REFINING GEOMORPHIC AND GEOLOGIC MAPPING OF ‘QUATERNARY ALLUVIUM' IN A GIS FORMAT USING HIGH-RESOLUTION LIDAR, FARMVILLE QUADRANGLE, COASTAL PLAIN, NORTH CAROLINA

Mapping geomorphic features on the North Carolina Coastal Plain using high-resolution LiDAR (Light Detection and Ranging) elevation models in a Geographical Information System (GIS) format improves the analysis and interpretation of Quaternary landforms in river basins. Here, landforms interpreted from a LiDAR (25 cm resolution) base are compared with units interpreted from a Digital Raster Graphic (DRG) image of the 7.5 Minute Farmville Topographic Quadrangle. Results show that previously mapped units in drainages are morphologically complex and have different map extents. This suggests that the commonly used surficial geologic map unit, Quaternary Alluvium (Q al), is too generalized for accurate mapping from a LiDAR base.

ASCII tiles (10,000 ft X 10,000 ft, 20 ft DEMs) were downloaded from the NC Floodplain Mapping website (www.ncfloodmaps.com) for the Farmville area. These were converted into floating-point GRIDS, mosaicked into larger scale tiles (10 X 10), and reprojected from State Plane NAD 83 ft into meters. Hillshade, slope, and contour line layers were created from the reprojected elevation grids; contour intervals were 0.5 and 0.25 m. Color ramps were added to the elevation grids to enhance features. The DRG for Farmville (contour interval is 2 m), aerial photographs, and the coverage for geomorphic map units interpreted from the DRG was added to the map. Heads-up digitizing followed a system of rules to define map units from the LiDAR, starting from the lowest elevations in drainage bottoms.

The map of landforms derived from LiDAR provided high resolution details of the land surface that did not resolve on the 7.5-minute DRG and in aerial photos. Working from the bottom of the drainage up, topographic relief enhanced by hillshade, slope, color ramps and contour lines allowed new extents to be mapped for previously defined landforms, and for the floodplain to be subdivided into additional units. These include an extensive wetland flat, small-scale stream channels, meander belts, partly buried ridges and swales (meander belts), natural levees and crevasse splays along streams, and fan deltas at stream confluences. The boundary between the active Holocene depositional system and older Pleistocene systems was demarcated. Successive Pleistocene terraces step down in elevation from uplands to the drainage bottom.