Unexpected Uses of LIDAR Data in Archaeology.
When most people read about LIDAR data in the news, it is about amazing discoveries of road networks or layouts of “lost cities” in remote jungles. In my day-to-day work I use LIDAR in multiple ways – from identifying historic roadways, boundaries, decades old excavation areas, and measuring landscape change over time. All of this is done using free and openly accessible data from State and Federal data repositories.
For the unacquainted, LIDAR stands for Light Detection And Ranging – think RADAR or SONAR, but using light. All three methods utilize an emitter and sensor that measures the amount of time wavelengths take to reflect. Unlike RADAR and SONAR, which use different forms of electromagnetic waves, LIDAR uses focused lasers. Laser light has a shorter wavelength and can be targeted, resulting in more detailed data than could be acquired with RADAR.
LIDAR, being so detailed, has unique applications that simply can’t be achieved with RADAR. LIDAR data, in its raw form, consists of a point cloud of millions of individual points that contain horizontal and vertical position information. These point clouds can be used to interpolate Digital Elevation Models (DEMs). Since LIDAR utilizes laser light, it can thread the needle between leaves in tree canopies, allowing you to make a detailed model of a forest floor.
A DEM can further be manipulated to create Hillshade models, where changes in elevation can be exaggerated and/or visualized using shadows. Hillshade models make the subtle changes in elevation pop. Downloadable LIDAR-derived DEM Hillshade models are accurate enough that you can pick up plow lines in open fields quite easily. Doing this you begin to look for patterns amidst the natural topography or agricultural noise. This is where the fun begins.
When you go to a park, you may see signs like “Leave No Trace,” but as humans, in a general sense, that is an impossible thing to do. We change our environment in both subtle and not so subtle ways. Not so subtle ways could be the construction of a building, which likely would involve the displacement of soil in noticeable ways. The repeated act of walking along a path creates depressions in the ground that can be picked up from LIDAR.
Jefferson Patterson Park and Museum has had numerous archaeological investigations since the 1980s. The excavations consist of a patchwork of arbitrary site grids, which would be difficult to relocate by attempting to rediscover decades-old datum points left in the ground. LIDAR can assist in mapping the locations of these excavations since the process of digging and backfilling excavation units leaves the ground surface in an altered condition. As seen in Figure 1, the outlines of individual unit squares from the King’s Reach Quarter site are clearly visible. To a casual observer, this is not readily evident when looking in the field.
Figure 1. LIDAR Map of Excavation Units at the King’s Reach Quarter Site.
Additional features picked up from LIDAR are paths and boundary ditches. These man-made features are among the easiest to spot from a Hillshade model. Remnants of a land bridge over a marshy branch of St. Leonard’s Creek can be seen in Figure 2. Like the backfilled units at the King’s Reach Quarter site, this feature could easily be missed in the field. This crossing may date to the late 18th-century where portions of a road extending from it are referenced in the 1770s as part of a boundary dispute. The total length of this road was also depicted on an 1848 US Coast survey map, connecting southern portions of Jefferson Patterson Park to present-day Mackall Road.
Figure 2. LIDAR Hillshade model and 1848 Coastal Survey Map of St. Leonard’s Creek.
Modern walking paths/hiking trails created without mechanical grading can also be seen in a similar fashion. Persistent walking along marked paths compacts soils, causing subtle but measurable changes in topography. Utilizing LIDAR data and high-resolution aerial orthography, the entire trail system within Jefferson Patterson Park was digitized without the need of a physically survey.
LIDAR is not only a tool for discovery, but it is an important tool in land management and planning. While not as glamorous as the headlines you may read, it is still nonetheless a unique tool for interpreting the landscape.