AVT’s plane sits ready for a photogrammetric flight in the Alps.
needed to first establish both a precise control network and a network of ground control points (GCPs). In an effort to minimize atmospheric disturbances, they chose to create the control network through a static observation, night survey. Using two permanent base stations situated near the center of the AOI, crews set out 30 GNSS receivers on pre- determined locations and the units simultaneously collected measure- ments for 12 hours. The permanent survey established a base network precise to 0.5 cm (0.2 in). For the GCPs, teams painted markers on concrete or other hard sur- faces at 2 km (1 mile) intervals around the 40 km area and measured the center points of each with shorter, static observations of about two hours. They set five points at a time and laid out a total of 50 GCPs with a horizontal accuracy of 1 cm (0.4 in). With the control networks set, AVT could dispatch their flight crew to collect aerial imagery with a ground sample distance of 2 cm. Flying at an altitude of 450 m (1,476 ft) and an average speed of 200 km/h, they covered the entire AOI in two hours. They flew 21 flight paths in an east- west direction and collected 1,300 images with their Vexcel large-format digital camera. The images had a 60 percent overlap and the average lateral overlap between the flying segments was about 50 percent. Per ÖBB’s request, they also carried out a secondary aerial Lidar mis- sion to produce de-tailed point clouds to generate digital terrain and surface models. Reducing the altitude to 200 m, a crew collected Lidar data with a point density of 25 points per square meter and a side- overlap of 70 percent. Completing the data collection campaign was a terrestrial survey using a Trimble MX7 mobile imaging system. A vehicle-mounted photogrammetric system, the MX7 is equipped with six, 5 megapixel cameras, and Trimble Applanix GNSS and inertial georeferencing modules. The system was mounted on the roof of a small van that was placed on a truck-transport wagon which in turn was pulled by a spe- cial locomotive of the ÖBB. Maintaining an average driving speed of 50 km/h, the MX7 captured a 30MP panoramic image every 4 m (13 ft) along each of the KB tracks and acquired important features such as switching boxes, passenger benches and electrical housings that were not visible in the aerial photogrammetry. Along the railway there is a 600-m-long (1,698-ft) tunnel which required the AVT team to install The lush, narrow valleys of the Austrian Alps. Photo: Daja Gellerova, Pixabay.
six LED headlights to compensate for the low-light conditions and en- able them to capture features inside the tunnel walls. In total, the MX7 system collected 20,000 images. “This was our first experience with the MX7 so we weren’t sure how well it would perform,” says Legat. “It not only complimented our aerial campaign and captured the essential ground elements but it performed surprisingly well in the tunnel. We knew acquiring features with LED spotlights would be challenging, but the cameras’ rapid ex- posure times enabled us to capture darkened features.” Into the Ortho After downloading and processing the aerial images and aircraft tra- jectory data, AVT personnel imported that data as well as the GCPs into the Match-AT georeferencing module of Inpho to automatically triangulate the images. Using an image pyramid process, the software analyzed the 1300 images and automatically pinpointed 15,500 com- mon features or tie points (TPs) across the images, with an average of 200 TPs per image. The precisely surveyed GCPs were measured in the images and the Match-AT module used a bundle-block adjustment pro- cess to automatically and precisely orient the imagery. The accuracy of the GCPs in the AT was around 1 cm in planimetry and altimetry. “The triangulation needed to be done as precisely as possible to ensure we could achieve the vertical accuracy,” says Legat. “So investing in our ground control was crucial but we also needed image process- ing software that could deliver on the precision requirements. As a long-term user of Inpho, we haven’t found any software that can rival Match-AT’s triangulation abilities or its intuitiveness.” With the OrthoMaster module, the software automatically orthorecti- fied the individual images with a ground resolution of 2 cm. Switch- ing to Inpho OrthoVista each orthophoto was then stitched together to create a 2D orthomosaic for the whole AOI. Experienced AVT opera- tors used the OrthoVista Seam Edit tool to manually check the seam lines to ensure they didn’t cross objects like bridges which would be distorted in the mosaic. Any imperfections were fixed to create a seamless, color-balanced and geometrically correct orthomosaic of the 40 km corridor. AVT personnel used the Inpho interface to export the aerial images into
50
csengineermag.com
october 2020
Made with FlippingBook Annual report