Lidar on the Tracks

Mobile lidar data is the key ingredient supporting infrastructure improvements that will transform a hi-speed rail corridor in Pennsylvania.

By Aaron J. Morris, GISP

The Pennsylvania Railroad built what is today’s Keystone Corridor as the main line of public works, and its use has changed over the past 100 years to primarily a high-speed passenger rail service. This requires the railroad line to be rebuilt using modern technologies, allowing for the flexibility and speed improvements necessary to continually improve passenger rail service.

Amtrak’s Keystone Service (Harrisburg to Philadelphia to New York) has grown to be the fourth-busiest corridor in Amtrak’s system, with approximately 1.4 million annual passengers. Michael Baker Jr., Inc. has worked extensively with the Pennsylvania Department of Transportation’s (PennDOT) Bureau of Public Transportation and Amtrak to support higher speed trains and to make operational improvements that accommodate the needs of Amtrak, Southeastern Pennsylvania Transportation Authority (SEPTA), Norfolk Southern, and a half-dozen shortline operations. Baker’s role since 2005 as PennDOT’s program manager for the Keystone Corridor Improvement Program has been to plan, design, and manage infrastructure improvements to the 105-mile section of the corridor between Philadelphia and Harrisburg. 

Project Conditions

With the limited access, logistics, safety concerns, and other site conditions present on a high-speed (110 mph) rail corridor, as well as through a conscious effort to reduce surveying costs and minimize impact to rail operations, PennDOT examined alternative methods for collection and delivery of high-quality survey data. Baker was awarded an initial work order in 2011 to deploy its mobile lidar technology to replace traditional surveying operations in support of ongoing and planned engineering design upgrades at 22 interlockings along the corridor. (Interlockings are integrated signal and switch systems that are interconnected so that each of their movements follows the other in a proper sequence to safely and efficiently move trains between rail lines.)

To leverage the comprehensive coverage and high-resolution data yielded by mobile lidar surveys, PennDOT acted on Baker’s recommendation to scan the entire 105-mile corridor at one time to eliminate unnecessary future field visits, ultimately saving tens of thousands of dollars in conventional survey and mobilization costs. This approach enabled PennDOT to obtain a complete 3D, 1:1 scale model of the entire corridor in just four days of collection that is being  used for current and future projects. The availability of this data and the many products derived from it will continue to dramatically reduce coordination, time, and efforts that would otherwise be required to perform field surveys for both PennDOT and Amtrak.

Baker’s task consisted of four primary assignments: 1) logistics and stakeholder coordination, 2) mobile lidar survey operations, 3) mobile lidar data extraction, and 4) aerial/mobile lidar fusion. Following the successful use of mobile lidar data from the initial work order, PennDOT has since realized additional cost savings by reusing the captured point-cloud data to derive products for seven additional project locations along the corridor, with numerous being planned.

Logistics and Safety

Two advantages resulting from the use of mobile lidar are the ability to perform accurate surveys of the entire corridor while riding on the rails and the significant reduction of delays for passenger and freight rail operations. Given the nearly constant railway operations by Amtrak, SEPTA, and Norfolk Southern along the corridor, Baker staff met with PennDOT and Amtrak representatives in Philadelphia to establish points of contact, review project requirements, establish operating logistics, and ensure that data-collection staff participated in Amtrak’s safety training course to attain certified credentials prior to entering the Amtrak rights of way. The safe and successful completion of the project depended on track logistics, stakeholder coordination, and a robust safety plan.  

Accurate Surveys

PennDOT engineering designs required survey-grade information. To achieve these results from the captured mobile lidar data, ground control was applied at key locations within the corridor, in a manner similar to that used in aero-triangulation for digital imagery. The initial locations where ground control was established directly corresponded to the 22 interlocking points where track and signal improvements were planned. Establishing the ground control required traditional surveying staff within the active corridor, which further dictated coordination with two divisions at Amtrak to secure required protection crews (flaggers, watchmen, and the safety foreman). To limit the duration of exposure, RTK GPS and a total station were employed by safety-certified survey staff to establish horizontal and vertical control on well-defined objects within the collection corridor.

The primary challenge of placing a vehicle on the line was mitigated by Amtrak’s provision of a hi-rail equipped vehicle, pilot, and driver. Baker designed a precision mounting platform that was fabricated by Amtrak to mount the system to the hi-rail, which shortened calibrations and testing prior to performing the on-track scanning.

Over four days, collections were performed along multiple tracks and in opposing directions to minimize laser shadowing caused by temporary and permanent obstructions, resulting in the total capture of over 220 linear miles of mobile lidar data. During collection, GPS field crews were situated along the corridor for subsequent post processing. Most of the corridor lacked sufficient sidings, and, with only three at-grade crossings along the entire 105-mile length, removing the hi-rail collection vehicle from the track during the collection was not an option. The on-rail collections were facilitated by the Amtrak pilot, who maintained real-time and constant communications with track officials for clearances and routing of other train traffic.

Innovative Applications

The use of mobile lidar technology represented a paradigm shift in how effective surveys should be performed for railway projects in active corridors. The deployment of the mobile lidar system allowed seamless operation at various speeds to scan all natural and as-built features visible along the travel route, including rails, cross-ties, turnouts, ballast, drainage, bridges, retaining walls, vegetation, signage, signals, switches, graffiti, electrical catenaries, fences, trees, and other features that overhang the rail corridor.

To generate the engineering-grade data, the system employs two class 1 (eye-safe) lasers that blanketed the corridor with a combined 400,000 laser readings every second, each with a precision of < 8mm. To facilitate the accurate georeference of each laser reading, the system is equipped with an on-board, survey-grade, dual-antenna GPS receiver to produce the initial X, Y, Z position. The position is further refined by the integrated Inertial Measuring Unit (IMU), the same that is used in the guidance system of Patriot Missiles, which provides real-time corrections based on the vehicle’s current attitude (pitch, yaw, and roll) at the time of each laser reading—providing the framework for accurate, repeatable, and defensible results.

In addition to the wealth of data obtained from the lidar survey, the system also captured digital images from two on-board, five-megapixel cameras at three frames per second while simultaneously capturing lidar data. Image captures were concurrently georeferenced for downstream use during secondary validations for positive feature identifications as well as to support robust quality assurance/quality control regimens.

Operating at allowable hi-rail speeds, the system delivered high-density scan information that provided remarkable visualization capabilities. Resulting products were not limited to standard two-dimensional line drawings and reports generated by conventional survey methods; they yielded a seemingly limitless array of products and services (DEM, TIN, contours, 3D visualizations, 3D modeling, planimetrics, as-builts, etc.), while remaining more affordable than conventional surveying. 

Fusing Mobile and Aerial Data

Traditional control surveying operations and subsequent point-cloud constraints were initially restricted to the predefined interlocking locations. These areas represented approximately 12% of the total track length collected. To enhance the spatial accuracy of the remaining areas and to normalize the mobile lidar collections with pre-existing aerial lidar data provided by Amtrak, Baker performed a fusion of the disparate point clouds, whose sum was truly greater than the parts.

The fusion of mobile and aerial lidar data was unique, providing additional benefits to the project. Where higher resolution data was required, mobile lidar data was exploited to prepare detailed topography and planimetrics for engineering design. Conversely, aerial lidar data provided the foundation for development of digital terrain models, building footprints, road surfaces, tree canopies, etc. for areas outside of Amtrak rights of way. As a direct result of this project, mobile lidar and data fusion will be used on future transportation corridor projects to reduce surveying costs, enhance personnel safety, and promote reuse of point-cloud data for engineering and design.

Future Value

For the original project, mobile lidar provided an economic solution for compiling survey-grade information while minimizing exposure and safety hazards for personnel. It eliminated safety hazard exposures for thousands of worker hours compared to traditional surveys. Throughout the project, cost benefits were recognized by PennDOT (and ultimately taxpayers), as well as by Amtrak through the reduction in safety protection resources and track access logistics coordination. By reducing the requirement for support, these valuable resources could be allocated in support of other important Amtrak projects.

Since initial delivery, PennDOT District’s 6 and 8 have repurposed the data to support numerous engineering and design/planning efforts, including several station redevelopments and the reconstruction of the massive “state” interlocking in Harrisburg. Every time PennDOT repurposes the captured data, cost savings are immediately realized through the reduction of conventional survey staff, while simultaneously improving safety and shortening project lifecycles. The multitude of products derived from the lidar data that are achieved at a lower cost than traditional surveys, which provides major social and economic benefits to taxpayers.

Aaron J. Morris, GISP is the director of mobile lidar operations and assistant vice president for Michael Baker Jr., Inc.  A 15-year veteran with Baker, Morris has operated out of Baker’s Jackson, MS office for the past 10 years, from which he has applied his extensive background in GPS, mobile collection systems, and linear referencing systems for state and nationwide projects.


Project Wins Grand Award in PA-MAPPS Geospatial Excellence Awards Competition

Michael Baker Jr., Inc. was selected as the grand award winner in the second annual PA-MAPPS Geospatial Products and Services Excellence Awards competition.

“This project was selected as the grand award winner due to the complexity, innovation, value to the geospatial profession and public, and the client satisfaction that was exhibited,” said Angela Cuthbert, Millersville University, the chair of the judges who evaluated the submitted projects.

The award was presented at a ceremony during the PA GIS Conference on May 23 in Grantville, PA.

» Back to our July 2012 Issue