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Paper No. 18-4
Presentation Time: 1:30 PM-5:30 PM
A SCALE-BASED APPROACH TO MAPPING SURFICIAL DEPOSITS IN VIRGINIA
Unconsolidated deposits of alluvial and colluvial sediment are widespread in Virginia. These deposits have physical characteristics that are different than residual soils and important to consider when developing, conserving, or remediating land. The recent availability of high-resolution LiDAR data has allowed these deposits to be accurately delineated and portrayed on geologic maps at multiple resolutions to meet specific customer needs. In response, the Virginia Department of Energy, Geology and Mineral Resources Program has developed a scale-based approach to mapping surficial deposits that maintains a consistent nomenclature but varies the resolution of the LiDAR DEM used to identify deposits, the map scale at which deposits are digitized, and the degree of subdivision within each deposit type. Our approach also establishes minimum widths and areas of deposits to be mapped. Based on recent experience creating statewide (1:250,000-scale), regional (1:100,000-scale), and detailed (1:24,000-scale) surficial deposit maps through the National Cooperative Geologic Mapping Program, we make the following recommendations: 1) for statewide mapping use a 10-meter LiDAR DEM, digitize surficial deposits at a scale of 1:24,000, establish a minimum deposit width of 150 meters (500 feet) and a minimum area of 0.05 km2 (12 acres), and do not subdivide map units; 2) for regional mapping, use a 5-meter LiDAR DEM, digitize at a scale of 1:10,000, establish a minimum width of 60 meters (200 feet) and a minimum area of 0.025 km2 (6 acres), and use up to 3 subdivisions for complex map units if warranted; and 3) for detailed mapping, use a 1-meter LiDAR DEM, digitize at a scale of 1:6,000, establish a minimum width of 30 meters (100 feet) and a minimum area of 0.01 km2 (2.5 acres), and use up to 5 subdivisions of complex map units if warranted. At all scales, we recommend only mapping deposits that are obvious to a viewer of the LiDAR DEM at twice the resolution of the desired map product.