Why LiDAR indeed! We have many methods of mapping the ground surface. Archaeologists use different types of data to create Digital Elevation Models (DEM, usually mapped with ground cover) or Digital Terrain Models (DTM, ground surface elevation without ground cover). Three-dimensional ground surfaces help archaeologists interpret archaeological sites, landscapes, and how people interacted with their physical environment. Archaeologists have been using these types of landscape models in analysis since the 1980s. All sorts of data are used to create these maps and models: aerial photographs (orthographic), satellite imagery, and land survey methods (total station, 3D laser scanning, etc.). Many times we specifically want a map of the ground surface, without any ground cover (DTM). This can be challenging for large landscapes due to ground cover such as buildings, trees, and brushes. Multi-frequency LiDAR data can be processed to remove ground cover and create a "bare earth" model.
Why might we want this type of "bare earth" model? Imagine mapping ground surface detail beneath the trees (miles and miles of it!), being able to peel back this traditional barrier of DEMs created from airborne instruments. Removing the trees can reveal the lumps and bumps, even just slight changes in the ground surface, and with thorough processing we are able to enhance LiDAR models to help interpret archaeological features and entire landscapes. A very helpful tool for archaeologists!
How LiDAR works
Airborne LiDAR, or light detection and ranging, measures the height of the ground surface and any features (i.e., trees, buildings) that may be on it and provides accurate high definition models of the landscape to a resolution of 1 m to 0.5 m in archaeological applications. LiDAR uses a pulsed laser beam that scans from side to side as a plane flies at a low altitude over the survey area. 20,000 to 100,000 points per second build the ground model. In post-processing the first returns can be removed from the data providing a "bare earth" model (DTM) that accurately represents the ground surface.
The airborne LiDAR data for Time Team America were acquired by the National Resource Analysis Center (NRAC), West Virginia University. NRAC operates an OPTECH ALTM-3100C airborne laser (small-footprint) mapping system. For positional accuracy the system integrates a laser altimeter, a high-end Applanix Pos/AV Inertial Measurement Unit (IMU), also called an Inertial Navigation System (INS), and a dual frequency NovAtel GPS receiver. This integrated system is capable of 100 kHz operation at an operating height of 1,100 m. LiDAR technology offers fast, real-time collection of three-dimensional points that are employed in the creation of Digital Elevation Models (DEMs), Digital Terrain Models (DTM), landscape feature extraction, forest stand structure analysis, flood plain mapping, and many other applications.
Data were collected in multiple, low altitude acquisition passes over the core areas of Badger Hole and the Dillard sites. The ground point densities of the LiDAR survey were equal to 15 – 20 points per square meter with a vertical accuracy of 15 cm or better.
What the LiDAR told us
The resulting "bare earth" models from the Time Team America LiDAR flights for Badger Hole and Dillard provide great information and landscape models.
The LiDAR model highlights the erosional effects of the Beaver River (A) and identifies preserved palaeo-arroyos that may contain intact bone beds that date to the same period as the Badger Hole, Jake Bluff, and Cooper sites (B).
At the Dillard site in Crow Canyon, the LiDAR model confirms the site’s orientation to significant landmarks and views of the Prehistoric Mesa Verde Region. Consideration of regional archaeological evidence in relation to the LiDAR model helps provide insight on the Dillard site’s role in the larger community.
Here are a few great resources for LiDAR applications in archaeology:
- USGS (US Geological Survey) LiDAR for Science and Resource Management
- Open Topography, Access LiDAR data
- English Heritage
- English Heritage, Free download for guidelines, The Light Fantastic
- Journal of Archaeological Science, Airborne LiDAR, archaeology, and the ancient Maya landscape at Caracol, Belize
- LiDAR and Landscape Archaeology, Challis, K.
- The Role of LiDAR intensity data in interpreting archaeological landscapes
- Documenting Native American Monuments at Effigy Mounds National Monument
- LiDAR data source, Wisconsin