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Full Frontal Mapping in 3D

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Cliff surveys are traditionally performed with fixed-wing aircraft that collect nadir photos. However, one particular California bluff required the cliff's face be surveyed for an erosion study. A photogrammetry company used the opportunity to test the viability of accurately triangulating oblique images and mapping them in 3D stereo. The company not only met tight data specifications, it developed a new technique for cliff-face surveys. overview Location CALIFORNIA, USA TRANSFORMING THE WAY THE WORLD WORKS TRANSFORMING THE WAY THE WORLD WORKS Photogrammetry specialist Robert Lafica and his company, Central Coast Aerial Mapping, have been producing detailed 3D stereo-based topographic maps, orthophotos and orthomosaics and surface models since 1977. Although he's performed many traditional cliff surveys, he'd never tried oblique-image mapping. "With oblique images, the land that's closer to the camera is quite clear but it's less clear the further away it is," said Lafica. "I wasn't sure if our photogrammetry software could calculate those odd angles to properly triangulate. And without precise triangulation, I couldn't accurately map or mosaic the cliff." The unique project targeted a 40-ft-high bluff with a sweeping residence perched on top. It's located along the Pacific Coast Highway 1 about 12.5 miles northwest of San Luis Obispo. Lafica needed to produce a 0.5-inch orthomosaic of the entire area of interest (AOI), a 1.2-in 3D topographic contour map of the cliff face and top-of-bluff surface and a vector-based digital terrain model (DTM) accurate to 1.2 in. The topographic map needed to have 1-ft contours and also clearly show the mean high water (MHW) line, which delineates private and public foreshore boundaries. FROM ALL ANGLES Lafica and a survey colleague, Paul Reichardt, surveyed the site, timing their arrival with the lowest tide window. Reichardt set out six ground control point (GCP) targets around the property and along the beach and then used his Trimble R8 GNSS receiver to measure their positions to within 0.04 ft accuracy. They also established four photo ID points around the AOI for quality control for the photo triangulation process. Lafica flew the UAS and collected 158 nadir and oblique photos at an average ground sample distance of 0.5 in. To capture the cliff side, he flew the UAS with the camera

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