Photo: Trimble
Cliff surveys are traditionally performed with fixed-wing aircraft that collect nadir photos. However, a photogrammetry company accurately triangulated oblique images and mapped them in 3D stereo, developing a new technique in the process.
The erosion survey — along Pacific Coast Highway 1 in Cayucos, California — required imaging the side of the cliff to produce a precise orthomosaic and topographic map of its structure and integrity. The project required a 0.5-inch orthomosaic, a 1.2-inch 3D topographic contour map and a vector-based digital terrain model accurate to 1.2 inches.
Surveyor Paul Reichardt and Robert Lafica, owner of Central Coast Aerial Mapping, placed ground control points around the property and beach, and then used a Trimble R8 GNSS receiver to measure their positions to within 0.04-foot accuracy. They also established four checkpoints for quality control in the photo triangulation process. The R8 and a Trimble 5600 total station were used to collect property corners and top-of-surface elevations to integrate into the 3D topographic map.
At an altitude of 131 feet, the UAS covered the site from both nadir and oblique camera angles in nine passes, collecting 158 photos at an average ground sample distance of 0.5 inches. To capture the cliff side, Lafica flew the UAS about 90 feet from its face and angled the camera at 40 degrees.
The photos and position data were loaded into Trimble’s Inpho UASMaster photogrammetric software to automatically triangulate the images. The software pinpointed 6,368 common features with multiple connections to match images to each other. After initial triangulation, precise coordinates were attached to each control point, a final triangulation was completed to create the maps, and a new technique for mapping cliff faces was born.