LIDAR AS A MAPPING TOOL FOR LANDSLIDES IN PENNSYLVANIA

Lidar has revolutionized many aspects of geologic mapping, but as with other technologies, its optimal use requires understanding of the data and adaptation to specific conditions. Usefulness of lidar comes both from its utility in visualizing surfaces and the ease and accuracy of measuring and calculating distances, slopes and similar statistics.

Pennsylvania has a variety of types and geologic settings of landslide and other mass wasting features. 1970s and 1980s vintage landslide mapping is useful for identifying regional issues, but less useful for site specific analysis because of tree cover and landscape changes since the mapping. The availability of statewide lidar acquired in 2006 - 2008 offers a way to improve our mapping with reasonable investment of time and effort.

Common ways to visualize the surface represented by the lidar data are hillshade and shaded slope maps made from a Digital Elevation Model. Shaded slope maps are good for showing scarps and toes of classic slump-earthflows. They highlight vertical changes in slope very well but are of little use in seeing debris avalanche chutes on steep slopes. Hillshade images can show subtle irregularities in surfaces, but are direction-dependent. Hillshades with a false sun angle from the northwest are easy for most people to recognize, but a sun angle that skims the slope, especially from a low angle may highlight features better. Contours, including closed depression contours, can be very useful in confirming features suggested by other visualization techniques. Coloring by elevation can be useful in visualizing complex topography where optical illusions of false sun angles cause confusion. Combined semi-transparent layers can enhance visualization.

Comparison of 1980s landslide maps made with stereo pairs of aerial photographs to lidar images suggests that some features previously identified are likely not landslides. Older photo-only techniques had to make assumptions based on large landforms when the ground was obscured by trees. Being able to visualize the bare ground surface without trees makes a big difference.

Lidar alone is not an adequate tool – aerial photos: either older analog and/or more current digital orthoimages, allow recognition of artifacts and anomalies. As always, field checking is critical to confirm what we think we see on imagery.