Mobile Mapping Collects Geospatial Data From A Safe Distance
When it comes to surveying for road construction projects, speed matters. Surveyors need speed to be efficient in the field and to reduce costs. But the speed of cars racing by matters too – particularly in Germany where there are no speed limits on some Autobahn sections. Here, surveying is not for the faint of heart.
International construction company Strabag AG is well versed in the complexities of collecting survey measurements along active highways.
“It can be quite unnerving to capture ground measurements when cars are racing past you at 150 km/hour,” says Thomas Gröninger, division manager of digitalization in Strabag’s office in Regensburg, Germany. “Transport infrastructure projects challenge us to acquire precise field data as efficiently as possible while protecting our crews’ safety. Most conventional methods still require us to close roads or re-route traffic to work safely, which adds time and cost to projects. And it’s difficult to get permission to fly drones over highways. Ideally, we’ve needed a system that would allow us to be safely on the road.”
Enter mobile mapping. Strabag viewed vehicle-mounted 3D mobile mapping technology as a promising solution to allow crews to work in traffic, giving them the tool to acquire centimeter-grade spatial data at highway speeds without sacrificing data quality or the crews’ welfare.
Turns out, their view was correct. One year after first mounting its system on a minivan, the mobile spatial imaging technology has not only become a needed complement to its traditional survey instruments and unmanned aerial systems (UAS), it’s helped Gröninger’s division redefine the business of road and railway projects, discover new applications for mobile mapping and steadily drive them toward new opportunities.
The Case for Mobile Mapping
According to Gröninger, the case for acquiring mobile mapping technology crystalized in 2018. The company, which Gröninger says is one of the biggest construction companies in Europe and the leader in Infrastructure construction in Germany, had become increasingly focused on integrating building information modeling (BIM) into its roadway projects – both by choice and by request – which require frequent measurement updates for planning and for monitoring construction progress. Crews were not only challenged to collect that information on a weekly, and sometimes daily basis with their conventional survey methods, they were also at risk each time they returned to the roadway.
With its reputation for delivering complicated transport projects on time and on budget, Gröninger says they needed technology that would allow teams to keep up with the breakneck pace of their design/build assignments.
“Mobile mapping can be used to map everything in your path, enabling you to extract features and objects whenever you need them,” he says. “All of the features are in the point cloud and imagery, so there’s no need for follow-up visits or rework. Additionally, the technology enables you to collect data in traffic over long distances. A conventional surveyor, for example, would need about three weeks to measure a 50-km highway. With mobile mapping, we can set ground control points, drive that highway and process the 3D data in about a week – the driving itself would only take two hours. It’s a great tool for construction, as-built surveys and rapidly changing sites.”
After testing a few options, Strabag purchased the Trimble MX9 mobile mapping solution, a field-to-finish system that combines high-density laser scanning, a spherical camera for panoramic and multi-angle imagery, and a high-precision Applanix GNSS IMU (inertial measurement unit) component. All sensors are time synchronized with precise GNSS time tags and are linked to the trajectory that is recorded with the GNSS/IMU subsystem. This synchronization allows all recorded points and images to be properly aligned in a post-processing step.
“We trialed other systems that only offer one laser scanner,” Gröninger says. “If you drive 100 km/hr with one laser scanner, you get a less-dense point cloud, which makes it difficult to extract features like the edge of a curb. The MX9 provides two laser scanners that each capture one million points per second. That gives us an incredibly high-density point cloud. It’s also quite compact and lightweight, which makes it easier to set up.”
Since acquiring the MX9 in late 2018, the company has wasted little time in getting it on the road, establishing efficient workflows for collecting and processing the geospatial data and building its 3D mobile mapping business – both in responding to tenders and proactively creating work opportunities through demonstrating the technology’s capabilities.
Mapping the Highway
One of those educational opportunities came soon after Strabag began using the MX9. The Northern Bavaria Motorway Directorate (NBMD) was planning to renovate a highway near Regensburg; Gröninger’s division viewed this as a chance to show the public authority how a mobile mapping, multi-sensor approach could be of benefit in designing and constructing the new highway.
“Part of our job is to educate existing and new clients on what new technologies can do for them, and in particular, how multiple technologies can be integrated for surveying and design projects,” says Gröninger. “So we went to the Directorate and offered to map the highway segment planned for renovation using a mobile mapping system and UAS to show them the data and the benefits of the technologies.”
Intrigued, the NBMD launched a pilot project, tasking Strabag to scan and collect imagery of a section of the two-lane A93 highway and create a DTM with an absolute accuracy of 1.5 cm horizontal and 2 cm vertical. Covering a 20-km stretch, they would also pair the 3D mapping data with UAS imagery that captured features 200 m either side of the highway.
To achieve the required data accuracy and quality, a field team used a Trimble SX10 scanning total station to survey and mark 120 ground control points (GCPs) to provide control for the mobile mapping. A traffic safety vehicle protected the ground crew as they marked and measured the GCPs with a transparent spray paint only visible to the MX9.
After the two-day GCP process, they were ready for the data-collection drive. With the MX9 mounted on their 2-m-high vehicle, the team drove the highway in two directions, with each trajectory totaling 10km. As they traveled the highway at 80 km/hr, the system scanned structural features such as break lines, pavement edge lines, road and building signs and road markings – any feature within 50 m of the side of the road – and captured panoramic and multi-angle photos every 5 meters. In two hours, they collected the entire point cloud of the 20-km section.
“A significant advantage of the MX9 is the so-called ‘butterfly configuration’ of its two lasers,” says Gröninger. “That unique positioning enables them to scan ‘cross-wise’ and capture features that might otherwise be blocked. For example, if you’re driving on the highway and a car passes, if we have only one scanner, then we’ll have a shadow in the point cloud. But because the MX9 has two scanners, if one is blocked because a car passes, the other scanner will capture the points behind the car. It’s perfect for optimizing data capture.”
Back in the office, a team used Trimble Business Center (TBC) software to integrate the GCPs with the scanning points to process the 30GB point cloud. From the 3D view, they extracted a host of features such as break lines, pavement edge lines, markings, crash barriers and curbs and produced a more user-friendly vectorized dataset. In a final processing step, they created the centimeter-accurate DTM of the A93 highway, and then they delivered the vector dataset, the UAS images and the DTM to the NBMD.
“Many people think that they can only use one technology for a project, but that’s not true,” says Gröninger. “This project proved that you can combine mobile mapping with other technology to give clients a rich dataset. Having a 3D model of the highway can greatly assist the Directorate in designing the new roadway, and once construction is underway, mobile mapping will allow them to quickly and precisely acquire as-built data to support their progress.”
Carrying out this pilot project will also put Strabag in a strong position to respond to the NBMD when it issues the official tender for the A93.
Racking Up Kilometers
In addition to demo projects, Strabag has been putting on hundreds of kilometers on the MX9 with numerous other transport infrastructure projects both in and out of Germany.
They’ve taken it to planned construction sites to capture 3D measurements of existing sites and for calculating quantities, they’ve scanned buildings, they’ve driven roads to produce DTMs of routes and buildings to support planning and BIM processes, and they’ve navigated railway lines with it.
Austrian transport company, Graz-Köflacher Bahn- und Busbetrieb (GKB), commissioned Strabag to use the MX9 to scan and measure a 51-km-long stretch of railway for a planned electrification of its route network in Styria, Austria. To avoid disrupting rail operations, the point cloud needed to be collected at night. Mounted on to a special locomotive of the GKB, the system scanned all visible features, including those overhead along the track between Lieboch and Wies.
The data was again processed in TBC and the 3D dataset was provided to the GKB to help simplify and inform the planning and design of the new electrical infrastructure that is scheduled to go into operation by 2024.
“The ability of the MX9 to capture data at night gives us great flexibility for projects like railways where operations can’t typically be stopped,” says Gröninger. “And because it scans overhead, we can measure power lines, which are a very complicated to survey with a tachymeter.”
Although one of Strabag’s specialties has been road and rail infrastructure projects, mobile mapping has enabled them to successfully venture into new territories such as airports.
Gröninger’s division has been on the ground floor of Munich Airport’s Terminal 1 expansion project, a ¤455-million undertaking that will construct a new pier and create a 95,000-sq-m apron to accommodate six wide-bodied aircraft or twelve smaller aircraft.
Before construction could begin, the lead engineering company needed a precise survey to retain an accurate as-found map of the existing space and its structures before it was demolished.
The fieldwork began with setting out a network of GCPs across the 95,000-sq-m concrete apron and measuring them with the Trimble SX10. With ground control set, a team used the MX9 to scan the whole area of interest, capturing all the buildings, markings, lights, signage and concrete joints and seams in one hour.
They integrated the survey measurements and scanning points in TBC to produce the 3D model of the site, clearly showing the real-world view of the terminal’s infrastructure, from the smallest concrete joint feature to the tallest light pole. The point cloud provided the critical foundation for planning the massive construction project, the first phase of which began in July 2019.
“Surveying such a large site with conventional methods would’ve required several days of work and added cost,” says Gröninger. “With mobile mapping, we collected all the needed field data in one hour. And there’s no question about data quality or choosing which features to measure because every object is contained in the detailed point cloud. If the MX9 can see it, it captures it. That gives us a lot of confidence and security in the field, which is particularly important on highway projects and airports where revisits are difficult.”
As pressures to cut field time and reduce costs are as common as survey tools themselves, technology that enables crews to be nimble and productive, and keeps them safe, is a smart approach. Mobile mapping has proven to be a clever choice for Strabag as it drives them toward a long highway of success.
A version of this article original appeared in the May 2020 issue of POB.