Five Years Under Water

At the Magdeburg Waterways Junction in Germany, laser scanning made quick work of surveying during a short maintenance down time.

For decades, Germany's extensive networks of rivers and canals have played a key role in the nation's commerce. A key connection point in German waterway transit lies 75 miles west of Berlin near Magdeburg, where the Mittelland and Elbe-Havel Canals meet the Elbe River. Until 2003, the intersection was a trouble spot. Ships traveling the east-west canal route had to make a 7.5-mile detour and pass through three locks. Navigation was difficult, and canal traffic could be limited or even stopped by seasonal variations in the flow of the Elbe. Opened in 2003, the Magdeburg Waterways Junction today provides year-round access connecting Berlin to the North Sea port of Hamburg in the north and points west to Wolfsburg, Hannover and the industrial regions of the Ruhr.

The junction showcases modern waterway design, as it includes two locks that use new water-saving technology and a trough bridge to carry canal traffic over the Elbe. The facilities are sized to handle Europe-class freighters up to 360 feet long and barge trains up to 610 feet in length. The bridge can carry vessels up to 105 feet wide in water 14 feet deep, matching the normal water levels in the Mittelland and Elbe-Havel canals.

Five years after construction was completed, the bridge was to undergo scheduled maintenance and inspection. The work would uncover any defects and document the bridge's condition at the end of the construction warranty period. The requirements called for a detailed survey of the entire trough, inside and out. Shutting down the bridge would cause significant delays and inconvenience to ship traffic on the canals. The schedule for the maintenance work was tight and allowed no flexibility for the surveyors involved. The bridge's size, rigorous schedule, and complex construction made it an ideal candidate for spatial imaging.

Surveyors Frank Jachan from Magdeburg Waterways and Shipping Authority (WSA) and Andreas Petter from Magdeburg New Waterways Construction Authority (WNA) initiated the project. "Our principal concern was to compare data for the bridge in loaded and unloaded states," says Jachan, "and we wanted to have scanning data to assist us in subsequent work."
In addition to Jachan and Petter, the project team included Sandro Muller from GeoSurvey in Schonefeld (near Berlin) and two survey engineering students from the University of Applied Science (HTW) Dresden, Martin Wiesenhutter and Roberto Kasche. For the field work, the team selected the Trimble GX 3D Scanner and the Trimble VX Spatial Station. Wiesenhutter, who is writing his diploma thesis on terrestrial 3D scanning for construction inspection, explains the choice of multiple instruments: "In terms of our work, we will be able to add a comparison of the two systems used. The data determined and the process steps can bring interesting information and experiences to light."

The centerpiece of the junction is the trough bridge. By allowing canal traffic to cross above the Elbe River, the bridge simplifies navigation and eliminates difficulties caused by variations in the Elbe's flow. The bridge is divided into two sections: the foreshore bridge and the river bridge. The foreshore bridge carries the canal from the Mitteland Canal to the edge of the Elbe. It is 2,260 feet long and 112 feet wide and sits on 17 pillars shaped to emulate the ribs of a ship. Extending the foreshore bridge, the river bridge carries the canal over the Elbe and connects to the Elbe-Havel Canal on the east side of the river. Built in three sections, the river bridge measures 750 feet long. Its central portion spans 348 feet over the Elbe. The bridge provides clearance of 21 feet above river's highest water levels, allowing year-round passage under the bridge by three-tier container ships. Control gates at the ends of the bridge abutments allow the entire trough to be closed off from the canals.

Two-Phase Operation

The scanning took place in two phases. In the first one, conducted in March 2008, the team surveyed the bridge under normal operating conditions. To prepare for fieldwork, Wiesenhutter drew up a plan of the individual sections to be scanned. The range of scanner made it possible to scan the entire structure from four points, two setups at the north end of the bridge and two more at the south end. But Wiesenhutter decided to use a different approach. "As we wanted to achieve a homogeneous object resolution, we divided the bridge into several scan areas," he explains. The scanner's software allows the user to set the vertical and horizontal spacing of the measured points. This spacing is a function of the distance from the scanner. "This is why we divided the bridge into several areas; otherwise, we would have scanned an uneven grid on the bridge. We were aiming to achieve a grid size of two to four inches. I could have scanned the bridge in one scan, but the grid size would have been too fine in close-up range and too coarse in long range."
Wiesenhutter points out that to obtain an even grid, they scanned the trough from locations on the bridge near the center of the river and each side of the bridge from points on the left and right banks of the Elbe River. The team occupied eight different stations and made between four and eight scans from each setup. Each scan covered a section of bridge of about 400 feet. When the field work was completed, the data was processed and stored using Trimble RealWorks Survey Advanced software. It took less than two days to scan the entire bridge and produce the desired grid. Muller says, "Each station setup took 10 to 15 minutes, including setting up the scanner, inputting data, and measuring the backsights. The individual scans took about 10 minutes, depending on the view angle to the bridge."
Near the end of March, the control gates were closed and the trough drained to allow work crews full access to the structure. Maintenance workers removed more than 100 tons of silt, algae, and mussels that had accumulated on the floor of the trough. They also installed a more powerful system of air jets used to prevent ice from forming in the trough during the winter.

With the maintenance and improvements complete, the surveyors returned to the bridge on Thursday, April 24. The schedule called for the bridge to reopen the following week, so they faced a short timeframe to complete the second phase of the survey. And they had only one chance to do the work. Once the trough was refilled, there could be no going back.

The difference between the filled and unfilled trough was apparent even without survey measurements. When filled, the trough contains more than 173,000 cubic yards of water weighing 146,000 imperial tons. "Take a look over the railings," says Muller. With the weight of the water gone, the team saw that the railings made a clear upward arc in the middle part of the bridge. "We could hardly see the curve in our scans of a few weeks ago."
To match the first phase data, the scanner had to be set up and oriented using the same control as the March field work. The team used survey control pillars on both sides of the river. Because of the long distances between control points, the team used 5.5-inch spherical targets placed on the reference points. They used the Trimble VX in reflectorless mode to measure the positions of the spheres and transformed the coordinates into the state reference system (Saxony-Anhalt 42/83). At each setup, the team used Trimble PointScape software to control the scanner. They measured spheres on at least three control points to compute the scanner's position in the state reference system. As a result, all the scans would link into a single point cloud, reducing processing time and providing flexibility during the editing and analysis work. As in the initial survey, the abutments, piers, and sides of the trough were scanned from points on the river banks.

The work then moved onto the bridge itself. When standing next to the now-empty trough, the advantages of 3D scanning became clear. To make a detailed survey of such a complex structure would normally require weeks, and the three surveyors had just two days. They completed the second phase of the field work on Friday, April 25. The following Sunday, the control gates would open, and the bridge would resume carrying canal traffic.

The process for each scan proved straightforward. Once the scanner's position was established, Wiesenhutter created an initial view of the bridge. Using the touch screen computer, he defined the area to be scanned and parameters for the work. As the point clouds were collected, the team used 3D views to verify the density and coverage. Each scan was saved to an individual file. Muller guided the students in using a naming process to provide easier data processing when the team returned to the office. Before leaving the site, the data files were copied to an external hard drive and burned onto a CD.

Comparing Results

To expand their knowledge, Wiesenhutter, Kasche, and Muller wanted to compare the results from the 3D scanner with information gathered by the Trimble VX Spatial Station. They used the Trimble VX to duplicate each of the scans taken by the Trimble GX. The results compared favorably. The images taken by the Trimble VX were used for visualization, 3D modeling, texturing, and inspection of the interior of the trough. MŸller reports that the instrumentÕs EDM was helpful in measuring specific points on the long spans. The team used the video interface as well as the click-and-move function to identify and measure needed points that could not be measured using conventional methods.  

Back in the office, the careful field work paid off. Because each setup had known coordinates and orientation, the work to register and combine the scans into a single 3D model went quickly. Images and additional points collected by the Trimble VX were added to the point clouds. Using the surface-to-model inspection tool in Trimble RealWorks Survey, the surveyors examined the deflection of the bridge against horizontal and vertical plane surfaces placed along the top, sides, and bottom of the trough. And they used the surface-to-surface comparison to examine the deflection of the filled bridge. When empty, the trough deflected upward about 10 inches. This matched the design values and agreed with deflection monitoring measurements made by the WNA.

As the fieldwork neared completion, Muller reflected on the project and the importance of the survey technology. To make a detailed survey of such a complex structure would normally require weeks. Instead, the team was able to collect points at 5 to 10-centimeter intervals over the entire bridge, twice, in just four working days. But the speed was not the biggest benefit.

"When the water returns, they will no longer be able to see everything that's down below," Muller says. "The pipes and technical installations will be under water. But our scans will still show every detail, not only as a photographic image but also as a three-dimensional point cloud accurate to the last millimeter. If someone needs to know the exact path of the new air supply tube, they can retrieve it on these scans." MŸller notes that the initial investment and processing time might be seen as drawbacks. 'But for companies and institutions like the WSA, the advantages of complete three-dimensional documentation outweigh any drawbacks. An investment of this kind will pay for itself in the short term."
John Stenmark, LS is a writer and consultant working in the AEC and technical industries. He has over 20 years experience applying advanced technology to surveying and related disciplines.

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