C+S September 2020 Vol. 6 Issue 9 (web)

To coordinate with the flight plan and to achieve consistently high ac- curacy over such a wide area, BSF Swissphoto used a base station, GNSS receiver and differential GPS (DGPS) technology to establish control for each sub-area, and they set out a network of ground control points (GCPs). For the GCPs, teams used a combination of colored, physical targets and painted markers on hard surfaces at set intervals within each sub zone and measured the center points of each with a GNSS receiver. They set GCPs in groups of two, each placed close together for point redundancy, and laid out a total of 164 GCPs with a horizontal ac- curacy of better than 3cm. After setting control, BSF Swissphoto dispatched their flight crew to collect aerial imagery. Flying at altitudes of both 335 meters (1,100 feet) and 427 m (1,400 ft) at speeds between 120 to 155 km/h, they covered the entire AOI in 30 hours over six days. They flew 261 flight paths and collected 30,000 images with their large-format digital camera. The images had a 60 percent endlap and the average sidelap between the flying strips was about 30 percent. “Flying at such low altitudes is risky because the slightest deviation–– like movement from a sudden gust of wind––can impact our defined overlap or clarity in a photo,” says Beckmann. “We were fortunate to After downloading and processing the aerial images and aircraft trajec- tory data, Beckmann and colleagues imported them together with the GCPs into the MATCH-AT georeferencing module of Inpho to auto- matically triangulate the images. The software processed the 30,000 im-ages in batches and automatically pinpointed 254,000 common fea- tures or tie points (TPs) with multiple connections across the images. The precisely surveyed GCPs were measured in MATCH-AT, and in a second quality control step, the team used MATCH-AT’s Stereo mod- ule to manually verify and measure all the GCPs in stereo. After that the imagery was precisely oriented automatically. “The automatic triangulation and tie-point identification capabilities in Inpho are very good and give us the essential foundation for creating have good weather for the flights.” Orchestrating the orthophotos A view of the Stad van de Zon housing and building project in Heerhugowaard, The Netherlands. Heerhugowaard is one of the 17 municipalities included in BSF Swissphoto’s AOI.

precise orthophotos and orthomosaics,” says Beckmann. “You can’t build an accurate result from an inaccurate base.” After the final triangulation was done, the team downloaded an exist- ing LiDAR-based digital terrain model (DTM) of the entire AOI and analyzed it for land-cover changes that needed to be corrected or up- dated. The DTM was then integrated into the Inpho software. With the Inpho OrthoMaster module, the software used the DTM to automatically orthorectify the individual images with a ground resolu- tion of 4cm. Switching to Inpho OrthoVista, the photogrammetry tool for creating seamless orthomosaics, each orthophoto was stitched to- gether to create a 2D orthomosaic of the entire AOI. Experienced BSF Swissphoto operators then performed quality control checks on each orthomosaic and flagged any potential issues such as incorrect seam lines. They could then use editing tools such as the OrthoVista Seam Edit tool, in coordination with ancillary data like building outlines, to manually check the seam lines and ensure they didn’t intersect build- ings or cross objects like bridges that would be distorted in the mosaic. Any imperfections were fixed to create seamless, color-balanced and geometrically correct orthomosaics of the 900 sq km AOI. The shore along the seaside resort town of Bergen aan Zee, about 9 kilometers west of Alkmaar.

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