Railway Track & Structures - February 2008 - (Page 32) Mapping The photos show LiDAR points fused with orthophoto (above left), rendered by elevation (above) and rendered by data classification (left). crews, which are accustomed to establishing ground control in preparation for LiDAR missions,” Golombek pointed out. “Prior to the BNSF project, they identified two surveying issues they would have to address. The first was the proper placement of GPS base stations within the long right-of-way to maintain accuracy of the LiDAR points during post-processing. The second concern was dealing with the fact that the railroad corridor passed through three California state plane zones and required 15 modification factors within the state plane zones. “Placement of the GPS base stations focused on making sure the aircraft would always be close enough to one receiver on the ground so that its coordinates could be used to differentially correct the onboard GPS data,” he said. “The points collected in the air would be used during processing of the LiDAR data, and their accuracy dictated the quality of the elevation points. Proximity to a GPS base station is not typically a problem in LiDAR because most of these elevation mapping missions occur in relatively small contiguous areas.” The survey technicians determined that to achieve the desired one-foot contour interval in the final mapping products, the aircraft should never fly more than 25 miles from a base station. To optimize data accuracy, a 15-mile 32 Railway Track & Structures February 2008 limit was set for the California project. This meant that multiple base stations would have to be set up about 30 miles apart in the corridor. On any given day, up to six base stations would be placed so the aircraft could cover more than 30 miles in one flight. “Fortunately, there were well-established, consistent National Geodetic Survey control points in most of the project area, so the crews densified the survey points from there,” Emison said. “During the flights, the survey teams would have their choice of control point for use with any base station. At the railroad’s request, the survey crews also established control at one-mile intervals on the track line. Thinking ahead to when GPS-based machine grading would be done during construction, the client wanted to have base station validation points every mile in the corridor. “Because constant train traffic causes significant ground vibration in the track bed, the crews took great care to make sure the control markers would not move,” he said. “At every fifth mile, they drove seven-foot-long stainless steel rods into the ground near the track. Each mile in between was marked with 24-inch-long No. 5 rebar. Both types of rods were topped with two-inch aluminum caps bearing the control point number.” As the ground control was being established, the survey crews dealt with the issue of crossing three state plane zones and 15 modification factors. Although the actual GPS surveying would be done in UTM coordinates, this required the crews to calculate multiple combined factors to move from state plane to ground coordinates without introducing significant error during LiDAR processing and map generation. The decision was made to divide the 453-mile corridor into segments determined by combined factors that would yield less than one-tenth of a foot error along its length. To create the segments, Merrick personnel used the Corpscon (Corps of Engineers Coordinate Conversion) software to calculate a combined factor at every point where the BNSF right-of-way crossed a township or range line. Mean combined factors were then calculated and tested for distances away from the crossing points until the error exceeded one-tenth of a foot. This marked the end of a segment, which meant that most segments had different lengths because their mean combined factors varied with gain www.rtands.com www.rtands.com
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