Mobile LiDAR plays a key role in moving the Honolulu Rail Transit Project forward safely and efficiently.
The Island of Oahu in Hawaii is known for its spectacular beaches, scenic mountain views, incredible natural beauty, rich history and, increasingly--gridlock. The traffic congestion in the island’s city of Honolulu is so bad that it earned the dubious distinction of first place in the ranking of Top 10 Worst U.S. Traffic Cities on the 2012 INRIX National Traffic Scorecard, with Honolulu drivers wasting an average of 58 hours in traffic in 2011.
It’s not a new problem. Planning for a mass transit system to connect the urban center of Honolulu with the outlying areas began as far back as the mid-1960s. However, securing the required public support and funding proved challenging. In 2005, the Oahu City Council voted to implement a tax increase that would make mass transit possible, and in 2008, the citizens of Oahu narrowly approved a charter amendment to construct an elevated rapid transit line serving the city and county of Honolulu. A ground-breaking ceremony was held on Feb. 22, 2011, to signal the beginning of construction.
Led by the Honolulu Authority for Rapid Transportation (HART), the Honolulu Rail Transit Project consists of a 20-mile elevated rail line with 21 stations that will connect West Oahu with downtown Honolulu and Ala Moana Center via Honolulu International Airport and downtown Honolulu. The plan also includes extensions west through Kapolei and extensions east to the University of Hawaii-Manoa campus and Waikiki. The line will use 128 foot trains carrying about 390 passengers each, similar in size to light rail systems elsewhere in the United States as opposed to larger trains typically found on heavy rail rapid transit systems like the New York City subway. As planned, the trains will operate with up to 20 departures per hour.
|A rendering of a station boarding area, courtesy of the Honolulu Authority for Rapid Transportation (HART).|
Global service provider AECOM was selected to oversee the design-build contracts for the first two segments of the rail system in addition to the final design of the guideway and utilities for the third and fourth segments. In early 2012, with construction on the first segment already under way and construction on the second segment about to begin, the AECOM engineering team was faced with choosing a method for the data acquisition and initial survey on the third segment of the project, a 5.2-mile portion of the elevated rail guideway from Aloha Stadium to the Middle Street Transit Center in Kalihi. Honolulu is the most populated city in the state of Hawaii, and this segment of the project encompassed the heavily congested Honululu International Airport and Pearl Harbor areas, making the safety of the field crews an important consideration. The timeline was also a concern. Completing the design survey with conventional practices would take too long, cost too much and expose the survey field crews to hazardous conditions.
Instead, AECOM Technical Services in Honolulu contracted Surveying Solutions Inc. (SSI) of Standish, Mich., to deploy its MoLi mobile mapping system, which uses the Riegl VMX-250, to survey the third segment of the project for use in the engineering design. “Having researched and investigated mobile mapping and mobile LiDAR over the past few years, AECOM felt that it was a viable technology for acquiring very accurate survey data,” says Steve Paul, senior land surveyor with AECOM. “With the timelines demanded by the HART project and hazards that our field crews would have been subject to, we felt that mobile LiDAR was a perfect fit for the project.”
|Planning ensured efficient data capture.|
Planning was crucial to ensure an efficient workflow. The AECOM and SSI project team worked together to divide the workload for the survey portion of the project. The AECOM survey crews would handle the primary control network in addition to any secondary control that was needed for the mobile mapping. SSI would offer guidance to the mobile mapping control by providing a file with scan acquisition targets (SCATs) and validation acquisition targets (VATs) to illustrate the placement of targets along the project corridor. SSI would also perform the data acquisition using its MoLi mobile mapping system and would do all of the feature extraction to produce the CADD deliverables in AutoCAD Civil 3D. AECOM would then handle the final QA/QC and validation of the data using additional validation “check shots” that were collected through conventional survey methods throughout the project. All additional and supplemental surveying that would be needed in obscured areas would be conducted by the AECOM survey field crews and added to the final deliverable files.
The mobile mapping control network is the most important consideration for achieving engineering design accuracies when using mobile LiDAR. For the Honolulu Rail Transit Project, AECOM provided control positioned horizontally using RTK GPS to an accuracy of 0.03 foot and vertically using digital levels to an accuracy of 0.02 foot. The control points were set in pairs at 500-foot intervals along the projects areas to be driven, based on the Caltrans MTLS Type A specification.* All control points were set on hard surfaces in flat areas that were free of debris or cracks to create a 1-foot by 1-foot plane. Because of the unique configuration of this project, approximately 170 control points were set by the AECOM field crews. Only eight targets had to be painted; the rest of the control point locations were placed in unobscured areas that would be within the LiDAR sensors’ line of sight during the scanning process.
The final deliverables would need to include all above-ground visible features within 50 feet of each side of the proposed rail centerline route that could be driven. Approximately 90 percent of the route was drivable; the remaining 10 percent of the route would be collected conventionally by AECOM. The data would be collected in NAD83 HARN and delivered on the Hawaii State Plane Zone. To avoid the data gaps typically caused by vehicles (both parked and in motion) in the heavily populated urban area, SSI proposed driving the route at least twice at different times of day. Driving the project twice would also ensure redundancy in the trajectory files, and it would provide a higher level of confidence that the data met the design specifications.
With the control network in place and the scanning parameters set, SSI was ready to begin collecting the data. In August 2012, the firm deployed two field personnel with its MoLi system to Oahu. The compact, modular, fully integrated Reigl VMX-250 mobile mapping system was easy to ship and install, allowing for rapid deployment. AECOM provided two field personnel and use of multiple base stations for post processing the GPS/INS data.
Working in two- to three-hour windows in the late evening and pre-dawn hours to avoid disrupting traffic, SSI began driving the route. At times, the weather proved challenging; several sporadic downpours required the crew to stop driving and wait until the rain cleared. Fortunately, the flexibility of the MoLi system allowed SSI to stay on schedule.
Within two days, all of the data had been collected and was ready to be processed, constrained to the control network and used to generate the AutoCAD Civil 3D files. Accuracy was verified by AECOM, proving the viability of mobile mapping for this type of project. “This segment of the project was very complicated,” Paul says. “The railway crosses a double-decker limited-access highway several times and has to be in perfect alignment. Additionally, we had to meet an incredibly tight schedule while keeping our surveyors safe and minimizing the impact of the survey on traffic. Mobile LiDAR was key to allowing us to achieve all of these goals, and we were confident in the accuracy of the resulting dataset.”
|SSI's MoLi system features a compact, modular fully integrated Riegl VMX-250 mobile mapping system.|
Honolulu has waited more than 40 years to get a mass transit system that reduces traffic congestion and improves travel throughout the region. In 2019, when the Honolulu Rail Transit becomes fully operational, such a system will finally be a reality. By 2030, the rail is expected to take 40,000 vehicles off the road each weekday while expediting travel for thousands of commuters and visitors.
Keeping the project on track has required AECOM’s project team to apply innovative approaches for design, surveying and construction. The third segment of the railway included streets, overpasses, assets, curbs, gutters, ROW alignments, centerlines and all above-ground features, including an extensive network of overhead utilities. Collecting the data on all of these features using conventional survey methods would have taken months. By using mobile LiDAR on this project, the field time needed to complete the entire data collection was reduced to a fraction of the time, allowing the project to move forward as scheduled. Importantly, surveyors were kept out of harm’s way.
As an additional benefit, all raw LiDAR point cloud data was archived and remains readily available should the need arise for additional information by the AECOM Technical Services engineering team. Having access to this data has already proven useful as the project has progressed.
Following the success of mobile mapping on the third segment of the Honolulu Rail Transit Project, AECOM contracted SSI to collect mobile LiDAR for a portion of the fourth segment. AECOM is also exploring other projects that might be able to benefit from the advantages of mobile LiDAR. “This technology gives us a tool to meet aggressive timelines safely and effectively,” Paul says. “We intend to use mobile LiDAR wherever it’s a good fit on other projects moving forward.”
*The Caltrans MTLS Type A specification can be found in the Caltrans survey manual, pages 15-22.
The SCAT/VAT Plan
The acronyms SCAT and VAT are industry-standard terms that describe control network surveys for mobile mapping projects.
SCATs, or scan acquisition targets, are the targeted control points established throughout an entire project that are used for controlling the mobile LiDAR dataset. These photo-identifiable, highly reflective targets are often placed as chevrons with reflective taping or reflective paint striping (see photo). Although the targets themselves are not new, the SCAT term was introduced by Surveying Solutions Inc. (SSI) in 2010. Since then, the acronym has increasingly shown up in conference presentations and client requests as a mobile mapping standard.
VATs, or validation acquisition targets, are the untargeted check shots that are collected throughout the entire project to ensure that the required accuracies of a project are obtained for the complete dataset. With project timelines constantly condensing, project teams must use an efficient approach to establishing control. In addition, the field crew’s time on a project needs to be minimized and return visits to the field eliminated whenever possible. The use of standardized VATs provides a reliable way to establish these values and control the data.
To meet the needs of clients, SSI develops a complete control plan during mission planning on all projects that outlines both the initial SCATs and all of the VATs needed for complete project success. Commonly referred to as a SCAT/VAT plan or SCAT/VAT report, this plan allows field survey crews to collect the entire control survey in a single field visit. In addition, since many of the VATs are collected on recognizable features such as roadway paint striping, they can later be converted and utilized as SCATs if a particular project area needs additional control. This process greatly minimizes any return visits to the project area by the traditional survey crews.