By Russ L. Tamblyn
Engineering Consultant
Special to AECnews.com
In June 2002, the Nebraska Department of Roads (NDOR) selected Lamp, Rynearson & Associates, Inc. (LRA), to conduct a bridge clearance height survey, as part of a new Internet-based oversized load permitting system. LRA was asked to collect clearance data on over 400 structures across the state without closing a single lane of traffic. The LRA team was confident it could complete this project without major, unforeseen problems. For the most part this was true, but it wasn’t easy.
LRA initially considered using a Leica Geosystems TCRA 1101 reflectorless total station for the survey work. But when the firm compared a total station to 3D laser scanning, LRA determined it could survey each bridge site faster and more thoroughly with 3D scanning. NDOR’s initial response was skepticism, and requested a test survey. A Cyrax 2500 3D laser scanner (from Leica Geosystems) was used in scanning a NDOR-specified bridge. After capturing more than 1 million data points, the vertical measurements were extracted and checked in the field. The NDOR engineers matched known measurements with the new data to within 1/8 of an inch. The tolerance satisfied NDOR, and 3D laser scanning was approved.
Several factors to consider
The project required LRA to analyze several important factors. The first was safety; how could LRA keep its field personnel out of harm’s way while collecting survey information on an Interstate system? “The most important feature was safety,” comments Ellis Tompkins, a rail and public transportation engineer with LRA. “Most of the structures are located on Interstate 80 with 75 mph traffic. The laser scanning could be accomplished without lane restrictions or extensive signage. The equipment was located in the median with no disruption of traffic and therefore safe for both the highway user and the crew completing the data collection.”
Accuracy played a close second to safety. LRA needed to maintain a high degree of accuracy throughout the entire project. Speed was a third important consideration, because less time spent on or near highways meant less time in danger. Other considerations involved point cloud registration, data management, efficient data extraction, and delivering an end product that the client would find beneficial above and beyond traditional methods.
By addressing these concerns at the beginning of the project, LRA was able to source the right equipment and methods. Relying on previous experience and assistance from various vendors, LRA tested and designed a workflow that would finish the project within the projected time.
Equipment
LRA compared the top 3D laser scanners then available. The two standouts were the Cyrax 2500 and the Riegl LMS-Z360 from Riegl USA. After field-testing both scanners, LRA decided the Riegl scanner was best suited for the project. It could be set level using a conventional tribrach, it had a larger field of view (360 x 90) and it was almost 10 times faster than the Cyrax 2500. [Editor’s Note: These are specifications from 2002; products from both companies have since improved.] In addition to the point cloud data, LRA also needed to collect new latitude and longitude points, and high-resolution digital images of each structure. Other field equipment initially selected included Leica Geosystems GS 50 backpack GPS, Panasonic Toughbook notebook PC, and a Kodak DX4900 digital camera.
It is typical for several instruments to be on-site at a scanning project; the scanner and a total station or the scanner and GPS. Checking the instruments against each other is the most common method of quality control. In this case, the GPS unit was found to be not accurate enough to conduct the QC checks. It was replaced by another Leica Geosystems device, the Leica Disto 4 Pro hand-held laser distance meter.
Data Processing
The project required collecting two scans per bridge over a 25- to 30-minute period, totally around 80 megabytes of field data for each bridge site. Eighty megabytes per bridge times 400 bridges meant the project would be gathering 32,000 megabytes of point cloud data (32 gigabytes). The survey team needed a fast and accurate way to transfer the raw data from the field to the Omaha home office for processing, and it needed both software and hardware that could work with such large amounts of data.
After transferring field data back to Omaha, there was plenty of post-processing work to be done. First, multiple scans on each bridge had to be registered into a common coordinate system. Next, the vertical measurements were extracted and exported to a spreadsheet for NDOR. At the same time, the team needed to organize the latitude/longitude data and 700+ digital images.
The LRA team created a post-processing workflow using both PolyWorks point cloud processing software from InnovMetric Software, and LASERGen from BitWyse Solutions. This was the first time these data processing software packages were used together on a large infrastructure project. LASERGen is commonly used in AEC environments, but PolyWorks is more at home in manufacturing. [Editor’s Note: NASA used PolyWorks last week to analyze 3D laser scan data taken in space by the Space Shuttle Discovery.] Why LRA used both PolyWorks and LASERGen is explained later in the article.
Project Management
With bridge sites throughout Nebraska, LRA needed a way to manage the logistics of the project as well as the data being accumulated. LRA’s Geographical Information Systems group recommended a database management system to monitor the project, so ArcPad GIS software from ESRI was installed onto the Panasonic Tough Pad notebook PC. It was used for navigation and collection of updated latitude and longitude coordinates. While in the field, the operator used ArcPad to graphically display a map of the state and locations of each bridge site and to monitor completion status from a geographical perspective.
The Field Work
Initially, the project utilized the 3D laser scanner, the notebook PC, the GPS unit, the distance meter, the digital camera, a 4x4 truck, and one field operator. A typical bridge site scan project averaged approximately 40 minutes of actual equipment set up, data collection and tear down. The scanner took a little over four minutes to collect three million data points from one side of the bridge. The operator needed to collect and update the structure’s location using the GPS unit, collect a digital image of one-half of the structure and highway, and collect a vertical height sample for quality control measurements (1 out of 5 bridge sites).
At the end of every field day, the scan data, digital images and ArcPad database were written to CD and sent back to the office. “The laser scanner collected data from every point on the roadway to every point on the bottom of the structure and provided a graphical view,” comments Tompkins. “Though the project was used to implement an automated truck permit system, there are other uses for the data. For example, if a structure was damaged by a traffic accident or natural disaster, we can have an after-scan taken for comparison and determine the amount of change in the structure.”
After reviewing the initial fieldwork, LRA decided to add a second field operator to the one-man crew. This shortened the elapsed time per bridge to less than 15 minutes. On its best day, the field crew was able to capture 31 bridges in a 10-hour period, including drive time between bridge sites. “The system is fast,” notes Tompkins. “Conventional surveying would have required approximately two to four hours per structure. It provided accuracy down to a hundredth of a foot and still maintained the speed of setup and collection of data.”
Working in Nebraska can wreak havoc on instrumentation. Field operators often worked in freezing weather, and needed to be creative to keep the instruments in working order. One time the team created a heat assembly that kept the scanner operating within its recommended temperature range even while the ambient temperature was well below freezing.
The Office Work
Once data started arriving in the home office, LRA post-processing specialist Jack Cudaback was given the challenge of keeping up with the field crew. The raw data was collected and stored in Riegl’s native format using Riegl’s 3D RiScan Pro software. Post-processing included a number of different steps to build the final deliverables. “The raw data sent back to the office averaged 1.5 gigabytes per day,” notes Cudaback. “Some engineering and surveying firms would not manage that much data over a one-year period, much less in one day.” The field crew was collecting data from an average of 15 bridge sites per day. Cudaback would then align various scans into one coordinate plane, verify accuracy, do minor clean-up, and extract vertical height measurements for the NDOR spreadsheet.
Aligning the Scans
Before alignment, Cudaback needed to convert the raw 3D RiScan Pro data to the PolyWorks format. The raw data files were large and time-consuming to process. In the field, an operator took two scans of the bridge site that required point cloud-to-point cloud alignment in a coordinate plane. PolyWorks has algorithms within the software that can analyze the geometry of every point within the cloud, conducting a best-fit alignment procedure over millions of corresponding points. Because it is based on the shape of the natural features in the scenery, the PolyWorks alignment technique did not require the use of reflective targets in each scan to align the point clouds. This method allows for faster and safer field operations. The alignment of two scan locations took approximately 10 minutes. Once the scans were aligned into a single data set for each bridge site, it was easily geo-referenced to real world coordinates.
Vertical Clearance Extraction
The next step was to extract the vertical clearance data from the aligned data set. Initially extracting vertical measurements with PolyWorks took approximately 40 minutes per bridge site. In order to expedite the process, LRA took the advice of the InnovMetric support team and created PolyWorks macros to automate the bridge height extraction process. This lowered the initial 350+ mouse clicks down to 12 and reduced the time to process each bridge site from 40 minutes to 10 minutes. This one change alone saved 200 hours of processing time.
Bringing the data together
Next Cudaback created a spreadsheet using the existing structure names and modified it to incorporate the vertical height measurements. The digital images were named and organized in corresponding project directories, and the updated coordinates of the structure were uploaded into the spreadsheet. By properly managing the incoming data, one 3D application specialist was able to process almost 400 bridges and keep up with the hectic pace of the field crew.
The quality control to check accuracy involved manually measuring the bridge height with a hand-held laser distance meter. The Riegl Z360 data with extracted measurements were compared to the manual field measurements with a typical accuracy of +/- ¼”.
3D Deliverables
When the project began, the NDOR did not realize the power of collecting millions of measurements at these bridge sites. During the project, NDOR had a chance to view the 3D data in raw form at periodic update meetings. To provide final viewable deliverables, LRA used LASERGen to create multiple 3D models at one time, suitable for viewing in MicroStation. None of the familiar CAD programs used in civil engineering are capable of efficiently displaying or manipulating the millions of 3D points created by laser scanning; conversion utilities are required. LASERGen eliminated most of the user involvement related to converting the data sets into models. After the two sets of data conversion in PolyWorks and then LASERGen, the final total was a massive 140 gigabytes of data—enough data to fill four hard drives on today’s average desktop computer.
3D scanning technology allowed LRA to not only achieve NDOR’s goals, but to do so with increased productivity. LRA took the time to thoroughly plan the project in the beginning, eliminating costly mistakes. The NDOR project is one example of how 3D laser scanning technology can be used to replace traditional survey methods with an alternative that is faster, safer, and more useful. Proper project planning, quality control procedures and project management skills play a vital role in the success of all scanning projects. By using previous experience and sourcing knowledgeable support, LRA was able to ensure a smooth project. LRA met and exceeded the NDOR’s project requirements, and did so within budget and on time.
Russ L. Tamblyn, formerly with Lamp, Rynearson & Associates, is now in private practice. He has worked on more than 50 successful laser-scanning projects in a variety of fields including civil/infrastructure, architectural, industrial, and a variety of hybrid applications.
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