The survey crew used four Leica ScanStations, registered together, to gather the required information from the scene. Within a day, the crew was able to create a 3D point cloud survey using Leica’s Cyclone software to develop animations and extract the critical dimensions of the site.

With that kind of speed, accuracy and flexibility, Wiethorn was hooked. He immediately began working with Stantec’s 3D laser scanning response team on a number of other crane-collapse accident scenes. The attributes of laser scanning have proven to be a perfect fit for this type of forensic work, especially compared to traditional survey processes and techniques.

With 3D laser scanning, the response team at the Fernley levee break was able to develop cross sections of the levee within days of the failure.

Speed

In the past, says Wiethorn, survey crews working on his projects first would put down a grid in the debris field and then measure in two dimensions where everything was located, entering all of the data points into CAD drawings. The process required just to complete the field measurements could take up to two weeks. “That system was so time-consuming, you’d only include the critical items,” he says. “But you’d always miss something and later wish you had it.”

Using 3D laser scanning, the survey crew can step right in, scan the site and have usable 2D and 3D products of the entire scene, usually within 24 hours.

“The response time for projects like these is crucial,” says Ken Stigner, a vice president in the Survey and Geomatics Division at Stantec. “When the phone call comes in, you have to be able to drop everything and be there tomorrow. With every day that goes by, evidence and circumstances can change.”

Such was the case last January when a levee in Fernley, Nev., gave way, threatening more than 800 homes and forcing nearly 3,500 people to evacuate. Stantec’s Reno, Nev., office had worked with the state’s reclamation bureau in the past and was able to mobilize a survey crew and laser scanner overnight to begin producing the topography models needed to determine the cause and circumstances of the break. Using Leica’s ScanStation 2 (selected for its reliability outdoors), the survey crew went through a similar process to that used on the Philadelphia site, including collecting point cloud data and creating models with Cyclone software. With 3D laser scanning, the response team was able to develop plans and cross sections of the levee within days of the failure.

Stantec’s scan team downloaded information to the main office on the Fernley site to deliver crucial topographic data to the land bureau.

Accuracy

Another benefit of using laser scanning in forensic work is the accurate detail it provides of every aspect of a site, along with its spatial relationships. Traditionally, forensics specialists take photos of all angles of a site and piece them together to recreate the positions of key items on the scene. But that technique requires a degree of interpretation. 3D laser scanning avoids that element of guesswork, however small it might be, by representing spatial relationships that are exact and can be rotated to view every possible vantage point.

“You can use animations and modeling, but when you use the exact field data with that high degree of accuracy, it eliminates a lot of the challenges you might run into,” says Wiethorn. “When you’re on the stand [in a courtroom] or in a deposition and they ask you how you determined particular dimensions, you have the data right there and know it’s accurate.”

What’s more, 3D laser scanning allows for previously removed objects to be reintroduced to the scene digitally, more accurately recreating pre-event circumstances. For instance, one of the New York crane collapses involved pieces of the crane equipment falling into a condominium building. Those pieces of equipment were taken off-site to a warehouse, where Stantec’s survey crew was able to scan them and model them back into the as-built scene. The crew used a combination of ScanStation and ScanStation 2, again using Cyclone software to develop 3D models, which were then imported into 3D Studio MAX software (Autodesk) to patch the models together into a comprehensive whole. With the traditional photographic method, the more elements that are introduced to the scene images means more questions that can be raised about the accuracy of the recreation.

Raw point cloud data of the levee break. These data were later developed into 2D topographic plans and cross sections, along with 3D surfaces and models for the land bureau.

End Products

Perhaps most important to users like Wiethorn is the resulting variety of products the 3D laser scans generate. Not only do they produce rotating 3D scans of a site, but they also can be converted quickly into 2D drawings that are compatible with engineers’ in-house CAD systems. In other words, one scanning session using 3D laser scanning can provide 3D images that can be manipulated to show all points of view, 2D AutoCAD drawings that the engineers can work from, and 3D models that can be used for stakeholder presentations, public participation events, litigation and court proceedings, and other purposes.

Moreover, the technology also can perform scans of undamaged pieces of equipment identical to those that have broken or failed. These scans are then superimposed onto the scans of the damaged equipment, providing a better understanding of how the accidents occurred.

For accident sites like crane collapses, this variety of quickly procured deliverables can help the forensic engineers immensely. “The most useful thing [about laser scanning] for us is court presentations,” says Wiethorn. “In a crane collapse, we use CAD drawings in combination with 3D scans. These show where the structures and components were at certain points and where those structures hit other objects, and then the computer can help us work backwards to determine how everything happened.”

Wiethorn notes that 3D laser scanning technology provides the following key benefits for his line of work:
  • Preservation of evidence. Scanning the site as close to the time of the event as possible helps ensure that the pre-accident conditions are documented accurately and are useful and admissible as evidence.
  • Spatial relationships. Accurate depictions of the site allow the engineers to clearly determine where the crane or other elements were located after the accident occurred compared to where it started.
  • Mechanism of failure. Depicting all of the site conditions together in as-built images allows the engineers to work backwards to determine what happened to the structures as they collapsed (order of failure).
  • Initiation. Because forensic engineers trace the steps from the end point back to the beginning, accurate imagery and modeling are vital in helping them match up the contact points, starting and ending locations, and everything in between to determine the causes and effects.
In addition, laser scanning can capture the profiles of surrounding buildings and other structures, which can help put the pieces of the puzzle together even further. “Traditional survey still gives you the location points,” says Stantec’s Ken Stigner, “but 3D laser scanning gives the visuals a jury or attorney would be looking for.”

Untapped Potential

Wiethorn and Stantec are now looking into more forensic applications for laser scanning, including television tower collapses, explosions and even the origins of fires. In such cases, these scans can show burn and debris patterns, the conditions of walls and exteriors, and other site circumstances.

“Preservation of evidence is so important in our line of work,” Wiethorn says. “That’s why, companywide, we’re really beginning to get more and more into scanning and looking for other potential applications.”

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