Absolute Accuracy in the Alpine By Mary Jo Wagner
When a Cessna fixed-wing aircraft took off over an Austrian alpine region one August morning, it ascended carrying two crew, a large format digital camera, a bit of uncertainty, and a whole lot of risk. The plane’s mission was part of an overall project to provide a precise, as-built survey of a 40 kilometer (24 mile), narrow stretch of railway in the Lower Inn Valley east of Innsbruck. Although this wasn’t an unusual task for a surveying company, the required accuracy of the project was: 2 cm (0.8 in) horizontal and vertical accuracy for the en- tire area of interest (AOI). “Airborne mapping surveys under 5 cm (2 in) accuracy are rare in alpine environments because the mountainous terrain is treacherous and unpredictable,” says Klaus Legat, head of the photogrammetry and aerial survey department at Vermessung AVT, a surveying company based in Imst, Austria. “To achieve 2 cm resolution we would have to fly about 500 m above ground within a very narrow air space, making maneuvering tricky. And for this specific railway section, we’d need to supplement the aerial imagery with ground imagery, precise control points and software that could integrate all the data into an accurate, true-to-life map of the AOI. Neither we nor the client had ever taken on such a task before so this would be a proof of concept test to see whether modern surveying techniques could deliver such high accu- racy over a large area. It was a risky proposition.” But, it proved to be a successful gamble. Integrating GNSS technology, aerial imagery and Lidar data, mobile mapping technology, and advanced image processing software, AVT proved not only that its multi-sensor, data fusion approach was a solid bet, it’s given the company the confidence and experience to pursue similar high-precision alpine mapping projects, pioneer new business developments and reap the rewards of an expanded service area, proj- ect portfolio, and revenue stream. Placing Their Bets Based in Imst, about 50 km (30 miles) west of Innsbruck, the Alps are in AVT’s backyard, making mapping and surveying mountainous envi- ronments a natural focus since the company’s inception in 1970. With such a perfect testbed for researching methodologies and technologies, AVT has become quite comfortable and experienced in the notoriously difficult alpine regions, pushing their photogrammetric capabilities to the point where they routinely provide ground-resolution accuracies of 5 cm or better. The Lower Inn Valley (LIV) railway project would push them further. A double-track high-speed main line of the Austrian Federal Railways (ÖBB), the LIV railway is a core part of the Trans-European Transport
Networks (TEN-T), a high-performance railroad that will eventually connect southern Italy with northern Europe. AVT’s target was the 40 km section between the Austrian towns of Kundl and Baumkirchen (KB), the first segment of the LIV that opened in November 2012. Designed for speeds up to 220 km/h (124 mph), the KB dual railway crosses the Alps and is the northern connection to the Brenner Base Tunnel, a 64 km long (40 mile long) tunnel between Austria and Italy scheduled for operation in 2026. ÖBB Infrastruktur AG issued a tender for a final as-built measurement of the above-ground areas of the new line, the converted sections of the existing line and any objects within 100 m (328 ft) of the tracks themselves. In addition to the 2 cm vertical and horizontal accuracy requirement, ÖBB specified that access to the tracks was prohibited so a purely terrestrial measurement technique wouldn’t be possible. There was also another complexity: the tracks were lined by up to 6 m high (19 ft high) noise-prevention walls. “Because we couldn’t access the track, we had to choose aero photo- grammetry,” says Legat. “But the prevention walls and other obstacles would hide many along-track features from the plane’s nadir-looking camera. So, our approach was to pair an aerial survey with mobile mapping. The aerial data would give us both the railway detail and the area outside the walls and the mobile mapper would give us the ancillary features not visible from the plane.” Critical to the multi-sensor approach, however, was the ability to in- tegrate the diverse data formats into one image processing software to create precise orthophotos and a seamless orthomosaic. AVT selected Trimble’s Inpho Suite to process its data. Inpho is a set of photogram- etry modules for transforming aerial imagery into orthophoto mosaics, point clouds and other 3D datasets. “Inpho can work with both analog and all brands of digital cameras,” says Legat. “That flexibility and interoperability is quite important for us and saves us significant data processing time. It also is quite good at triangulating and multi-ray image matching which is the foundation for producing accurate results.” Rolling the Dice To achieve consistently high accuracy over such a long distance, AVT The peak of Rough Horn in the Tannheimer Mountains in the western Austrian state of Tyrol where AVT is based.
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october 2020
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