On the Face of It Project at Port of North Charleston Wins Industry Honor By Mary Jo Wagner
When surveyor Paul Reichardt approached Robert Lafica, owner of Central Coast Aerial Mapping, about acquiring a cliff-face survey, he was frank in his response: “I’ll try, but I’m not sure it will work.” As a specialist in photogrammetry, Lafica is skilled in producing de- tailed 3D stereo-based topographic maps, orthophotos and orthomosa- ics, and surface models––his CCAM company has been providing such products since 1977. And he’s provided photogrammetric services many times for traditional cliff surveys using fixed-wing aircraft that collect nadir images. This California bluff survey, however, was unique. It required imaging the side of the cliff to produce a precise orthomosaic and topographic map of the cliff’s structure and integrity. Lafica was confident he could capture the 800ft long bluff with his unmanned aerial system (UAS); the challenging unknown was the ability to accurately triangulate oblique images and map them in stereo. “A drone would allow me to capture the caves and pockets along the cliff face, which you can’t get with a traditional nadir angle,” says Lafica from CCAM’s office in San Luis Obispo, Calif. “My photo- grammetry software is top notch at triangulating nadir images but I’d never tried oblique drone images. With oblique images, the land that’s closer to the camera is quite clear but it’s less clear the further away it is. I wasn’t sure if the software could calculate those odd angles to properly triangulate. That was a big question mark because without accurate triangulation, I couldn’t properly map or mosaic the cliff.” It seems he needn’t have worried. Lafica not only met the tight data specifications, he proved the untested oblique stereo mapping to be a viable methodology for cliff-face surveys, and surprised some en- lightened surveying traditionalists who are ready to apply this proven concept along other California cliffs. No more crazy, please The bluff in question sits just off the famed Pacific Coast Highway 1 in Cayucos, Calif., about 12.5 miles northwest of San Luis Obispo. Perched precariously on top of the nearly 40ft high bluff is a sweeping residence. Reichardt knows the property well. Thirteen years ago, he performed the same coastal cliff survey with a decidedly different methodology––reflectorless scanning using a Trimble 5600 DR robotic total station. “Because of the length and steepness of the bluff, you have to be about a quarter of a mile offshore to get the full face,” says Reichardt, a pro- fessional land surveyor and photogrammetrist with Terrain Group, in
Certified photogrammetrist Robert Lafica prepares the UAS for launch.
San Luis Obispo. “We took a boat out to a rock, set the instrument up, established vertical and horizontal parameters, and captured measure- ments to the house and the lot and as much of the edge face of the cliff that we could. It was precarious and crazy, but it worked.” However, when he was requested to capture the bluff again, Reichardt tapped into his long working relationship with Lafica and approached him for a safer, more efficient and more accurate technique. Reichardt needed 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) ac- curate to 1.2 in. The topographic map needed to have 1ft contours and also clearly show the mean high water (MHW) line, which delineates private and public foreshore boundaries. Given the small AOI, the ability to tilt the UAS’ camera for oblique imaging, and Lafica’s willingness to push his photogrammetry technol- ogy, the conditions were favorable for testing and potentially proving a new photogrammetric mapping concept. From all angles Lafica and Reichardt went to the bluff in late February 2020 to survey the site, timing their arrival with the lowest tide window. While Lafica prepared the UAS, Reichardt set out six black and white ground con- trol 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 checkpoints, or photo ID points, around the AOI as quality control points for the photo triangulation process. After setting ground control, Lafica immediately launched the UAS before the tide washed away the GCP targets on the beach. Flying at an altitude of 131 ft, the UAS covered the site from both nadir and oblique camera angles in nine passes, collecting 158 photos at an aver- age ground sample distance of 0.5 in. To capture the cliff side, Lafica flew the UAS about 90 ft away from its face and angled the camera at 40 degrees. While Lafica was capturing the aerial survey, Reichardt used the Trim- ble R8 and 5600 total station to collect the property corners and some top-of-surface elevations to integrate into the 3D topographic map. In less than two hours, they were packing up to go home.
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csengineermag.com
december 2020
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