Detecting Dune Features: Coast Guard Beach, Massachusetts
Tall and contiguous coastal dunes can substantially mitigate storm surge and flooding near our coasts. Understanding the position and height of dunes, as well as how these features change over time, is critical to the USGS mission of predicting impacts to coastal infrastructure and habitat caused by storms and hurricanes. To this end, the USGS operates a number of small aircraft and unmanned aerial systems to map our coastline using either lidar (an advanced laser rangefinding system) or aerial imagery. For instance, when images are collected in a certain way, they can be processed with a photogrammetric technique called structure from motion, which generates highly-detailed 3D models of the imaged areas. Then, using a series of transects digitally cast over the resulting elevation model, an analyst can carefully determine the location of the dune crest and toe, or direct an algorithm to automatically delineate these features. Ultimately, this data is combined with near-real-time information about water levels and incoming wave height to predict how a given storm may affect coastal communities.
Aerial mapping is an integral part of the USGS science mission - collecting data from flying aircraft is an efficient means of mapping topography, bathymetry, and habitat. At the USGS's Coastal and Marine Science Centers, data products derived from aerial missions are used to delineate the extent of changing coral reefs, measure volumes of sediment that accrete and erode from our beaches, and delineate dune features that protect our infrastructure from storms. Traditionally, the USGS has used aircraft equipped with either lidar or digital imaging arrays to map landscape elevation. To derive distances (and by extension ground elevation), lidar works by directing very rapid laser pulses at the ground, measuring the time it takes for each discrete pulse to reflect off of surfaces and return to the emitter. The digital imaging systems are simpler, consisting of one or more digital cameras that take overlapping photos of the landscape. The photos from these missions are processed with a recently-popularized photogrammetric technique called structure from motion (SfM), which automatically detects the same features in overlapping images, ultimately using these overlapping images to determine 3D depth in a way analogous way to how we use our two eyes. Data from both lidar and SfM are often processed into digital surface or digital elevation models, where each pixel represents the ground or surface elevation underlying that pixel. It is often these digital elevation or surface models, such as the one shown in the video, that become the basis for a coastal manager’s or scientist's analysis. When SfM is used, as in this video, the elevation model can be accompanied by a high-resolution color map of the study area.