By using geospatial remote sensing tools, archaeologists and researchers are discovering, documenting and monitoring countless sites around the world that are meaningful to our cultural heritage. For example, previously unknown ancient civilizations have been detected under rainforest canopies in Central America, Civil War cannons have been uncovered on the banks of the Hudson River, and hundreds of temples throughout Southeast Asia have been located and hopefully preserved for generations to come. These sites all share a common theme: access is difficult and sometimes dangerous for people on the ground, and the sites can be nearly impossible to detect with the naked eye. The use of satellites, airplanes, helicopters and drones in the air, and a host of sensors and devices on the ground, has opened up many new opportunities for studying and understanding our past.
Discovering Hidden Sites
Archaeology requires some detective work. Anomalies in satellite and aerial imagery, which can be as subtle as a change in vegetation or an out-of-place rise or fall of the ground, provide clues that, at some time in the past, the area was disturbed by humans. Traces of buildings, foot paths, garbage dumps, graves, etc., remain long after people are gone.
After likely areas are located, other forms of sensing on the ground can be utilized, such as ground penetrating radar (GPR), sonar and LiDAR, to further identify exactly what lies above and below the surface. The various kinds of information are brought together as layers in a geographic information systems (GIS) matrix to complete the picture, and time and money is saved by focusing efforts on the best areas for finding archaeological elements.
“I examine aerial imagery to detect vegetation differences to prioritize and limit the areas that need to be investigated on the ground,” says Joel Grossman, an archeologist with a long history of embracing geospatial tools. “In a heavily overgrown area, I use false color analysis and multispectral images to look for secondary vegetation growth, and indicators of past disturbance, which could suggest that it has been cleared and re-grown. For one project at West Point, I identified dense thorn bush vegetation, an indicator of historic walls and foundations made with lime mortar. Upon further ground investigation, we uncovered important Civil War houses and artifacts that were successfully excavated under heated shelters in deep wintertime. Remote sensing also was invaluable for the West Point project due to it being a cadmium-polluted Superfund site. By using remote sensing technology, we reduced the duration, level of exposure and proximity of the archaeologists to contaminated materials during both the detection and recording work at the site.”
Grossman continues, “The variety of remote sensing technology available is incredibly helpful for our work. … On land, we use magnetometers, conductivity machines and ground penetrating radar to map subsurface cultural material. Over water, we use bathymetry and side-scan sonar to locate large artifacts and submerged maritime structures, such as historic boats and barges that are buried under silt and water.”
In addition to ground-based LiDAR for surveying and mapping, aerial LiDAR has become increasingly valuable as a tool for discovering and mapping archaeological sites covered with trees or other vegetation. With up to 600,000 light pulses per second, LiDAR records the contours of objects on the ground and creates a 3D representation, greatly speeding up the research process in hard-to-access locations.
“One of the most exciting technological breakthroughs for archaeology recently is aerial LiDAR,” Grossman says. “We now have the ability to identify anomalies such as plazas, rectangular walls, causeways and large compounds, all covered with almost impenetrable vegetation under rainforest canopies, such as in Central America. Airborne LiDAR technology has changed from being a recording technology to a remote sensing detection technology, which opens up many new opportunities for research and target-specific survey and investigation.”
Cost-efficient drones are also proving themselves useful for recording excavation areas, structures, pyramids, artifact distributions and topography. In the past, archaeologists were hampered by the need to build kites, balloons, overhead bi-pods and/or robotic mono-pods to mount high-resolution metric cameras and site-specific photogrammetric equipment. Drones provide access to otherwise inaccessible places and can scan archaeological sites in 3D color with sufficient resolution to reconstruct a precise XYZ, 3D model. Archaeologists also benefit from the unique perspectives, elevations and details provided by drone imagery.
Monitoring Known Heritage Sites
In addition to searching for new archaeological sites, much effort is directed at monitoring the ones we already know about. Unfortunately, in many parts of the world irreplaceable and invaluable heritage sites are threatened by terrorism, military conflicts and natural disasters. Remote sensing is a valuable tool for tracking activities in areas that are otherwise too dangerous for on-site inspections. Important work is being done to raise awareness and promote preservation by the American Schools of Oriental Research Cultural Heritage Initiatives (ASOR CHI).
ASOR CHI is an international cooperative organization tracking conditions at heritage sites. Its research zone includes all of Syria (about 72,000 square miles) and Northern Iraq (around 85,000 square miles). An inventory of more than 12,000 archaeological or historically significant sites and buildings is being monitored for changes and damage.
“At ASOR CHI, we use both satellite imagery and other media for assessment of damage to sites, including photographs taken on the ground,” says Susan Penacho, project manager of geospatial initiatives. “The advantage for us with satellite imagery is its availability in areas where it is dangerous for people to travel. We are also able to corroborate reports of damage from local sources as soon as satellite images become available. We identify change over time through successive satellite images that are often recent or only a few days or weeks old. Fortunately our collaboration with the U.S. Department of State allows us access to current satellite imagery; otherwise it would be cost prohibitive to cover our entire research area.”
ASOR CHI uses imagery provided by DigitalGlobe’s GeoEye and Worldview 1, 2 and 3 satellites. The highest resolution is 30-centimeter pixels from Worldview 3. “Through the Internet interface we are able to download geotiffs or orthophotos of everything we need,” Penacho says. “The 30-centimeter resolution provides enough clarity to identify damage on individual buildings. By streaming directly from the Web interface or via a tileserver into ArcMap, we can watch a large landscape without needing terabytes of storage space. Rather than using automated detection software, we have been most successful using analysts who are trained to look for particular change indicators at specific locations in our research zone.”
Documenting Heritage Sites in 3D
There are other organizations taking preservation a step further and documenting every inch of heritage structures to create 3D models, viewable in a virtual reality environment or even a simple Web browser. CyArk, a non-profit with the goal of building a free, 3D online library of the world's most valuable and unique cultural heritage sites, uses the most up-to-date technology available. Its 3D models are used in classrooms and research institutions for educational purposes, but also provide detailed and measurable information about each structure in the event there is damage that needs to be repaired.
CyArk has been using drones for the past four or five years to improve collection of data in inaccessible and delicate places. The organization currently operates a quad-copter DJI Inspire 1 Pro with an X5 camera system for large-area coverage, and a Phantom 4 for more detailed requirements, such as up-close capture of architectural details on fragile buildings. Terrestrial LiDAR scanning continues to be an important part of CyArk’s methodology. However, a 360-degree color panoramic photo and aerial photogrammetry collected by a drone, combined with terrestrial photogrammetry, results in a higher quality model. Also, since time and money can be tight, the Inspire 1 Pro is able to capture site-wide detail of several acres at 1- to 2-millimeter resolution in just four or five passes, compared to several days of terrestrial scanning. In addition, the drones are considerably less expensive than a survey-grade laser scanner.
CyArk recently used drones in Bagan, Myanmar, before and after the August 2016 earthquake. “In May 2016, we partnered with the University of Carleton and Yangon Technical Institute to fully map just three temples out of the 2,000 in the valley,” says Ross Davison, field manager with CyArk. “On Aug. 24, a devastating earthquake hit, causing structural damage to at least 416 of the temples. In particular, damage to original structures occurred when repairs made in the 1970s broke away and fell on the old sections.”
CyArk returned to Bagan to assist with a damage assessment after the earthquake, leveraging data captured earlier to provide a quantifiable comparative analysis. During the two-week trip, CyArk collected imagery of eight sites with a suite of reality capture equipment, including drones. Staff also worked with the Myanmar National Museum’s Department of Archaeology, training them on drone image collection skills and processing so that local efforts could continue with their own equipment.
“It will take about three months for the initial assessment and to shore up what’s there,” Davison says. “Monsoon season is causing water damage, so it’s important to prioritize by cultural significance. This period will be followed by a five-year plan to reconstruct all of the temples, this time working within the UNESCO guidelines with the goal of being named a UNESCO World Heritage Site in the future.”
Very high-resolution 3D mesh is CyArk’s final product for virtual reality and other applications. The high-resolution imagery from the drones, combined with the laser scan data, also supports 3D printing of heritage sites, which helps with visualization. “This year we’re focusing on using technology to share our 3D models with more people,” says Scott Lee, director of programs at CyArk. “It has always been difficult due to limitations on transferring multi-terabyte datasets and viewing the models on normal equipment. Exploration into virtual reality allows us to more easily share without losing resolution. We’re excited about making the models accessible online in the cloud to extend our reach, particularly for education and virtual tourism.”
Remote Sensing in the Field
Geospatial remote sensing tools are having a tremendous impact on the work done by archaeologists and researchers worldwide. The technology makes it possible to detect unknown heritage sites, monitor existing areas and document down to the smallest detail these irreplaceable structures. Remote sensing provides visibility into dangerous or physically inaccessible places, while building a record of our cultural heritage that will support research in the future.