USING GIS TO CORRELATE DIGITALLY MAPPED BEDROCK FEATURES WITH AIRBORNE LIDAR DATA OF FREEPORT MAINE

The geology of Wolfe's Neck Woods State Park in Freeport, Maine was mapped using digital techniques and GIS in the fall of 2007 including handheld GPS for data entry, half-foot orthophotos of the area for spatial reference and electronic total stations for detailed surveying. Features mapped include gneiss layers and granite contacts as well as stretching lineations in the metamorphic rocks. The resulting map shows greater detail than earlier quadrangle maps as it includes individual dike intrusions. Granite intrusions and one prominent basalt dike are visible along the exposed southeastern shore, where older, deformed granites are intruded parallel to the metamorphic layers, while younger granites tend to cut across those layers. Along 2100 feet of shore 33 individual cross-cutting granite intrusions have been mapped, with widths from 1-120 feet. These younger granite intrusions tend to be more resistant to erosion than the surrounding gneissic host rock and so should be visible in the area topography.

Airborne LiDAR data covering 584 square miles along the coasts of York and Cumberland Counties, Maine were collected for NOAA in 2006. These data have a vertical accuracy to 1.2 feet and are delivered in 100,000 square foot tiles as first return points, last return points and bare earth grids. Bare earth grids are produced from last returns which have been filtered to remove vegetation and buildings and interpolated to represent the elevation of the unvegetated surface. Triangulated irregular networks (TINs) of bare earth grids and filtered last returns are also provided.

A GIS is used to correlate LiDAR data with geologic data collected in the field. Using filtered last returns and bare earth data that have been rasterized and hillshaded, large-scale geologic structures within the wooded terrain are visible. These structures include the more erosionally resistant steeply dipping beds, with potential for seeing more areally extensive folds not picked up from scattered outcrops; cross-cutting granite intrusions, which offer the most contrast in erosional resistance; and glacial streamlining, grooving, and stoss and lee topography. As a tool for geological mapping, LiDAR has potential for tracking bedrock fold patterns and intrusions in areas of vegetative cover and in interpreting glaciated topography.