Archaeology from Space Read online

  Cracks appeared in the pot, showing its broken pieces.

  After half an hour of careful work, a 3-D jigsaw puzzle emerged of a flattened beer jar. A ghostly white slip coated its exterior. These jars, common in the Old Kingdom,4 would not be out of place alongside the stylish serving options for mixed drinks on Bourbon Street in New Orleans. More than just an object, I had a possible story. Perhaps relatives of the deceased had brought the jar to the cemetery. They had said the ancient offering formula, a magical recital ensuring the dead received a bounty of bread, beer, and goods for eternity, and drank to their memory.5 Looking closer, while brushing the pot for an official photograph, I saw something near its mouth: a fingerprint, from the potter who had made it 4,200 years ago.

  In my imagination, the gulf of time separating us compressed.

  The print seemed to be from a robust thumb. A middle-aged man appeared, sweat on his brow, bent over the wheel he rotated by hand. The ancient Day of the Dead, the Feast of Wagy,6 was approaching, and he had a deadline to meet: he would need two sets of fine ware for the mayor and his family and 200 beer jars for the townspeople of Per-Banebdjedet.7 Nearby, his sons stoked the kiln fires; too hot, and the pots would crack, too cool, and they would crumble. His daughter brought him a small cup filled with cool water, and he smiled, grateful for the blessing of the gods. Praise Djedet,8 he would make his deadline!

  Excavation’s Greatest Challenge

  Once you taste a drop like this from the spigot of history, you never forget, and your thirst is never quenched. Stories, not things, lie in disjointed sentences below the ground, and it is the job of archaeologists to coax them out and weave them into prose. But when facing a featureless sea of brown silt, or modern fields, or a mound beneath dense rainforest, the challenge is where to begin.

  This is the exact question space archaeology has evolved to answer.

  On most unexcavated archaeological sites, few hints exist aboveground of what features might be hidden below. This varies widely depending on where in the world you work. In Belize, towering mounds emerge from the rainforest floor, looking out of place in an otherwise gently rolling landscape and suggesting the presence of structures. Stone fragments may appear in straight lines beneath olive groves in Greece, showing the locations of 3,000-year-old walls. Archaeologists feel fortunate when they have these conspicuous clues to guide their excavation efforts.

  When the clues are not so conspicuous, we have a setback. While archaeologists may live to dig, they cannot be in the field more than a few months each year, unless they work for a cultural resource management firm or as professional archaeologists for a cultural ministry. Even Indiana Jones taught class. Tight schedules and financial restrictions mean archaeologists must plan for every moment and penny spent: responsible publicly funded excavations do not want to report a fruitless season.

  All applications for archaeological funding, public or private, now require well-formulated research questions, state-of-the-art project design, and evidence such as a preliminary site assessment that the team is digging on target.

  Some sites are found by luck or by accident. In 1900, for instance, a donkey transporting a gentleman from a quarry in Alexandria, Egypt, fell into an abandoned shaft. What the poor donkey landed in were second-to-fourth-century AD Roman Period catacombs containing hundreds of individuals, now on the list of must-see tourist sites in Alexandria.9

  Such sites underlie modern towns all over the world. When I was in graduate school, doing survey work in middle Egypt, I needed local help to verify hints from the satellite imagery that an ancient town might be lurking under modern urban landscapes. In the city of Dalga, the priests from a Coptic church led me down two flights of stairs to sacred rooms used for baptisms. They were decorated with sixth-century AD reliefs removed from the earliest Coptic church in the town, located some 20 feet below where we stood. No donkeys were harmed in the making of that little surprise.

  Deny it though they might, most archaeologists pray to all gods ancient and modern who might heed our requests for success. It never hurts to spread the love, sample bags in hand! Aside from unexpected discoveries, archaeologists rely on diverse techniques to figure out what lies underfoot.

  The simplest is fieldwalking. Moving along equally spaced lines, either in a group or alone, is a way to see how surface remains may change over a site or region. Sudden dense concentrations of slag, the by-product of metal production, may indicate an industrial zone. Tiny limestone chips and bone fragments found together could pinpoint a high-ranking cemetery, with the limestone pieces coming from sarcophagi or tomb structures. Larger limestone fragments in heaps, perhaps with inscribed and/or intact blocks, may locate a long-destroyed sacred or palatial building. And any ancient pottery or other remains flag the range of time periods below.

  Fieldwalking—as well as ungulates—may represent an important step in site survey, but only an aerial perspective allows us to see the entire picture, not only of a single site, but of its relationship to the surrounding landscape. Aerial photos have proven invaluable for assessing ancient sites, and those taken from drones today are nothing less than spectacular. However, images from a far greater height, 200 miles higher than the International Space Station,10 have paved the way for the subfield of archaeology that has already transformed our understanding of the past and of the potential for future discovery.

  How Space Archaeology Works

  Whenever archaeologists apply any form of air- or space-based data to the assessment of modern landscapes, attempting to locate long-buried rivers or hidden ancient sites, they are doing “space archaeology,” also called “satellite archaeology” or “satellite remote sensing.” NASA shoulders the ultimate name blame. In 2008, NASA began its “Space Archaeology” program,11 funding scientists to apply satellite data sets to large-scale archaeological research projects. If NASA calls what I do space archaeology, who am I to disagree?

  Interpreting satellite imagery is part science and part art. All remote-sensing specialists must start by learning the language of light, and it is not easy: what appears as a simple high-resolution photograph on your computer screen is so much more. Each pixel on the image is representative of an exact area on the ground.12 The light composing the pixel represents not only the visible part of the light spectrum, but the near, middle, and far infrared, depending on the satellite-imaging system. Additionally, everything on the Earth’s surface has its own distinct chemical signature that affects the light it reflects: much as we all have distinct signatures when we write our names, different materials show up uniquely in the light spectrum.13

  For example, sand appears very different from forest on the satellite imagery. We can see this easily with our own eyes. When you need to discern different tree species within the forest, this is where chemical signatures come into play. A group of oak trees emits a different chemical signature than does a group of pine trees. Visually, they might appear as the same green to us, but using different parts of the infrared spectrum to visualize subtle vegetation health differences, we can perceive color variation.14

  Remote-sensing specialists can exaggerate these differences by assigning “false color” to the images,15 to highlight individual classes of surface features. Within remote-sensing programs (like Photoshop color replacement with an attitude), you can choose any color for any cluster of pixels. While it’s recommended that users choose classes closely resembling their real-life counterparts—for example, green tones for vegetation, gray for buildings, brown for soils—you can choose any colors you want. Satellite images shown at conferences or in publications sometimes look like bad acid trips.16

  Scientists shop for specific types of satellite images to suit the data they need. Each satellite is different, and there are over 1,700 of them up there.17 Most are lower-resolution weather or large-scale satellites, with resolutions of 15 to 30 meters. These are the images most used, not just because they are free, but because there are millions of images going all the way bac
k to 1972 that highlight short- and long-term landscape changes.18 In addition to these free images, there are high-resolution images recorded by sensors such as DigitalGlobe’s WorldView-3 and -4 satellites, with resolutions of between .31 and 1 meter, where a single pixel represents an area between the size of an iPad and a bodyboard.

  Everyone looking at satellite imagery extracts pixel-based data to detect subtle short-term versus long-term changes, or to detect features. We tweak and test algorithms depending on our research questions, and eventually, through sheer dumb luck or a moment of genius, we find something of interest, usually because we’re scraping the barrel bottom of possible techniques. When it turns out to be dried snot on our computer screen, this being science, we go back to the drawing board and try again.

  It Isn’t All “Aha” Moments, Except When It Is

  People think that remote-sensing work is all about the “Aha” moment, the moment when a single click of a button reveals secrets hidden in plain sight. It isn’t. A typical remote-sensing specialist will spend dozens of hours per week in front of a computer screen, often cursing due to program crashes. When something does work, there is additional swearing, because you have forgotten to record the exact steps you took to reach that point. And you must start over. It’s about learning, about refining the process.

  Then again, “Aha” moments do happen. One of my favorite remote-sensing stories unfolded at the well-known Maya site of Caracol in Belize, which dates back over 1,000 years.19 In 2008, a new laser-imaging technology called LIDAR, for LIght Detection And Ranging, was just warming up at the starting lines.

  Diane and Arlen Chase, a gregarious and generous archaeologist couple at the University of Nevada, Las Vegas, had worked at the site for nearly 30 years.20 When a keen biologist, John Weishampel, of the University of Central Florida, first asked the Chases about using LIDAR at Caracol, they told him that they were skeptical. They had never heard of it, but they were understandably enthusiastic about the idea of bringing more funding to their site. After decades of toil, they almost hoped they hadn’t missed anything major.

  They told him to go ahead with his grant application—he could try and peer beneath dense rainforest canopies using LIDAR if he wanted. It sounded like fun and wouldn’t do anyone any harm.

  John, now grant in hand, commissioned an airplane from the United States to collect the point cloud data, or hundreds of thousands of points from the top of the vegetation down to the forest floor, in a large area surrounding the site.21 If you were to look at the area on Google Earth, all you would see is rainforest—a sea of green, with nothing suggesting anything ancient, aside from a few well-known limestone pyramids peeking through the tops of the trees.

  After he had processed all the data, John displayed the images for Arlen and a small group. Arlen’s exact words were: “Holy shit!” The same thing was on everyone’s minds. Another astonished colleague said that this was the data to launch a hundred PhD dissertations.

  The next day, Diane called John: “Arlen’s been stuck to his screen all night looking at the images. And he’s missed dinner and breakfast.” In a single night, the entire field of Mesoamerican archaeology had changed permanently: Arlen had found more ancient Maya sites than he had in 30 years of combing the jungle. Today, he can find 500 new Maya features before lunch from his desk in Las Vegas.22

  Such wholesale rethinking is not the product of a single flash of technical brilliance, but rather the result of decades of often serendipitous developments in the field of archaeology. To understand this takes a brief nosedive into the history of seeing ancient sites from afar.

  It All Began with Balloons and Airplanes

  Technically speaking, one of the first ancient sites to be viewed from an aerial platform was Stonehenge.23 The famous Neolithic (ca. 2500 BC)24 circular formation of large stones stands on a grassy down in southern England and is long beloved of modern pagans on summer solstice. Stationed near Stonehenge in 1906, Lieutenant Philip Henry Sharpe of the Royal Engineers’ Balloon Section used a tethered balloon to take three photos of the site.25 Published in the Society of Antiquaries shortly thereafter, the photos caused quite a stir. Archaeologists could see the site and its relationship with the surrounding landscape, and curious darker patches appeared on the ground, suggestive of possible buried ancient features. A new world had been opened up from on high.

  World War I saw the creation of the Royal Flying Corps, with pioneering aviators flying across Europe and the Middle East. Used for establishing artillery ranges and the enemy’s positions, their aerial photography efforts formed the crux of their operations.26 Photos of the front used to plan attacks have become essential archaeological data today.27

  Later, Father Antoine Poidebard, who had the rather fabulous nickname of “the Flying Priest,”28 flew a biplane across large swaths of Syria and Lebanon from 1925 to 1932, recording many ancient sites from above.29 Aside from these invaluable early photos, he created the foundations for aerial archaeology in the Middle East, emphasizing the importance of timing for revealing ancient structures clearly. Images taken in the morning, for example, when the ground contained more moisture, revealed more sites than those taken in the afternoon, when the drier ground had more uniform colors.

  Meanwhile, in England, Osbert Guy Stanhope Crawford, better known as O. G. S. Crawford, pioneered the application of aerial photography to long-occupied landscapes. After serving in the Royal Flying Corps during World War I, he joined the Ordnance Survey as an archaeological officer.30 While a prisoner of war in Germany during World War I, he had completed Man and His Past, a seminal volume emphasizing the importance of maps to define culture.31 Affectionately called Uncle Ogs by the rising generation of British archaeologists,32 Crawford located hundreds of previously unknown sites across the United Kingdom.33 Even today, his aerial photographic archive of Britain remains an invaluable resource for archaeologists.34

  Crop Marks—It’s Not Just Aliens

  Most features show up on these early aerial photographs, and later on satellite images, as crop marks. Crop marks do what the name says: vegetation grows faster, slower, or, in some cases, not at all, based on what is beneath the ground, revealing possible buried walls or even entire buildings.35

  Let’s break that down. Imagine the foundations of a stone wall, slowly covered by earth over time. As grass takes root over the top, its roots simply cannot go as deep as the grass a few feet away. Growth becomes stunted and it is less healthy than the other grass. In a time of drought, it might die altogether.

  Conversely, over time, a ditch fills in with rotting vegetation. This forms fertile mulch and is an ideal place for new vegetation to grow. Grass and other crops thrive, growing taller and healthier over the ditch than the surrounding area.

  The shadows formed by the taller or shorter vegetation can easily be seen on aerial photos, while more subtle differences in vegetation health can be picked up by satellites recording information in the near infrared. Chlorophyll content, for instance, can best be seen in the near infrared—in which all vegetation appears red.36 Try explaining it that way to your child the next time they ask you why grass is green. My son’s response was: “You’re a weirdo, Mommy.”

  These crop marks have a fascinating history, and people can actually see them while walking across fields. In Britain, observant walkers mentioned them over 500 years ago, as noted by British antiquarian William Camden, who named them “St. Augustine’s Cross” after the earliest missionary to England.37

  I regularly receive emails from people in Europe who send me snapshots of crop marks that they have observed on Google Earth, and I am usually impressed. People have great eyes. Straight lines rarely occur in nature, and it is even rarer for them to form rectilinear features, so when multiple connected boxes show up on a satellite image of a field in Britain, France, or Italy, there is a high degree of probability you have discovered a Roman house.38 Even when a field has been plowed over for millennia, the stone foundations survive and
affect crop growth. Despite the pub lunch at the finish line, I can no longer idly go on pleasant Sunday rambles across these fields in England. I’m always on the lookout, just in case.

  From World War II to the Start of the Space Age

  Following World War II, remote-sensing technologies underwent a great revolution as archaeologists and other scientists recognized the value of emerging color and infrared technologies. In fact, my grandfather, Professor Harold Young, wrote in a 1950 publication: “In a relative sense, the use of aerial photos in forestry has only reached an early adolescent stage. Many research workers are trying to determine the limits of aerial photos, as well as the many ways that aerial photos can be profitably used. Today the possibilities of color film are scarcely known at all.”39

  This was only 70 years ago. Now, we can 3-D scan objects and take thermal infrared photos on sites using our cell phones.40 These technologies have emerged over only two generations—a blink of the eye of our human history.

  During my grandfather’s era, archaeologists could access thousands of military images of Europe and the Middle East and use them to plan new surveys. Aerial photography had become a standard archaeological tool, led by pioneers like J. K. St Joseph of Cambridge University. Trained as a geologist, St Joseph learned about the field of aerial photography during World War II, when he served in the Ministry of Aircraft Production. After the war, he commenced major photography of landscapes around the United Kingdom via Royal Air Force training flights, leaving over 300,000 photographs to Cambridge University. He gave wonderful lectures with amazing images but spoke in such a way to earn the nickname “Holy Jo.”41 That’s one way to preach from on high.