Mapping with Lidar-Based DEMs — A Geologist's New Tool
Whitfield, Thomas G.
Geologists use a variety of tools in their work: their previous work, work by others, topographic maps, aerial photography, borehole data, and good old boots-on-the-ground field work. Each has advantages and disadvantages, but together they enable the geologist to think in 3-D and discern a logical geologic model of the area. Sometimes natural barriers such as forest cover, steep terrain, indignant landowners, etc., hinder what can be observed, detracting from the model.
Between 2004 and 2008, the entire state of Pennsylvania was overflown with high resolution (1-foot pixel) digital orthoimagery and lidar (Light Distance and Ranging) using a high pulse laser. The resultant lidar-based DEMs (Digital Elevation Models) derived are 3.2-foot resolution pixels in 10,000 by 10,000-foot tiles. The DEMs are processed to produce a bare-earth model DEM, effectively stripping away any forest cover, vegetation, buildings and infrastructure.
The most common derivative of a DEM is a hillshade raster, showing solar values of sunlit and shaded terrain. The high resolution of the base data shows intricate detail of even hidden and/or inaccessible features. There are shortfalls to the hillshade, such as brightly sunlit or deeply shadowed areas hiding detail. This presentation demonstrates a lesser known DEM derivative called a "slope-shape". It is a raster grid based only on slope. The key aspect of a slope-shape is how it is displayed. Slope-shapes reveal a much more detailed view of features, without regard to sun or shade.
By using different DEM visualization techniques, features like bedding, contacts, faults, landslides, sinkholes, etc., are visible. Other natural features such as glacial boundaries, kettles and eskers are also revealed. Manmade features like quarries, ghost towns, overgrown forest trails, etc., are also visible, despite heavy ground cover. Complex geology that was not visible, even in high-resolution orthoimagery, is now visible.
In the geologist's quest to see the entire "big picture" and close-in detail offered by high-resolution surface models, a lidar based DEM is absolutely essential. It may prove to be one of the most valuable tools in the geologist's toolbox.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013