Caltrans Uses LiDAR Data to Meet Federal Retroreflective Guidelines

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The California Department of Transportation (Caltrans) District 4 developed an innovative process to replace overhead signs using LiDAR technology. By using LiDAR to map six hundred miles of freeway assets in the San Francisco Bay Area, Caltrans was able to significantly improve sign data. Caltrans has been incorporating LiDAR technology into more projects, beginning with the 2011 San Francisco survey-grade scan of the Doyle Drive replacement project, now Presidio Parkway. This survey-grade scan was conducted in a matter of hours rather than the months traditional survey methods would have taken, not to mention the impact to traffic with periodic highway closures throughout those months.

As with all LiDAR projects, the opportunity to use the data mined for various uses and customers beyond its initial intended use. In May 2012, the Federal Highway Administration (FHWA) released updated guidelines for the retro-reflectivity of overhead and roadside signs along freeways and highways throughout the United States.

At the onset of the project in 2014, Caltrans District 4 Office of Surveys needed to develop a process to collect the data that Caltrans Traffic Operations needed to meet the updated federal guidelines. When analyzing the project’s core problem to be solved, team members in Caltrans Surveys asked, "How can we use Mobile Terrestrial Laser Scanning (MTLS) to capture the overhead sign data needed to comply with the new FHWA guidelines?" Team members in Surveys recognized the potential of LiDAR technology and readily identified the solution. Surveys’ proposed solution to use the MTLS to capture LiDAR data was the beginning of what is now known as the Overhead Sign Project (OHS).

Caltrans Surveys typical end-products are survey-grade data in the form of a digital terrain model. However, in this case, Surveys needed to create a process for gathering LiDAR data that other non-design programs, besides Traffic Operations, within Caltrans could also utilize for future projects.

After the scope and budget was developed for the OHS, the team prioritized the order that counties would be scanned by taking into account the Project Approval and Environmental Document (PAED) dates. The goal was to deliver each county’s data 45 days before the PAED dates. This would allow time for Caltrans Traffic Operations and Design to review and evaluate the data.

The project planning team was comprised of members from the California Highway Patrol and various Caltrans offices, including Traffic Operations, Design, Field Surveys, and Right of Way Engineering GIS. This team started with identifying locations that needed to be covered, and mapping location runs for the MTLS. The Right of Way Engineering GIS group mapped each county’s specific upcoming capital projects and the locations of High Precision Geodetic Network (HPGN) for base station placement. This information enabled the team to identify the areas that needed survey-grade data for future projects. Due to the short turn-around, painting targets was not possible before scanning.

Running concurrently with the planning process, another team developed the workflow process for data extraction. The workflow process was quick, efficient, and software agnostic. Within two weeks of receiving the initial set of data, this team developed and tested the workflow with the processing team before full production began. The workflow relied on good communication from data acquisition to delivery. A master calendar communicated to all team members the deadlines and individual workloads. Project milestones scoped deadlines that were based on the strengths and weaknesses of each team member. Many large capacity harddrives safeguarded the raw files, made backup copies, distributed the data, and maintained knowledge of where the data was stored. Each data extractor team member was trained on all software programs used to ensure that progress would continue even when someone was away from the project. This dynamic arrangement enabled the team to provide quality assurance and control by checking all of the work. From August to September 2014, the Caltrans MTLS, along with coordination from CHP, scanned and collected the data throughout the San Francisco Bay Area. The Caltrans MTLS consists of a Trimble MX-8 with two VQ-450 laser scanners by Riegl, seven digital cameras with a five megapixel resolution, IMU POS LV 520 by Applanix, and two BD982 GPS receivers by Trimble. All work was performed during daylight hours to ensure that the signs were visible and legible. Caltrans and CHP coordinated the timing to ensure that the data collected would be free of noise and would not require additional work during the extraction process.

This project also emphasized that using LiDAR, and MTLS specifically, to gather survey data makes the surveying process safer and more time efficient for the surveyor.

Simultaneously, the teams collected and processed data. Data flowed to the processing team on the first day of scanning. Before the data could be mined for the overhead signs, the data needed to be processed. After post-processing, data extraction began and the workflow was put into action. The final step of the workflow was the delivery of a quality product where the images and dimensioning were visible to the GIS group and stakeholders. As routes were being delivered by point cloud images, the sponsor, Caltrans Traffic Operations, provided immediate positive feedback, stating that the LiDAR technology was "breaking new grounds on delivering projects."

When the project was finally prepared for delivery, the data was presented in a web-based GIS map format. The overhead sign data had a viewer which allowed the customer to visually see which signs had data ready for consumption, which also built upon information already archived. The web-based map [see Sidebar] showed the current inventory of overhead signs in green, or yellow which indicated that data was available for that particular sign. The website mapping application provided information on the sign identification, structure type, clearance, total frame width, county, route, and post mile, along with an image of the sign and its dimensions on the photograph. Customers could click on the photograph to bring up a larger .pdf version of the sign for viewing.

As identified by Surveys at the inception of the project, the goal was always to use the data for future projects. This is one of the most dynamic features of LiDAR technology. Currently, three capital projects are using this same data along with the new methods of post-collection control. The first two projects, one on Sonoma County Highway 101 and one on Napa County State Route 29, will have the mobile point cloud controlled by stationary scans using point cloud to point cloud registration method. The third project on Alameda County Highway 680 will use a new MTLS run to scan targets set on the shoulder and weaved in with the previous runs.

The success of the Caltrans Overhead Sign Project is leading the team into new ways of thinking about how LiDAR data can be used and by whom. Combining GIS with LiDAR is just the beginning of the different ways in which the data collected from this project will continue to serve Caltrans. It is expected that future customers from other Caltrans units such as Planning, Environmental, Maintenance, and Operations, will find additional innovative ways to use this data for their applications. By using LiDAR, this Surveys team has collected data that is flexible for future uses.

Rebecca Boyer works in Caltrans District 4 Office of Surveys on a training and development assignment to become a Caltrans Transportation Surveyor. She has been with the State of California for 17 years.

Surveyed Signs versus Non-Surveyed Signs

GIS provides the ability to visualize infrastructure assets within the surrounding environment and the empowerment to make more informed decisions. The planning and delivery phases of the Overhead Sign Replacement project made this clear. During the planning phase, geospatial data was successfully leveraged to minimize data collection efforts while maximizing benefits. Highway routes with the highest concentration of sign inventories were identified and intersected with planned project boundaries to determine the locations where high accuracy survey data would be needed for future projects.

The accelerated schedule of this project required streamline coordination amongst numerous participants during the design and construction phases. An interactive web mapping application was used to quickly deliver the processed LiDAR data to the project team. The ability to view the overhead sign information on a dynamic map allowed interdisciplinary team members two advantages: 1) the ability to easily and effectively communicate observed geographic patterns, and 2) the ability to quickly derive insightful project decisions. The asset data that was collected and displayed in the mapping application is maintained in a geospatial database which can be used for future day to day maintenance, operations, and project and system planning.

A 6.751Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE