Slope mapping, LiDAR data, and Macon County


In light of recent community discussions about regulating development on steep slopes, it is worth mentioning that Coweeta LTER researchers have increasingly begun to consider the scientific importance of landslides. Some of the recent debates about landslides and slope topography have related to the North Carolina Geological Survey’s (NCGS) maps. Many map-related technical questions have been raised in this newspaper, at planning meetings and elsewhere. This suggests that the maps themselves, aside from other the issues related to the steep slope development, have raised a series of concerns. In light of this conversation, we will discuss the research being done at the Coweeta LTER on landslides, including the LiDAR technology behind the maps and why scientists working at the Coweeta LTER and elsewhere use this mapping technology.

To create the maps, the NCGS used a technology called LiDAR, which stands for “Light Detection and Ranging.” LiDAR works using similar principles as radar, though it uses many laser beams and a global positioning system (GPS) to capture a variety of land cover characteristics in 3D. Here is basically how it operates: after being mounted on an airplane, a LiDAR unit emits laser beams toward the ground, then it calculates elevation based on how quickly the beams bounce back to the unit. Using this information, a LiDAR technician can create a number of things, including an extremely accurate elevation map for an area. Even in heavily forested areas like Macon County, LiDAR is incredibly accurate. One recent LiDAR survey used by University of Georgia cartographers surveyed Great Smoky Mountains National Park and recorded the entire park’s ground elevation within six inches of accuracy.

Professionals worldwide increasingly use LiDAR because of its precision, accuracy, and reliability. Other uses of LiDAR include automobile speed detection, transmission line placement and examination of vegetation near power lines. Because LiDAR is the state-of-the-art technology in mapping landscape characteristics like slopes, the NCGS and other government agencies charged with providing quality information in the public’s interest are keen to make LiDAR-based products widely available. Scientists like ecologists, geologists, geographers, seismologists, and physicists all routinely use LiDAR in their work. Local and state planners nationwide, too, are beginning to use LiDAR because of the practical information it can provide about local landscapes.

Coweeta LTER scientists have used LiDAR data from the NCGS to produce scientific models of land movements in the Coweeta watershed of Macon County. In this research, LiDAR information created and made public by the NCGS about portions of Macon County’s elevation was used to create a new computer model of how heavy rainfall, vegetation, and tree canopy are related to landslide movements. These high-resolution models allow Coweeta LTER scientists to also identify potential hazard areas based on both LiDAR information and the hydrological and vegetation data. The hazards they were able to scientifically analyze included the landscape damage done by flash floods, landslides and forest fires. The integration of LiDAR information with other forms of ecosystem science provides a new understanding of the landscape that would not otherwise be possible without LiDAR technology.

Scientific results using this information and published in a scientific paper (by Band, Hwang, Hales, Vose, and Ford) show that slope instability in various sites across the watershed tends to occur toward the head of hollows, in soils with shallow-to-medium depths, where slopes are moderate. The study also found, thanks in part to the use of LiDAR, that areas further downslope with thicker soils and greater drainage areas (especially hollows) seem to have lower root cohesion. Also, areas with lower slopes and higher thickness of leaves (referred to as Leaf Area Index or LAI) tend to produce greater slope stability.

This edition of our column was designed to give a broader, scientific perspective on LiDAR as one mapping technique that produces reliable, relevant, and reputable information used by Coweeta LTER researchers and other organizations in the production of information about steep slope development.

 

This column is produced by members of the Coweeta Listening Project (CLP), a branch of the Coweeta Long Term Ecological Research Program. Views expressed here are not representative of the USDA Forest Service or the Coweeta Hydrologic Lab. Please share questions, comments, or suggestions for future topics at cwtlistn@uga.edu or Coweeta Listening Project, UGA, 210 Field St., Room 204, Athens, Georgia 30602.

 

Original Citation: The Coweeta Listening Project. Franklin Press. Column on "Science, Public Policy, Community." Page B8. July 27, 2012.