Merging into the Fast Lane
High-accuracy helicopter and terrestrial mobile LiDAR datasets have proven to be accurate sources for engineering-grade digital elevation models. However, LiDAR data acquisition is a complex, highly innovative and rapidly evolving field. Because of the varying levels of accuracy and resolution that can be obtained, relying on one type of data acquisition is not usually sufficient for large-scale transportation or infrastructure projects. The combination of two or more methods is increasingly recognized as a best practice among service professionals, but merging the datasets from multiple types of methods has been challenging due to software limitations and other hurdles. Additionally, there has been a broad lack of understanding on the part of government agencies as to what results can reasonably be expected through the use of these advanced technologies.
In late 2010, the Texas Department of Transportation (TxDOT) embarked on a study of a 2,500-foot section of Interstate Highway 30 (IH 30) in Mesquite that was experiencing repeated water accumulation. The project presented TxDOT with a unique opportunity to closely evaluate the use of LiDAR as a suitable technology for data acquisition. “We wanted to figure out how much water was accumulating and how it was accumulating on the surface,” says Mark Eder, district survey coordinator for TxDOT. “We needed some very precise data, so we decided LiDAR was the answer.”
The agency decided that teaming several expert firms in the LiDAR field would provide the best approach. Woolpert, Aerial Data Service Inc. (ADS), Surveying and Mapping Inc. (SAM) and Tuck Mapping Solutions were contracted to collect and process datasets for the project using two Optech Lynx Mobile Mapper systems and a Riegl LMS-Q560 full waveform 164 KHz airborne laser scanner. “We had never deployed all of that equipment on one project before, much less tried to merge all the data together,” Eder says. “We didn’t really know what to expect.”
The introduction of LiDAR units to low-flying helicopters and, more recently, moving vehicles, has allowed mappers to cover many miles of roadway each day while also reducing costs compared to conventional surveys of the same area. In the past, surveying and mapping a 30-mile segment of road might have taken months. Today, this data can be acquired in a few hours and processed in a matter of weeks. Maps are also being produced with record accuracies; it is not uncommon to map a large segment of highway with an accuracy of .05 to .1 foot. This accuracy is allowing design engineers to provide construction volumes that have fewer challenges to quantities during construction.
Conversely, mobile mapping provides details of drainage structures such as curbs and gutters as well as breaks in the pavement--details that are difficult to capture from the air. When used with software such as TopoDOT from Certainty 3D, mobile mapping allows cross-sections anywhere along a corridor of dense point cloud data to be easily extracted for use at the engineering design and operations level.
Mobile mapping also provides for images to be registered to the LiDAR points, thus defining the roadway features in a way that was not possible in the past. By colorizing the LiDAR point cloud, an exact fit between the LiDAR point cloud and the imagery can be achieved. (Typically it is preferable to map point cloud data to the same view as the image overlays, thereby using full camera resolution for feature identification, extraction and modeling operations.)
Mobile mapping systems require ground control to be surveyed within the right-of-way and within the sight range of the mapper as it travels down the road, but this is one area that is expected to be refined in the near future. Improved methods of ground control will reduce the amount of surveying needed in and around the roadway.
Using a combination of mobile mapping and helicopter LiDAR, the project team working with TxDOT collected all of the data required from the 2,500-foot stretch of IH 30 in less than one day. No lane closures were required, and except for placing the control (handled by TxDOT Dallas District), no personnel were on the roadway, which provided a substantial safety benefit. The next step was to process and merge the mobile and aerial datasets to produce the final deliverables.
Combining the point clouds obtained along IH 30 improved the quality of the data and allowed the synergy between the aerial and mobile datasets to be exploited. Within TxDOT, Eder used Autodesk CAiCE Visual Transportation software to merge the datasets and create a seamless digital terrain model (DTM). Because this was an evaluation project, the consultants also processed the data and created deliverables. For Tuck Mapping, Riegl USA and Certainty 3D assisted with merging the datasets and constructing a DTM.
The mobile mapping data provided a high level of detail where it was needed, such as on the pavement, where several thousand points per meter were collected. With the addition of the airborne mapping data, a cross-section was extended beyond the right-of-way. Such detail is especially important when new construction is being planned and when roadway slopes will be modified during construction.
The TxDOT project provided ample evidence of the utility and synergy between airborne and terrestrial mobile LiDAR data as well as its superiority over traditional survey methods. Enough points per area were acquired to extract all features of interest, and the information was easily identified, extracted and modeled. What’s more, the data can now be easily accessed and used across engineering design and operations.
Based on the results of this project, it is clear that combining mobile and aerial LiDAR technologies can provide accelerated schedules, increased safety, streamlined operations and improved quality for highway design projects. “This project has definitely had an impact here in Texas,” Eder says. “Now that we’ve had a taste of the technology, it’s going to be utilized more and more. It gets people out of the roadway surface, so it’s safer, and the datasets are very accurate. We’re already using mobile mapping for some projects, and the combination of mobile and aerial will come into play on future projects. The data that we didn’t have before is now easy to gather with this technology.”