HOW THE NORTH CAROLINA GEOLOGICAL SURVEY MAPS LANDSLIDES IN 2020: LIDAR, DRONES, SATELLITES, AND BOOTS ON THE GROUND

Since 1990, the North Carolina Geological Survey (NCGS) has mapped over 3,900 landslides. The dominant type of landslides are debris flows triggered by high-precipitation events such as tropical cyclones or convective storm cells. Since mapping efforts began nearly 30 years ago, our technology and techniques have evolved to include the use of field computers, GPS, and digital mapping tools such as high-resolution digital elevation models (DEMs), public-domain satellite data, unmanned aerial systems (UAS), and a continued focus on field-based verification.

The NCGS landslide geodatabase is both an event- and historical-inventory, cataloging landslides of known vintages (e.g., from a particular storm event) and those that have persisted in the landscape with unknown origins. Collected in 2017, quality-level 1 (QL1) lidar point clouds (nominal 8 points/m²) are publicly available for all of western North Carolina. We reduce these data to 0.5-meter resolution bare-earth DEMs, providing a marked increase in terrain analysis efficiency and completeness compared to previously available data. Because landslides occurring since the QL1 data collection are not represented in these DEMs, we use public domain, rapid repeat-cycle, multispectral satellite imagery to observe losses in hillslope vegetation, a potential indicator of mass-wasting activity. In particular, the use of ESA Sentinel-2 data targets decreases in vegetative cover following large landslide events. During routine and emergency landslide responses, we utilize UAS to increase field efficiency and aid interpretation. A key component of the UAS technology is the generation of orthomosaic images to photo-document landslides for use in a GIS. The foundation of this effort, however, is the continued, iterative process of field-verification. More than half of our 2019-2020 landslide inventory entries have been field-verified. Given this level of scrutiny, we have increased the resolution of our mapping to >1:2,000, which would have been unreasonable at the county-wide scale a decade ago. In addition to traditional static maps, NCGS inventories are available as interactive web-map viewers, increasing the accessibility of the data to the public – a primary client in mapping and communicating landslide hazards in western North Carolina.