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When many people think of the field of archaeology they often think of lost cities and fedora hats thanks to the popularity of the "Indiana Jones" movies. Rarely does the general public think of LiDAR in connection with recording historical information and artifacts, but that is exactly what has been used in several cases–including one recent close-range LiDAR project for the Missouri Department of Transportation (MoDOT).
When investigating a rockslide in 2008, MoDOT archaeologists identified eight rock shelters situated in and adjacent to MoDOT right-of-way, according to MoDOT’s historic preservation manager Mike Meinkoth. One of these rock shelters was found to have prehistoric petroglyphs mixed with many layers of modern graffiti. The rock face was entirely covered with carvings of all descriptions due in part to the visibility of the cave near a state highway and its relative ease of access despite steep terrain. Visitors to the shelter during the past century continue to carve their names or leave a date behind to mark their passing–considered to be modern graffiti.
Despite all these more recent carvings, the site still presented significant historical value worth preservation. Meinkoth said, "American Indians have a great variety of places that are considered to be sacred. These sacred places can be locations associated with origin stories, oral traditions or have been used for ceremonies and other spiritual activities. A place may have something constructed or have symbols placed there such as pictographs (images painted on rock) or petroglyphs (images carved into rock)."
MoDOT explored traditional methods in which to document the discovered petroglyphs such as sketches, photographs and tracing. However, due to the density of petroglyphs and graffiti and the changes in the angle of the rock surface, these tracing methods didn’t work. Instead MoDOT called on the LiDAR services of Bartlett & West, an engineering and technology solutions firm with 14 offices across the nation including one in Jefferson City, MO. Bartlett & West is one of only two firms in that region with LiDAR technology and has an on-call LiDAR agreement with MoDOT.
"Archaeologists have always been interested in using new technology to be able to learn about a site without destroying it," said Bartlett & West archaeologist and data solutions manager Mike Flynn. In fact, there are a wide variety of examples of aerial LiDAR use to document historic sites. "The interesting part of our project was the close-range at which we had to set up the scans due to the nature of the terrain and the angles of the rock shelter walls," Flynn said.
Scanning the Site
During the initial site visit it was determined that the LiDAR scanner could not be safely positioned on the undulating floor of the rock shelter. However, the path along the base of rock bluff adjacent to the rock shelter yielded just enough space to set up a telescoping tripod and elevate the scanner to a vantage point above the rock carvings and approximately six feet from scanner to petroglyph. A Trimble S6 robotic total station unit was used to establish scan control. Two scans were taken at different angles to help identify the features. Each scan was registered to one another, and then to the control, therefore placing the scan data into the real world coordinates provided by MoDOT. After overcoming the challenging terrain, the actual scanning time was only about a half hour.
Bartlett & West used its Riegl Vz-400 tripod-mounted terrestrial LiDAR scanner to obtain these images and process them with a precision that allowed MoDOT archaeologists to document this history without disturbing the site. "This historic site was at major risk both from natural deterioration and modern graffiti," Flynn explained. "Yet, thanks to our scans the rock face is now preserved forever. We were able to not only document the petroglyphs but also to allow for their analysis and interpretation."
Processing the Data
Meinkoth and Richard Rahe from the MoDOT Historic Preservation team met with Bartlett & West’s processing technician to provide a crash course on petroglyphs that were already identified and other common symbols that might be encountered at the site. Using TerraScan, a Terrasolid software, several different categories were created to manage the sheer amount of carvings present in the rock shelter. While TerraScan provides a powerful toolbox of automated classification routines, the complexity of this rock shelter was such that manual classification was the only viable option. The classifications that were decided upon were the modern carvings, prehistoric symbols and unknown features, shelter roof and the bare rock face. Modern graffiti was any clear alpha or numeric character. Petroglyphs were those identified by archaeologists or similar to ones found reoccurring through other sites in the area. Unknown features were a mixture of natural cracks and weathering that were difficult to distinguish from intentional carvings and geometric shapes that could be from any point in history.
Several extra considerations had to be taken to with the data content. For example the shelter roof, which either never had carvings or have since been lost to weathering, was classified to allow a top down view without the need to look through superfluous points. In addition, as the site processing progressed it was found that a number of carvings that were clear in photographs were almost impossible to discern in the point cloud. This could have been due to how these carvings had weathered, how shallowly they initially had been carved or just how the rock surface undulates. To address this issue three additional classifications were added as subsets of the graffiti, petroglyph and unknowns. Using the photograph, lines were digitized on the point cloud and then used as guidelines to classify the points underneath.
Seeing the forest but for the trees was a daunting task, but having the points for the ancient rock carvings separated out of the point cloud was a powerful tool for analysis and clarity. The ability to view this site, in isolation from the rest of the data, provides unique ways of visualizing the data that cannot be achieved in any other known way. Other methods of study and preservation of petroglyph sites can often be highly dependent on the lighting conditions and angle of light at the time of observation. Carvings that might be readily apparent at one time of day can be completely camouflaged an hour later. LiDAR data does not have such restrictions. Being able to rotate the entire site in three dimensions can provide viewing angles that could never occur in the real world. Using color for the three layers displayed in the data allowed the prehistoric symbols to shine through or draw attention to areas of unknown features that might have otherwise been overlooked. One example of such an area was a meandering line with a number of circular depressions on either side, which could be a type of prehistoric map.
The petroglyph selected for scanning shows the torso of a human figure with birds sitting on each shoulder. According to Meinkoth, similar figures have been identified in other rock shelters and their meaning speculated in the archaeological community. One such thought has given the name of such a humanoid figure as "Red Horn," a mythical hero of the Siouan speaking tribes of the Midwest, or perhaps a legendary leader whose memory has been passed down through oral histories among the tribal entities.
Meinkoth said, "While this data analysis work is still in progress, what excites me the most is being able to preserve these petroglyphs within a 3-D model that also allows us to share the information with interested tribes, archaeologists and the public. Petroglyphs are more than marking on the rock; they are spiritual symbols that are associated with the rock and with the place where the rock is located."
Bob Gilbert is a professional engineer and the location manager for Bartlett & West’s Jefferson City, MO office. He has served as project manager, quality control/quality assurance reviewer, technical advisor, and project engineer on numerous projects.
A 416Kb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE