- SPECIAL REPORTS
- THE MAGAZINE
The Pulaski Skyway is a four-lane bridge-causeway in the northeastern part of New Jersey, carrying a freeway designated U.S. Route 1/9 (US 1/9) for most of its length. The landmark structure has a total length of about 3.5 miles; its longest span is 550 feet. Traveling between Newark and Jersey City, the Skyway rises high above the Meadowlands, reaching more than 135 feet above the Hackensack River, before it passes over an industrial area in Kearny on an elevated causeway. Then it rises 135 feet above the Passaic River and crosses the New Jersey Turnpike into Newark as it descends. Access ramps provide connections from the skyway and local streets in Jersey City, Kearny and Newark.
In 2011, the Texas Transportation Institute determined that the skyway had the dubious distinction of being the sixth most unreliable road in the United States, due to the unpredictability of traffic conditions. The existing bridge was in need of a major rehabilitation. During that time, the New Jersey Department of Transportation (NJDOT) completed short-term repairs to keep it functional. Short-term repairs were performed on critically deteriorated areas, while complete rehabilitation was prioritized, sequenced and funded.
Parsons Brinkerhoff (PB) was appointed to provide design services to perform rehabilitation on the Pulaski Skyway while minimizing or mitigating delays due to congestion to commuters in the corridor and region. In order to achieve this goal, PB commissioned Robinson Aerial Surveys Inc., based in Hackettstown, N.J., to provide color digital aerial mapping photography, analytical triangulation and topographic mapping services in MicroStation V8 format.
“We chose to utilize digital aerial imagery because its superior image quality allows for more accurate plotting of planimetric detail,” says Kurt Lutz, Robinson’s director of operations. “The sensor built-in airborne GPS and inertial measuring unit (IMU) technology facilitates a more precise aero-triangulation solution, while requiring less ground control. Its ability to meet the project accuracy requirements while flying at a higher altitude allowed for the flight crew to avoid Newark-Liberty International Airport’s busy airspace, enabling the project to stay on schedule while preserving safe flying conditions for the general public.”
With the goal of increasing the accuracy of the three-dimensional data on the effected roadways, Robinson proposed to supplement the conventional photogrammetric mapping with high-definition mobile mapping.
“One of Robinson’s goals is to provide the client with expedited services without sacrificing the quality of the work and end results. By implementing the Mobile Mapping Technology, Robinson was able to accomplish that goal,” Lutz says. “Understanding the dense traffic conditions on this bridge, the Robinson team proposed to implement mobile mapping technology in order to obtain highly accurate three-dimensional data that might otherwise be obscured by auto traffic or the bridge’s own superstructure.”
The topographic mapping area covered the Pulaski Skyway corridor as well as detour routes for routes 1 and 9, totaling approximately 580 acres. Robinson obtained color digital aerial photography with a Vexcel UltraCAM Digital Mapping Camera with forward motion control. The photography was taken at an altitude of 3,200 feet above the mean terrain and had 60 percent end lap. Forward motion compensation (FMC) systems have been designed to ensure the radiometric quality of motion acquisition in airborne cameras. Because the scene and sensor are moving relative to each other, the sensor perceives the scene as moving during image acquisition. This results in directional blur called the smear effect, present all over the image and proportional to the scene displacement during exposure time. To preserve quality in motion acquisition, FMC techniques appeared in the most recognized film-based cameras that ruled the photogrammetric world for decades, both in terms of radiometry and geometry.
The mobile mapping data was collected using Trimble’s MX8 Mobile 3D Laser Scanning/Mapping system. This system is equipped with two VQ-250 Riegl mobile laser scanners, six cameras, a military-grade Applanix POS LV 420 inertial measurement unit, two Zephyr GNSS/GPS receivers and a mobile spatial imaging system. This system is designed to collect continuous point-cloud data as well as georeferenced photography and video imagery along highways, secondary roads and other specified routes having vehicle access. The point cloud and video capture densities can be adjusted according to the amount of details required, storage space available or other project specific requirements.
“Minimizing the data collection schedule was key to this project, and utilizing mobile mapping allowed us to perform this task with minimal disruption to the traveling public,” Lutz says. “Since the laser scanners are ‘active sensors,’ meaning that it emits and receives its own energy, the system was ready, aside from inclement weather and very poor GPS satellite availability, to collect the required 3D datasets even at night, enabling the Robinson team to avoid gridlocked traffic, thus complementing the aerial mapping.”
Survey-grade data collection consists of using designated targets, spheres, etc., placed on accurate survey-control throughout the project area. The point clouds are then constrained to these control points in order to compensate for IMU drift or bad GPS data coverage. The number of control points needed depends on the site conditions of the area to be scanned, such as the amount of canopy or other obstructions creating interference with GPS signals. Robinson pretargeted and established about 410 photogrammetric ground control points for the entire project. About 250 were required for the Pulaski project, and 162 were required for the resurfacing project on routes 1 and 9. The horizontal and vertical controls were in NAD-83 and NGVD-88, and a NJDOT format control report was created.
As for the aerial mapping, the triangulation adjustment output data was processed through Aerosys software for the generation of a photogrammetric control file, which was migrated to Robinson’s analytical plotting systems. The digital file for the 1-inch-equals-30-feet scale mapping contained all land use features within the project limits, such as buildings, billboards, edges of pavement, curb lines, catch basins, utility poles, lane lines, signs and more visible on or interpretable from the aerial photography.
The digitizing of the stereo-model was comprised of points and break lines of cultural features such as curbs and varying terrain differences. The digital terrain model (DTM) was generated from the data collected. Digital files were delivered in Bentley MicroStation V8 format consistent with scale map specifications, and each feature will be located on its appropriate level. All editing was performed in MicroStation V8 format mapping specifications with respect to symbology, level structures, line styles and other.
This 80-year-old structure serves as a link for cars and bus traffic to and from Bergen and Hudson counties in New Jersey, as well as to New York City; it carries an estimated 67,000 vehicles a day. After discussions with representatives from both NJDOT and PB, the Robinson team understood the need for accurate, cost-effective, low-impact base mapping.
“By combining fully digital aerial base mapping and mobile mapping technology, this project approach resulted in many advantages for the client, including a quick turnaround time, immediate availability of digital imagery for image analysis after the flight, increased safety for the staff and general public and enhanced and more accurate point measurements,” Lutz says.