Railway Track & Structures - February 2008 - (Page 33) and loss of elevation and proximity to the next township/range crossing point. A total of 24 segments were created. Extra ground control makes both surveyors and LiDAR operators happy, and the ones on this project were treated to a rare source of correction data that ultimately enhanced the accuracy of the LiDAR elevation data. By coincidence, BNSF was planning to run its ‘geometry train’ through the Barstow-Oakland corridor during the project. BNSF invited Merrick to place a GPS receiver on the geometry train to collect an extremely accurate set of coordinates for the location of track centerline. These data proved useful later in the project for quality checking the LiDAR and for aligning the track segments during integration with the GIS. Acquiring elevation data The corridor width for BNSF and many other railroads averages about 300 feet, which falls well within a single-pass illumination swath of the Leica ALS50-II LiDAR sensor. “At first glance, this narrow corridor may seem to make flight planning easier than for a localized elevation mapping project that requires numerous back-and-forth flight lines,” Golombek said. “One might think that one long, continuous flight straight down the track would be sufficient, but, in fact, just the opposite is the case. “While it’s true that train tracks can extend for many miles in straight lines, they usually contain a number of gradual curves, which pose a challenge for the navigation software that controls the firing of the digital camera and the LiDAR sensor,” he noted. “The guidance system is designed for single-trajectory flight paths, and even a small turn of the aircraft can affect the software. “As a result, Merrick flight planners had to calculate multiple straight-line paths to get the aircraft through each turn in the railway track,” he said. “For example, if the track was turning to the left, the pilot would complete a flight line and then make an approximately 270-degree right turn to set up for the next flight line. This procedure would be repeated at least two to three times to complete one turn of the track.” “Another challenge in mapping the long BNSF railway corridor with LiDAR was the nearly constant variation in ground elevation,” Emison said. www.rtands.com “While this can be encountered in any type of LiDAR acquisition project, the situation almost always occurs when mapping right-of-way over long distances. Elevation change makes flight planning difficult because the ground sample distance of the laser is dependent on the altitude above ground level. “For this reason, the flight planners constantly had to think in three dimensions as they planned each flight line,” he explained. “The requested twofoot GSD required a flying altitude of 2,500 feet AGL with a small error budget. The pilots, therefore, had to constantly watch their flight plans to gain and lose altitude as needed to accommodate the terrain. Fortunately, the railroad bed itself always follows a gentle incline or decline, so there were no sudden changes in altitude for the aircraft. “Another concern that Merrick watched out for in the BNSF project was very long stretches of straight track,” Emison noted. “Just as many curves can wreak havoc on flight planning, so can perfectly straight flight lines. The onboard inertial measurement unit that collects data on the pitch and roll of the aircraft during LiDAR and imagery acquisition tends to drift if it senses no movement for 30 minutes. While Merrick has encountered this situation in other projects, it was not an issue in the BNSF corridor.” Integrating data with GIS “Many railways have become sophisticated users of geographic information systems technology, and BNSF is no different,” Golombek said. “The railroad requested all data products to be delivered in GIS-compatible MicroStation files. During the processing of elevation and imagery data to generate GIS mapping layers, the combined factors again took center stage, underscoring the importance of taking this issue into account from the start of surveying. “One of the most critical aspects of mapping a railroad for GIS is to make sure the track alignments match up properly when the segments are integrated during processing,” he said. “The difficulty in tying tracks together at segment divisions is the fact that each segment is in a different modified projection, and the combined factors break at the ends. The data sets don’t match up.” He continued: “GIS software is designed to handle differences in normal state plane systems, but not the modified projections, so Merrick GIS technicians developed a customized solution. Since every major projection system has a file that defines the data projection, the technicians created a process to modify the standard state plane projections to take into account the combined factor used in each segment. The result was a modified projection file for each of the 24 BNSF corridor segments.” Once these modified project files were built, they could be associated with the map data sets relating to that segment: contours, DTMs, orthoimages, etc. Maintained in their native ground coordinate systems, these layers could then be integrated into the GIS and reprojected on the fly back into a common state plane grid so the data sets lined up accurately. Prior to delivery of end products, Merrick developed a customized viewing application utilizing ESRI’s ArcReader so that any BNSF office can look at the contour maps and orthophotography even if they don’t have GIS software on their desktops. “We delivered preliminary data sets to executives in the BNSF engineering and construction department in early summer 2007,” Emison said. “They were impressed with the volume of detailed information in the GIS map layers that have been created with extremely accurate survey data, LiDAR elevation points and high-resolution orthophotography.” “I’m very excited and pleased with the results I’ve seen to date,” said BNSF’s Fleming. “The information is just tremendous.” While mapping the BNSF right-ofway presented some challenges unique to railways, Merrick believes the techniques developed in this project relating to surveying, LiDAR acquisition and GIS data integration have applicability not only for mapping railroads, but for pipelines, power lines, highways and other linear corridors. “Looking at the bigger picture, however, the BNSF project may represent a more important emerging trend: surveying and mapping projects that utilize the latest technologies require a high degree of coordination and planning among all of the technical groups involved,” Emison said. February 2008 33 Railway Track & Structures www.rtands.com
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