IDENTIFYING ACTIVE LANDSLIDES USING REPEAT LIDAR DATASETS, NORTH DAKOTA

Since 2001, the North Dakota Geological Survey (NDGS) has mapped over 45,000 landslides at the 1:24,000 scale; an inventory of slumps, earth flows, and soil creep which can have a substantial annual impact on statewide infrastructure. Historically, the NDGS has mapped landslides using aerial photographs from the 1950s and 1960s (viewed in stereopair) and early satellite imagery (Phase 1). Since 2017 it has been updating all its maps using LiDAR data (Phase 2), and landslide mapping is nearing completion at the 1:24,000 scale at the Phase 2 level for the entirety of North Dakota. This scale provides great geologic context to engineering firms, construction companies, and land developers as they strive to mitigate landslide risk. While these maps are useful, there was no temporal data associated with landslides and no sense of susceptibility was conveyed. North Dakota has statewide LiDAR (Light-Detection and Ranging) data available at approximately 1-meter horizontal spacing and now is getting repeat coverage, which is managed by the North Dakota Department of Water Resources. The timespan between coverages is approximately 10 years on average. Comparing the elevations of the two datasets allows NDGS geologists to observe any changes that occurred to the land surface over that time interval. An elevation difference between two raster layers is calculated by using the Minus Tool in ArcGIS (Spatial Analyst), resulting in X (Latitude), Y (Longitude), and Z (vertical displacement). There is often appreciable noise in the resulting raster, especially when comparing LiDAR DEMs of differing quality. Noise can be filtered out by making vertical difference +/- 1 foot transparent. There are different signals of vertical displacement observed, such as: farming practices, logjams, ponding water, snowdrifts, construction earthwork, stream erosion and many other artifacts and non-landslide features. The NDGS is now applying this vertical displacement method where multiple LiDAR datasets are available (which currently is the NE part of North Dakota, including the entire Red River corridor and the Pembina Gorge) to identify areas of landslides that have recently been “active”. This supplement of temporal data is now Phase 3 of the NDGS landslide mapping program.