Surveying a mountain highway with unmanned aircraft systems

On a mountain highway project in the Republic of Macedonia, a Greek company is saving time and money by using an unmanned aircraft system (UAS) to collect aerial imagery and produce high-value deliverables.

Surveying a mountain highway with unmanned aircraft systems
Surveying a mountain highway with unmanned aircraft systems

By Eric McCuaig

On a mountain highway project in the Republic of Macedonia, a Greek company is saving time and money by using an unmanned aircraft system (UAS) to collect aerial imagery and produce high-value deliverables.

In the steep canyons of the Republic of Macedonia—a landlocked Balkan nation of mountains, lakes and ancient towns—AKTOR ADT, a large international construction company based in Athens, Greece, is building a highway for the national government. It will run 28.2 km (17.5 mi) from the village of Demir Kapija to Smokvica, traversing a natural gorge and crossing the Vardar River. The highway will help complete Corridor X, the most important north-south route through the country, which provides a connection to the region’s modern, multi-national transport network.

Surveying a mountain highway with unmanned aircraft systemsSurveying a mountain highway with unmanned aircraft systems

Due to the area’s very challenging geomorphology—including steep slopes, ravines, rivers, forests, and protected areas—the survey project was designed and is being undertaken with great regard for the environmental conditions and protection of the local flora and fauna.

The effort, which began in March 2013 and is projected to continue through the end of 2016, requires construction of millions of cubic meters of earthworks as well as heavy structures. The project includes two twin tunnels, six bridges (four of which are balanced cantilever bridges), more than 100 culverts, five overpasses, seven underpasses, two interchanges, high embankments, and deep cuts. As a result, the work requires regular monitoring of the amount of material removed, landslides after heavy rain falls and other such variables, as well as progress reporting to government ministries.

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To conduct the survey, AKTOR ADT has contracted with AVIATOP pc, a company it has collaborated with for many years. AVIATOP deploys various unmanned aircraft systems (UAS) to provide aerial mapping and surveying services for corridor mapping and monitoring the progress of construction projects. For this project, the firm is using a Trimble UX5 UAS to fly over the project corridor and collect data every two months for the entire project timeline. Equipped with a high-resolution camera, the UX5 captures images needed to measure the slope edges of cuts and fills and the surface of the roadway. George Papastamos, a surveying engineer and AVIATOP’s founder and CEO, is the lead on the project.

The challenges
The highway’s path snakes through deep gorges, reaching an elevation of about 500 m (1,600 ft) above sea level with a 6 percent grade and crossing bridges up to 90 m (300 ft) in height. Following a river, the gorges’ corridors produce winds with speeds greater than 90 km/hr (60 mph). These conditions make aerial surveying very challenging.

Gorge Side View

“It is a quite large and demanding project,” says AKTOR ADT Civil Engineer Konstantinos Simou. “Our quantity surveys and 3D geometry need to be accurate within a few centimeters. The project’s shape is constantly changing and we are always looking for the most updated information. Anyone who tried to achieve this with conventional methods [e.g. land surveying] would realize that monitoring a project of this scale is expensive. However, using UAS we manage to have a full view of the activities and the progress in near real-time.”

According to Papastamos, the most challenging part of the project is the very rough surface and mountainous area through which the corridor passes. “This is very dangerous even for the aircraft itself,” he says. “The big differences in ground elevation can cause turbulence anytime, and the narrow passages make the landing procedure extremely difficult. Additionally, the limited line-of-sight minimizes the range of the radio link (loss of radio communications can initiate the fail-safe procedure, which automatically directs the aircraft to return immediately to its designated landing site). Finally, there’s the challenge of keeping the image resolution relatively homogeneous while being productive.”

In spite of the challenges, AVIATOP has been able to conduct the UAS aerial survey by closely monitoring wind conditions and carefully choosing takeoff and landing zones—thanks to the UX5’s remarkable performance.

The first five flights
The corridor’s edges are up to 400 m (1,300 ft) higher than its center and some of the cut slopes are more than 100 m (330 ft) high. In order to clear the tops of the hills along the project’s path, Papastamos flies the UAS just above the edges, which yields a ground sample distance (GSD) of around 12.8 cm (5 in).

Due to the flight’s elevation and the corridor’s length, each survey requires at least six flights, and Papastamos needs to find a suitable landing area for each one. Therefore, each aerial survey takes two to three days, with each flight lasting 30 to 40 minutes. So far, AVIATOP has conducted five flights and captured roughly 2,050 images.

The very rough terrain also requires densely-spaced ground control points (GCPs). Papastamos sets them every 250 m (820 ft) on each side of the flight area, for a total of about 150 points. He sets up his GCPs using Trimble R6 and R8 GNSS receivers and re-uses them, except in those very few cases when the benchmarks are destroyed by the progress of the project. In areas with sharp elevation differences, he installs as many as needed to produce a correct model. He sets the camera’s shutter speed to somewhere between 1,600 and 3,200, depending on the lighting conditions, with an 80-percent forward and side overlap.

Skouries High

The deliverables
Project deliverables include georeferenced orthophotos of the project, aerial videos, a 3D surface model, profile sections, quantity surveys, horizontal plans and virtual reality videos. To produce the required information, AVIATOP uses the collected point cloud data and break lines, as well as situation maps of areas where material can be deposited, and locations of planned access roads and stream or utility diversions. The firm also produces situation maps to document landslides after heavy rains and show break lines, boundaries, top and bottom banks, crests, houses, structures, volumes and other features.

It typically takes Papastamos about ten days to post process the data using Agisoft PhotoScan and create the required deliverables. From the 3D models, he makes volumetric calculations. The survey so far shows that AKTOR ADT has been removing about 500,000 cubic meters (650,000 cubic yards) of dirt every month.

Advantages of using UAS
The new highway is not visible from the existing roadway, Simou explains, so people driving by do not realize the progress being made on this big investment of their tax money. “Using the UAS, we are able to document the progress of our project month-by-month with photographs,” he says. “The virtual reality videos we produce and publish via the local media have greatly helped to inform the public that the project is progressing and their money is not wasted.”

In addition, the UX5 has significantly expedited the surveying of areas indirectly involved with the project—such as streams, rivers, ditches for large irrigation areas, dumping and depositing areas and access roads. This would typically be very expensive and time consuming to document by conventional methods. “It is also very helpful that whenever we need to measure, evaluate or investigate the project further, we can do it from our laptops, without spending time and effort to conduct a site visit,” Simou adds.

Cut 2

Lessons learned
“This project taught us how to optimize the installation of benchmarks to get accurate results in such a rough area,” says Papastamos. “It also taught us how to land the aircraft in very narrow and difficult areas and to pay close attention to weather and lighting conditions. The project area is very rugged and varied terrain is a good school. We are doing several other projects like this one. However, because we have been working on this one from day one, it is our guide.”

Simou explains that by providing clarity and transparency, the UAS survey enabled AKTOR ADT to reinforce the trust and the confidence of all the stakeholders and the public. It also saved time for the company’s top managers, because the videos minimized their need to visit the project in person. “From my experience, I estimate that a UAS survey is at least three times cheaper than a conventional survey and eight to ten times faster for large-scale projects,” he says. “In addition, we are getting all the other products—orthophotos, a dense 3D model, videos, and virtual reality—as well as the ability to zoom in on any area and get details up to a 1-2 cm resolution.”

Given that the project requires repeating the UAS survey every two months until completion, that’s a lot of savings.

About the Author
Eric McCuaig has been involved with mobile mapping and aerial surveying technologies for the past 12 years in a variety of leadership positions in North America and Europe. His most recent position is managing Trimble’s UAS, land mobile and aerial systems and he was involved in Trimble obtaining the first FAA exemption for a fixed wing UAS for flight operations in the United States. His areas of expertise include position and orientation systems, photogrammetry, remote sensing, mobile mapping and UAS. Eric graduated from the University of Guelph in Ontario, Canada.

About Trimble Geospatial
Trimble's Geospatial Division provides solutions that facilitate high-quality, productive workflows and information exchange, driving value for a global and diverse customer base of surveyors, engineering and GIS service companies, governments, utilities and transportation authorities. Trimble's innovative technologies include integrated sensors, field applications, real-time communications and office software for processing, modeling and data analytics. Using Trimble solutions, organizations can capture the most accurate spatial data and transform it into intelligence to deliver increased productivity and improved decision-making. Whether enabling more efficient use of natural resources or enhancing the performance and lifecycle of civil infrastructure, timely and reliable geospatial information is at the core of Trimble's solutions to transform the way work is done.

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