SCAT, VAT and EMAT—these three terms are changing the face of mobile LiDAR and how it is used in applications that require engineering design tolerances. The first two acronyms, SCAT and VAT, have become industry-standard terminology over the past 15 months in describing control network surveys for mobile mapping projects. The third, EMAT (pronounced “eemat”), is a brand new term that will have a big impact on mobile LiDAR projects.
For those unfamiliar with the first two terms, SCATs, or scan acquisition targets (technically the acronym should be ScAT, but SCAT seemed to stick), 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 at right). 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. The idea behind VATs is also not new to surveyors or the mobile mapping industry, but the times at which these validation points are collected during a project’s life cycle have varied tremendously. Teams on some projects might do their validation or check shots immediately after the LiDAR is collected, while others wait until after the point cloud has gone through initial processing. Still other teams pick up these validation points while they are in the field collecting the main control points used for the LiDAR dataset, and these extra control points are often targeted points taken in areas of GPS outages or concerns (such as around bridges, urban canyons, tunnels, etc.).
Any of these methods are acceptable with regard to the collection times; it is up to each service provider to decide how and when to validate the data. However, 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. Since most of SSI’s mobile mapping business is done through partnerships, and the company's partners are often the ones who collect the survey data under SSI's guidance, the need for a more efficient plan for establishing control and validation of the data on mobile mapping projects became paramount. SSI developed a standardized procedure, which it calls VAT, for establishing these values and controlling the data.
To meet the demands of its 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.
EMATs, or elevated mobile acquisition targets, are a new and exciting capability that is now being used by SSI as an option for establishing survey control on MoLi (SSI’s trademarked name for mobile LiDAR, pronounced “Molly”) and RaLi (SSI’s trademarked name for railway LiDAR, pronounced “Rolly”) projects. Although vertical targeting has long been a standard operating procedure for static laser scanning, the use of vertical targets in mobile mapping has been virtually nonexistent. There are a number of reasons why vertical targets aren’t used (and some will say shouldn’t be used) in mobile LiDAR applications. However, some projects and applications irrefutably require an accuracy that is far greater than what any off-the-shelf mobile LiDAR system can provide alone. Unwilling to accept that these projects cannot benefit from mobile LiDAR, SSI has begun the process of putting EMATs into the workflow to allow its clients and partners to achieve the required tolerances.
The use of vertical targeting for mobile laser scanning is not a simple undertaking. The variables that must be accounted for by a project team are quite substantial. Since it is well known that mobile LiDAR sensor point densities are far less on vertical objects at range compared to the point densities on the surface behind the vehicle drive path, proper planning and placement of EMATs is critical. LiDAR sensor laser pulse repetition rates, scanner (mirror) speed, the distance from the drive path, the speed of the vehicle, and the size of the targets only begin to scratch the surface of some of the variables that have to be considered. It also takes a very experienced and well trained operation crew with an extreme attention to detail to ensure the successful use of EMATs. An additional concern for using EMATs on mobile mapping projects is making sure that the project team has the proper processing workflow and capabilities.
After months of research, testing, innovation and deployment of EMATs, SSI has been able to successfully build the proper workflows to take full advantage of vertical targeting for mobile mapping. With standardized practices for field collection and the development of proprietary and specialized software, SSI now is able to offer a solution for projects that were previously categorized as “not the right opportunity” for mobile mapping technology. This development offers a significant advantage to SSI clients and partners, as well as the entire industry.
At HNTB Corp.’s Incubation Center, dedicated technologists have been closely studying EMATs as a possible standard for engineering design. “For HNTB, we see significant potential from SSI’s EMAT developments,” said Paul DiGiacobbe, director of strategic technology for the center. “We believe that this new advancement will provide improvements in the adjustment and overall quality of mobile mapping data, and we look forward to continued joint research and development with SSI.”
The higher-end LiDAR sensors that are currently available on the market are able to achieve the tolerances for engineering design. However, as a total solution, mobile mappers struggle to hit these accuracies without the use of traditional methodologies. The establishment of the survey control network is an area where innovation can directly and immediately impact the use of mobile mapping technologies. By progressing and improving how the control is established, mobile mapping service providers are opening the doors to new applications, uses and markets that require the highest levels of accuracy.