Photogrammetry of Lincoln Memorial Cemetery

By Helen McCreary, MSc, Archaeologist
ERG, LLC

            Archaeology isn’t just about digging. There are lots of ways we can learn about the past without ever sticking a shovel in the ground. One fairly new method of studying archaeological sites and artifacts is photogrammetry.

            Photogrammetry involves making digital 3D models out of overlapping 2D images. By tracking a single point across multiple pictures taken from different angles, software such as Reality Capture and Agisoft Metashape can calculate a coordinate in three-dimensional space. Add thousands of these coordinates together, and you have a 3D model of a projectile point, a pot, or an entire landscape. This enables us to more thoroughly analyze archaeological sites and artifacts.

            Take, for example, Lincoln Memorial Cemetery on Fort Knox, Kentucky. Environmental Research Group, LLC conducted a photogrammetry survey of the cemetery in May of 2019, in order to document the historic road and church foundations on the site. Normally, a landscape survey like this would be conducted using a drone. However, the Department of Defense has issued a moratorium on the use of commercial drones on military bases. This meant that we had to be creative! We mounted a camera to a long pole and took photos of the ground from three meters in the air. By walking systematically over the entire site, taking photos the entire time, we were able to collect enough photographs to make a 3D model of the entire cemetery. A model of a landscape is typically called a Digital Elevation Model, or DEM, and it can reveal subtle topography that is difficult or impossible to see from the ground, such as old building footprints or grave depressions. The DEM we made at Lincoln Cemetery has a 3cm resolution, meaning that every pixel in the model corresponds to 3cm on the ground.

            Included below are several figures. Figure 1 is the DEM of Lincoln Memorial Cemetery, visualized as a hillshade. Hillshade essentially casts false light across the model, allowing you to see the topography clearly. Figure 2 shows the same DEM after it was subjected to a high-pass filter. This filter essentially strips the larger, natural topography away from the DEM, leaving only the smaller topography that could be associated with archaeological features. Figure 3 is the archaeological interpretation of the topography, taking into account our knowledge of the site and historic maps.

Figure 1. Photogrammetry-derived DEM of Lincoln Memorial Cemetery, Fort Knox, KY
Figure 2. Surface extraction (high pass filter) of Lincoln Memorial Cemetery
Figure 3. Interpretation of the topography at Lincoln Memorial Cemetery
Figure 4. Lincoln Memorial Cemetery from the ground at the time of survey

            As you can see in Figure 4, the micro-topography across the site is not readily apparent from the ground. The camera and pole method proved to be effective, especially considering the tree cover, which would have made using a drone rather difficult.

            We also used photogrammetry to model one of the gravestones on site. This can help us make out carvings that are hard to see with the naked eye. Figure 5 shows the gravestone, Figure 6 shows the model in progress, and Figure 7 shows the final model in Sketchfab. The final model can be viewed and inspected here: https://skfb.ly/6MC6z.

Figure 5. A double-sided gravestone immediately adjacent to Bathsheba Lincoln’s (Abraham Lincoln’s grandmother) grave. This headstone has Rebekah Mumford’s name on one side and Mary Crume’s on the other. The names are visible on the gravestone, but the photogrammetry model makes the carvings easier to read.
Figure 6. The gravestone in Agisoft Metashape. The blue squares indicate the position and orientation of the photographs, and the grey points are what make up the final model.
Figure 6. The 3D model of the gravestone in Sketchfab (https://skfb.ly/6MC6z).

            This gravestone actually has quite a dramatic history! Gerald R. McMurtry, in a 1939 book titled Kentucky Lincolns On Mill Creek, related the story of a stone cutter named Mumford, who carved most of the sculptured stones in Lincoln Memorial Cemetery. His wife Rebekah died on April 12, 1846, and he placed a stone on her grave with the proper inscription. Sometime later he was short of stone, so he pilfered his wife’s gravestone, plastered over the inscription, and put Mary Crume’s name on the other side! He was paid $2.50 for his work, and his daughter claimed that he would never have done it had he not been “crazed with drink”. As he never replaced the stone on his wife’s grave, Rebekah Mumford’s final resting place is unknown (McMurtry 1939, pg 69, footnote 131). But now her gravestone is digitally preserved!

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