All about that LiDAR
Zada Law, Director, Fullerton Laboratory for Spatial Technology
Geospatial Research Center
Department of Geosciences
Middle Tennessee State University
LiDAR. Archaeologists everywhere are talking about it. It’s short for “Light Detection and Ranging,” and uses lasers to scan a surface and measure reflected light. Aerial LiDAR, collected by specially equipped aircraft, can be used to generate accurate three-dimensional elevation models of the bare earth even in heavily vegetated areas. This allows archaeologists to identify potential cultural surface features in areas that were previously inaccessible or obscured by vegetation in aerial photos.
I’m using LiDAR to assist in identifying where Civil War earthworks are located in Nashville’s Fort Negley Park. The Union Army built the fort in 1862 to defend their strategic occupation of Nashville. Period renderings as well as maps show defensive works extending from fort’s parapets.
Seventy years after the Civil War, the Works Progress Administration (WPA) reconstructed the fort’s limestone walls and wooden stockade for use as a municipal park (Johnson 1986). In reconstructing the fort, the WPA cleared the hill slopes and may have filled in the defensive lines. The fort was closed for repairs in 1945, and when it was re-opened to the public in 2004, dense vegetation had grown on the hill side, obscuring any remaining visible trace of these defensive works.
Since the construction involved with re-opening the park did not affect the hill slope, we didn’t conduct archaeological investigations in that area. But I always wondered if archaeological traces of the earthworks might still exist under that impenetrable vegetation. Could this be a use for LiDAR? I decided to investigate.
Getting the most out of LiDAR involves special software and training, but visual analysis of images created from LiDAR is an important first step. The image below is a bare earth model of Fort Negley created from a 2011 LiDAR dataset of Davidson County. Unfortunately, the pixel resolution of 4 feet means that features smaller than 4 feet won’t be shown. However, this image, even though it is coarse, shows several linear features northeast of the fort that might be remnants of the earthworks depicted on historic military maps.
When higher resolution LiDAR becomes available, we will re-examine this portion of the park, and perhaps conduct archaeological testing to evaluate our hypothesis. In the meantime, we are using the Davidson County LiDAR data set to identify Civil War earthworks elsewhere in the city.
At Shy’s Hill, earthworks are readily visible on the slope and hill top. In the LiDAR image below, the semi-circular berm on the crest of the hill is an earthwork that was rebuilt by a former landowner. A line of entrenchments extending east from the hill is not only very visible, but corresponds well to the location of Confederate defensive lines (red) that we digitized from georeferenced military maps.
However, the feature that caught our eye was a relatively straight line that is visible just to the west of the red redoubt. The yellow arrow points to it. Straight line features visible with LiDAR are usually not natural, and this feature may represent a portion of the earthwork that was previously thought to have been destroyed.
Can LiDAR definitively show that the features identified on Shy’s Hill and Fort Negley are earthworks? No. But LiDAR combined with other documentation can identify areas that are likely to contain archaeological features. This is especially important for conserving the remaining traces of Civil War earthworks in Nashville.
The Union Army occupied Nashville early in the Civil War and proceeded to make it one of the most heavily fortified cities in the Union (Horn 1956). Wilbur Foster’s map of the December 1864 Battle of Nashville shows the double ring of federal defensive works surrounding Nashville along with the Confederate defenses. The Tennessee Division of Archaeology’s Middle Tennessee Civil War survey identified numerous locations where these earthworks still exist (Smith, et al. 1990).
Our next step is to build on the previous archaeological work by plotting the site locations of known earthworks on the LiDAR image to identify how these features look on a bare earth elevation model – their “signature.” Then, we’ll add the layer of defensive lines we digitized from historic maps to our model as a starting point to look for additional earthwork signatures on the LiDAR image.
Even if we don’t see surface remnants of earthworks on the LiDAR image, the digitized historic lines of earthworks are “pointers” to possible archaeological remains of these features. And this information can be folded into the local planning and development process.
LiDAR is good for archaeology, but it has many other uses including flood mapping, precision agriculture, and power grid inspections, to name a few. But LiDAR data is expensive to obtain and not available for all counties in Tennessee yet. That is why our state’s Office for Information Resources is working to find the funding to invest in a statewide LiDAR dataset. But OIR – and more importantly, our state legislators – need you to tell them that it’s important for our state’s archaeological heritage as well. And what better time than Tennessee Archaeological Awareness Month!
Horn, Stanley F.
1956 The Decisive Battle of Nashville. Louisiana State University Press, Baton Rouge.
Johnson, Leland R.
1986 The Parks of Nashville: A History of the Board of Parks and Recreation. Metropolitan Nashville and Davidson County, Board of Parks and Recreation, Nashville.
Smith, Samuel D., Fred M. Prouty and Benjamin C. Nance
1990 A Survey of Civil War Period Military Sites in Middle Tennessee. Report of Investigations No. 7. Tennessee Dept. of Environment and Conservation, Division of Archaeology, Nashville.
U.S. War Department
1891 Atlas to Accompany the Official Records of the Union and Confederate Armies. Government Printing Office, Washington, DC.