NEWS FEATURE

News Feature: Mapping the lost megalopolisLaser imaging reveals long-lost traces of ancient civilizations hiddenbeneath tropical forest canopies.

Stephen OrnesScience Writer

Archaeologists are known for getting theirhands—and everything else—dirty. However,in April 2012, archaeologist Damian Evansboarded a helicopter and spent hours beingflown over the dense foliage surroundingAngkor Wat, Cambodia’s legendary complexof ancient stone temples. The heat insidethe helicopter was intense, reaching upwardof 40 °C, but Evans, a faculty member of theUniversity of Sydney based in Cambodia,much preferred flying over the trees thantrudging through the vegetation beneaththem. The visible, known temples in Angkorwere well-trodden and populated with visi-tors from all over the world. However, theoutlying forest beneath Evans’s ride, lush

and green from the air, hid land mines leftover from Cambodia’s tumultuous past.The small and sturdy helicopter swept low

and slowly over the area. A laser mounted tothe left skid of the chopper sent out a steadystream of 200,000 short laser pulses persecond and recorded how long it took thepulses to return; a GPS system tracked thelocation. A computer program uses thosetravel times to calculate how far a pulse hastraveled. Data from the pulses combine toproduce a high-resolution, 3D map of boththe canopy and the land hidden beneath it.In recent years, archaeologists have used

this technology, called LiDAR (a portman-teau of “light” and “radar”), to find ruins

of structures—roads, canals, temples, res-ervoirs—long overgrown with vegetationand hidden from easy observation. LiDARis revolutionizing what scientists think aboutthe size of ancient cities and how ancientcivilizations used the land. It has acceleratedthe pace of surveying nearly impenetrableareas to a rate that would have been unthink-able just a few years ago.After 20 hours of flying over two days, the

helicopter carrying Evans had surveyed 370square kilometers (91,400 acres) and Evanshad as much data about Angkor’s hiddenlandforms as he might have gathered duringhis entire career.“To achieve the same number of data

points as we did with LiDAR would havetaken decades on the ground,” Evans says.In addition, Evans says he and his colleaguessuspected that previous studies of the areawere incomplete. “Our concerns were thatprevious research had missed three-quar-ters of the downtown metropolitan partof Angkor.”The urban sprawl around the temples of

Angkor had already been identified as thelargest integrated settlement complex ofthe preindustrial world. However, Evans’sLiDAR map confirmed that existing surveyshad been vastly underestimating the size ofthe formally planned street grid in the centralarea of the city.In a 2007 PNAS study (1) that combined

ground surveys with airborne radar mapping,Evans and his colleagues first found a cha-otic, urban sprawl beyond the city walls ofthe Angkor Wat complex, and the templeswere the center of large, urban landscapes. Ina study based on LiDAR maps, published inPNAS in July 2013 (2), Evans et al. reportedthat the lasers illuminated in exceptional de-tail that the now-tangled land outside thetemple walls had once, 1,000 years ago, beendivided into neat rectangles like city blocks,with canals, ponds, and residences. The ur-ban grids found inside the walls had beenbuilt outside, too, covering an area of 36square kilometers.“It’s relatively easy to draw a line around

temples, but the revelation from LiDAR is

What hides beneath: Two views of Angkor Wat. The bottom image, created from LiDARdata, exaggerates the terrain beneath the trees and shows features hidden in the photo ontop. Reprinted from ref. 1.

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that you find this web of subtle traces ofurban networks,” Evans says.The new study suggests the Angkor cap-

itals were much more densely populated thanwas previously believed, offering more evi-dence to a growing idea that the Khmercivilization grew so large that it couldn’t growenough crops to keep up. Overpopulation,combined with the lack of sustainable agri-cultural methods, may have left the cities vul-nerable to decades-long droughts that struckin the 14th and 15th centuries, the same timethe Khmer kings abandoned Angkor.

From Space to the SurfaceAlthough it’s now upending the field of ar-chaeology, LiDAR isn’t new. NASA de-veloped laser-sensing technology decadesago, using it not to study the surface of Earth,but to peer inside clouds. As laser pulsestravel through a cloud, some are reflectedback to a waiting sensor by water par-ticles and other molecules in the cloud.Researchers can use that data to deter-mine the dimensions and constituent par-ticles of the cloud. In the 1970s, LiDARwent to space as part of the last threeApollo missions; astronauts used the tech-nology to measure the height of lunarfeatures near potential landing sites. Inthe 1980s and 1990s, scientists and engi-neers brought the lasers down to Earth,creating LiDAR systems that could bemounted on airplanes.

It wasn’t until the early 2000s that thetechnology became inexpensive enough to bewithin the reach of most researchers. RameshShrestha, now director of the National Centerfor Airborne Laser Mapping (NCALM) atthe University of Houston, first worked withairborne LiDAR in 1999 on a project thatused the technology to map coastlines alongthe panhandle of Florida. Shrestha and hiscolleagues used a machine that fired 3,000pulses per second, “which was great at thetime,” he says.Even with that first excursion, Shrestha

says, “we knew that this technology couldbe revolutionary in its applications inarchaeology.”Since then, the technology has pro-

liferated—and sped up—at a quick pace.In only 15 years, LiDAR lasers went fromfiring 3,000 pulses per second to 200,000,like the one used by Evans at AngkorWat. Additionally, within a few months,Shrestha says, the NCALM will start map-ping with a device that uses two lasers andsends out 900,000 pulses per second atthree different frequencies. Earlier this year,researchers at Lincoln Laboratory, a federalresearch and development laboratory atthe Massachusetts Institute of Technology,debuted a system that could have scannedthe Angkor project in 30 minutes. Withina few years, Shrestha expects to see LiDARwith full-color lasers, the data from whichcan be used to delineate among different

types of vegetation or even measure waterquality. UAV-ready LiDAR systems alreadyexist, which means drones either do have orwill soon be equipped with high-resolutionlaser-sensing devices.

Lasers in the RainforestLiDAR’s first significant foray into archaeol-ogy came in 2009, when the Caracol Ar-chaeology Project secured a grant fromNASA’s Space Archaeology Program to fundNCALM’s mapping of 200 square kilometers(nearly 50,000 acres) of tropical rainforest, at20 points per square meter, known to containMaya ruins. The Caracol project, an effort toexcavate and study the ruins at a site in thedense mountain forests of western Belize, waslaunched in the 1980s by husband-and-wifearchaeology team Arlen and Diane Chasefrom the University of Central Florida.The ruins had been discovered in 1937 by aBelizean logger searching for marketablemahogany. Since then, waves of intrepidarchaeologists have ventured to the site,creating maps by hand and documentingartifacts to better understand Caracol’ssignificance in Maya history. When theChases arrived in 1985, they found looters’camps scattered through the surroundingforests, the fire pits still warm. The epicenterof the ruins—including Caana (Sky Palace),still Belize’s tallest man-made building—wasblanketed in jungle.

LiDAR-based maps are helping scientists better understand ancient sites like the Mayan site of Caracol, in Belize. This colorized image showsthe degree to which the people changed the landscape. Image courtesy of the Caracol Archaeological Project.

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Making accurate maps of the area requiredblood, sweat, tears, and machetes. Teammembers established sight lines by hackingthrough the jungle, ever watchful for ta-rantulas and scorpions, as well as treevipers, coral snakes, and the fer-de-lance (ortommygoff), a deadly and aggressive pit viperwith fast-acting venom. However, the Chases,who cut their wedding cake with a machete,found themselves at home among the ruins.“Arlen and Diane are like little kids playing

in a sandbox no one has played in before,saying ‘no one has documented this!’ and‘no one has documented that!’” says Chasecollaborator and ecologist John Weishampel,also at the University of Central Florida,who has worked on numerous LiDARstudies of forested areas in Central America,including Caracol.Over the years, the Chase family grew—

their kids helped out at Caracol, naturally—and so did their maps. By 2001, the docu-mented study area had grown from 16 squarekilometers to 23, less than half the size ofManhattan. In the 1980s and 1990s archae-ologists had clashed over the significance ofthe lost city: some viewed it as a backwateroutpost with little significance on Maya civ-ilization as a whole. However, the Chase’spersistent work, which kept extending theknown boundaries of the settlement, sug-gested the city was bigger and more impor-tant than anyone had guessed, far outsizingthose 23 square kilometers.The 2009 study changed everything, says

Arlen Chase. He’d spent more than a quar-ter-century tromping over the same landcovered by the LiDAR mapping, and he sayswhen he first looked at the LiDAR map hewas bedazzled.“It was just mind-boggling,” Chase says. “I

had walked this ground and I knew what itwas going to look like, but it was like amagical world opened up.”The map, built from 4.28 billion individual

x, y, z point measurements, showed in sharpdetail monumental architecture, roads, resi-dences, and agricultural terraces, in perfectagreement with evidence collected by ex-plorers on foot (3–5). The LiDAR data also

turned up causeways and highways notpreviously known to link the center complexto outlying sites and it revealed that theCaracol settlement covered at least 180square kilometers (44,500 acres). As in thecase of Angkor, LiDAR gave researchersstrong evidence that Caracol was nothingshort of an ancient megalopolis.

The Future of Laser MappingEven so, Weishampel and the Chases felt likethey weren’t seeing the whole picture. InApril and May 2013, they again enlistedNCALM to conduct a LiDAR study. Thistime, the plane covered 1,057 additionalsquare kilometers (nearly 250,000 acres)—one of the largest regions recorded withthis technology to date—around Caracoland other regions in western Belize. Theresearchers anticipate mining the data fornew insights into Maya culture for years tocome, but already they’ve been able to seehow central Caracol was connected to out-lying settlements. Their findings have beenpublished in recent articles in Advances inArchaeological Practice (6) and in RemoteSensing (7).Weishampel says that most archaeologists

are chiefly interested in the map of theground, showing the location of hiddenmonuments and structures. “They’re onlyusing 5 to 10% of the data they’ve collected,”he says. In his own research, Weishampel’staking a different tack, looking at the foreststructure to figure out how ancient civi-lizations manipulated the landscape. “LiDARcan show how agriculture and terraformingled to changes in the way water flows and theway soil erodes,” he says. “It’s the thousand-year fingerprint that the Mayas basically leftto us.”

As with any burgeoning technology a littleLiDAR begets a lot of curiosity. Evans, inCambodia, is preparing for a 2015 LiDARmapping of the Angkor complex that willcover four to five times the area of hisprevious mapping. Shrestha worked withresearchers on using LiDAR to search for LaCiudad Blanca, a legendary lost metropolis inHonduras. The laser data revealed featuresthat could be ancient ruins of the city. Andfor their next big project, NCALM is takingLiDAR to Antarctica in a project fundedby the National Science Foundation thatuses LiDAR to map changes caused byclimate change.LiDAR is ushering in a new age of ar-

chaeology, as well as new growing pains.Arlen Chase points out that in Belize—aswell as other countries in Central America—“technology is outpacing policy.” In theUnited States, scientists have to release datafrom publicly funded projects to the public.However, in Belize such data would be tan-tamount to a roadmap for would-be looters.“Belize won’t let the LiDAR data be re-

leased, so we have to figure out ways to makeit accessible,” Chase says. “And the technol-ogy is moving so rapidly no one is dealingwith it.”Of course, in a few years those ethical

concerns might be a moot issue. Given therapid pace of development for LiDAR, it’sonly a matter of time before some companydevelops the ability to map from space: thinkof a LiDAR-based Google map. By then,Chase says, it will be too late to try to restrictaccess to the data.“What we need are guidelines in place for

the use of this stuff,” he says. “Politics needsto catch up with the technology.”

1 Evans D, et al. (2007) A comprehensive archaeological map of theworld’s largest preindustrial settlement complex at Angkor,Cambodia. Proc Natl Acad Sci USA 104(36):14277–14282.2 Evans DH, et al. (2013) Uncovering archaeological landscapes atAngkor using lidar. Proc Natl Acad Sci USA 110(31):12595–12600.3 Chase AF, et al. (2011) Airborne LiDAR, archaeology, and the AncientMaya landscape at Caracol, Belize. J Archaeol Sci 38:387–398.4 Chase DZ, et al. (2011) Airborne LiDAR at Caracol, Belize, and theinterpretation of ancient Maya society and landscapes. ResearchReports in Belizean Archaeology 8:61–73.

5 Chase AF, Chase DZ, Fisher CT, Leisz SJ, Weishampel JF (2012)Geospatial revolution and remote sensing LiDAR inMesoamerican archaeology. Proc Natl Acad Sci USA 109(32):12916–12921.6 Chase AF, et al. (2014) The use of LiDAR in understanding theAncient Maya landscape. Advances in Archaeological Practice 2(3):208–221.7 Chase AF, et al. (2014) Ancient Maya regional settlement andinter-site analysis: The 2013 West-Central Belize LiDAR Survey.Remote Sens 6(9):8671–8695.

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