Why I am skeptical about the new Maya LiDAR results from NGS

New Tikal LiDAR map

I am skeptical about the hype surrounding the recent press release from the National Geographic Society about the new findings of LiDAR survey in the Maya region of northern Guatemala. I have no reason to question the quality of the LiDAR survey, or its potential usefulness for understanding aspects of ancient Maya society in this region. Rather, I question two aspects of the way these new findings have been portrayed, both in the NGS press release and in the journalism that has resulted from the find. (1) This is portrayed as revealing brand-new ideas, when in fact earlier LiDAR work had very similar results; and (2) The work is portrayed as a major scientific discovery, when in fact it is only the first step of a process, the end result of which will be (one hopes) some major scientific discoveries.

LiDAR is a relatively new airborne remote sensing technology that permits detailed mapping of the surface of the earth at a detailed scale. It is far superior to earlier forms of satellite or airplane mapping in that LiDAR can penetrate dense vegetation. It is ideal for the Maya lowlands, where the jungle vegetation hinders traditional mapping. Wherever it has been applied, in the Maya area, the result is the identification of many new houses and features of the built environment. (1) This is my first misgiving: the lack of acknowledgement that Mayanists have been working with LiDAR for more than seven years (Chase et al. 2012; Chase et al. 2011).

The NGS story has breathless quotes about how suddenly we know about many new features and structures on the Maya landscape. Well, that is what LiDAR does in the Maya lowlands. It finds many more features than archeologists knew about previously. If archaeologists are surprised about this, they just haven’t looked at the prior work, both in the Maya area (Brown et al. 2016; Chase et al. 2014b; Chase et al. 2014a; Chase et al. 2012; Chase et al. 2011; Chase et al. 2016; Chase 2016; Chase and Weishampel 2016; Ebert et al. 2016; Prufer et al. 2015; Von Schwerin et al. 2016; Yaeger et al. 2016), in other parts of Mesoamerica (Fisher and Leisz 2013; Rosenswig et al. 2015; Rosenswig et al. 2013), and particularly at Angkor in Cambodia (Evans et al. 2013; Hanus and Evans 2016).

 

Lidar-identified small reservoirs at Cacacol. Chase 2016.

One difficulty with LiDAR data is that while it is easy to see large structures like pyramids in the output data, small features such as houses or agricultural fields are more difficult to pick out. They often require a combination of intensive, time-consuming searching by eye, and sophisticated custom computer algorithms that can pinpoint such features automatically. For example, my student, Adrian Chase, analyzed LiDAR data to identify small residential-level reservoirs at the Maya city of Caracol (Chase 2016). In areas that had been mapped previously by traditional methods, Adrian’s algorithm identified 25 times the number of small reservoirs at the site! These did not stand out on the LiDAR landscape like dropped pins in Google-Maps. They had to be painstakingly identified.

As far as I can tell, the intensive phase of analysis has not yet been carried out (or is not reported in this press release). It is easy to use LiDAR to find a bunch of new features and make a pretty map. But the next two steps are more difficult. For the first step, the archaeologist has to analyze the data—staring at maps and applying algorithms—so that one can be confident that most of the relevant small features have been identified. The pretty color maps one sees in all the press accounts are not the only way to portray spatial data in LiDAR; often other visualization methods are more useful. Adrian was able to identify all those small reservoirs only because he did two things: he spent countless hours staring at the output, and he applied custom computer algorithms to the data to identify the features. There is no indication that archaeologists have carried out this intensive level of analysis of the new Guatemalan data.

 

3 LiDAR visualizations. Chase 2016

A second crucial step is to analyze the results quantitatively and spatially to construct population estimates and study the on-the-ground patterning in settlement data. The NGS article subtitle says there were “millions more people than previously thought.” The report has this quote:

“Most people had been comfortable with population estimates of around 5 million,” said [Francisco] Estrada-Belli, who directs a multi-disciplinary archaeological project at Holmul, Guatemala. “With this new data it’s no longer unreasonable to think that there were 10 to 15 million people there—including many living in low-lying, swampy areas that many of us had thought uninhabitable.”

It will take quite a bit of analysis to turn this quick preliminary suggestion into rigorous population estimates for settlements and regions. These additional steps—technical application of algorithms, lots of staring at screens, and then quantification and calculation—are only beginning for the Maya lowlands (Chase 2016; Chase and Weishampel 2016; Ebert et al. 2016), and there is no sign that they have been accomplished for the new Guatemalan LiDAR results.

 

LiDAR of central Caracol. Chase et al 2011.

So, what is my beef? The new results are just in, and the analysis is probably only starting. This is the normal process of science. (2) My second misgiving is the idea—promoted by NGS, by the people interviewed in the article, and by secondary articles in the media—that archaeological advances consist of discoveries in the field. Yes, the fieldwork is essential. But without an often lengthy period of analysis, one typically cannot know the meaning or importance of the finds.

There is a kind of archaeology where the main discovery is made in the field. If one is looking for the tomb of a king or noble, and one finds it, that may be the essential defining moment of discovery. But I pursue another kind of archaeology. I have spent my career on the archaeology of Aztec provincial households. When I dig up another house or trash midden, it seems pretty much the same as countless I and others have excavated. They are pretty boring, I have to admit. But once I have spent months or years studying the artifacts, quantifying them, sending off samples of technical analyses, only then do I make my discoveries. When I argue that this household was well-off and that one was poor, or when I argue that conquest by the Aztec empire had little effect on local people, these are my discoveries. They rely on extensive analyses of artifacts. I had no idea about these things at the time of excavation. I discuss this issue—what is the real moment of discovery?—in more detail in my recent book (Smith 2016).

 

Social interpretations at Yautepec were based on study of 1 million potsherds

When one focuses almost exclusively on the actual uncovering of a find during fieldwork (for an excavation), or on the initial pretty maps of a LiDAR survey—before the hard work of analysis is done—one is distorting the scientific significance of our work. Will NGS have a big feature when the archaeologists involved actually publish a revised population estimate for northern Guatemala, or when they can quantify the amount of construction in rural vs urban areas?
We'll see.

A kind of archaeology based on extensive analysis

How can one spot a finding that seems spectacular but is actually a preliminary find, not yet analyzed, from a finding based on proper analysis and interpretation? Peer-review publication is the primary way to do this. The NGS piece was based entirely on interviews, not on a paper that has been peer-reviewed and accepted by a scientific journal.

Claims that LiDAR will revolutionize the study of Maya settlement and demography may very well be correct, but it is too soon to tell. The Guatemalan LiDAR has reached the stage of preliminary findings and pretty maps, but not the stage of solid architectural, demographic, and social findings. I look forward to the scientific results. I don’t care if they are an internet sensation; I’d rather see them published in a journal.

References

Brown, M. Kathryn, Jason Yaeger, and Bernadette Cap

2016 A Tale of Two Cities; LiDAR Survey and New Discoveries at Xunantunich. Research Reports in Belizean Archaeology 13: 51-60.

Chase, Adrian S. Z.

2016 Beyond Elite Control: Residential Reservoirs at Caracol, Belize. WIREsWater 3 (6): 763-797.

Chase, Adrian S. Z. and John F. Weishampel

2016 Water Capture and Agricultural Terracing at Caracol, Belize as Revealed through Lidar and GIS. Advances in Archaeological Practice 4 (3): 357-370.

Chase, Arlen F., Diane Z. Chase, Jaime J Awe, John F. Weishampel, Gyles Iannone, Holley Moyes, Jason Yaeger, and M. Kathryn Brown

2014a        The Use of LiDAR in Understanding the Ancient Maya Landscape. Advances in Archaeological Practice 2 (3): 208-221.

Chase, Arlen F., Diane Z. Chase, Jaime J. Awe, John F. Weishampel, Gyles Iannone, Holly Moyes, Jason Yaeger, Kathryn Brown, Ramesh L. Shrestha, William E. Carter, and Juan Fernandez Diaz

2014b        Ancient Maya Regional Settlement and Inter-Site Analysis: The 2013 West-Central Belize LiDAR Survey. Remote Sensing 6: 8671-8695.

Chase, Arlen F., Diane Z. Chase, Christopher T. Fisher, Stephen J. Leisz, and John F. Weishampel

2012 Geospatial revolution and remote sensing LiDAR in Mesoamerican archaeology. Proceedings of the National Academy of Sciences 109: 12916-12921.

Chase, Arlen F., Diane Z. Chase, John F. Weishampel, Jason B. Drake, Ramesh L. Shrestha, K. Clint Slatton, Jaime J. Awe, and William E. Carter

2011 Airborne LiDAR, Archaeology, and the Ancient Maya Landscape at Caracol, Belize. Journal of Archaeological Science 37: 387-398.

Chase, Arlen F., Kathryn Reese-Taylor, Juan C. Fernandez-Diaz, and Diane Z. Chase

2016 Progression and Issues in the Mesoamerican Geospatial Revolution: An Introduction. Advances in Archaeological Practice 4 (3): 219-231.

Ebert, Claire E., Julie A. Hoggarth, and Jaime J. Awe

2016 Integrating Quantitative Lidar Analysis and Settlement Survey in the Belize River Valley. Advances in Archaeological Practice 4 (3): 284-300.

Evans, Damian H., Roland J. Fletcher, Christophe Pottier, Jean-Baptiste Chevance, Dominique Soutif, Boun Suy Tan, Sokrithy Im, Darith Ea, Tina Tin, Samnang Kim, Christopher Cromarty, Stéphane De Greef, Kasper Hanus, Pierre Bâty, Robert Kuszinger, Ichita Shimoda, and Glenn Boornazian

2013 Uncovering archaeological landscapes at Angkor using lidar. Proceedings of the National Academy of Sciences 110: 12595-12600.

Fisher, Christopher T. and Stephen J. Leisz

2013 New Perspectives on Purapécha Urbanism through the Use of LiDAR at the Stie of Angamuco, Mexico. In A Primer on Space Archaeology: In Observance of the 40th Anniversary of the World Heritage Convention, edited by D.C. Comer, pp. 191-202. SpringerB riefs in Archaeology, vol. 5. Springer, New Yokr.

Hanus, Kasper and Damian Evans

2016 Imaging the Waters of Angkor: A Method for Semi‐Automated Pond Extraction from LiDAR Data. Archaeological Prospection 23 (2): 87-94.

Prufer, Keith M., Amy E. Thompson, and Douglas J. Kennett

2015 Evaluating airborne LiDAR for detecting settlements and modified landscapes in disturbed tropical environments at Uxbenká, Belize. Journal of Archaeological Science 57: 1-13.

Rosenswig, Robert M., Ricardo López-Torrijos, and Caroline E. Antonelli

2015 Lidar data and the Izapa polity: new results and methodological issues from tropical Mesoamerica. Archaeological and Anthropological Sciences 7 (4): 487-504.

Rosenswig, Robert M., Ricardo López-Torrijos, Caroline E. Antonelli, and Rebecca Mendelsohn

2013 LiDAR Mapping and Surface Survey of the Izapa State in the Tropical Piedmont. Journal of Archaeological Science 40: 1493-1507.

Smith, Michael E.

2016 At Home with the Aztecs: An Archaeologist Uncovers their Domestic Life. Routledge, New York.

Von Schwerin, Jennifer, Heather Richards-Rissetto, Fabio Remondino, Maria Grazia Spera, Michael Auer, Nicolas Billen, Lukas Loos, Laura Stelson, and Markus Reindel

2016 Airborne LiDAR acquisition, post-processing and accuracy-checking for a 3D WebGIS of Copan, Honduras. Journal of Archaeological Science: Reports 5: 85-104.

Yaeger, Jason, M Kathryn Brown, and Bernadette Cap

2016 Locating and dating sites using Lidar survey in a mosaic landscape in Western Belize. Advances in Archaeological Practice 4 (3): 339-356.

Archaeology in PNAS

Area of the Puchituk Terminus at Caracol

I just found two Mesoamerican articles in the online-first section at the Proceedings of the National Academy of Sciences. It's great to see good archaeology receive high-profile coverage in places that are seen by a wide range of disciplines. These papers describe early steps toward larger research goals, although they tend to be phrased as if they were reaching those goals right now.

Chase, Arlen F., Diane Z. Chase, Christopher T. Fisher, Stephen J. Leisz, and John F. Weishampel
2012    Geospatial revolution and remote sensing LiDAR in Mesoamerican archaeology. Proceedings of the National Academy of Sciences (published online first).

Scarborough, Vernon L., Nicholas P. Dunning, Kenneth B. Tankersley, Christopher Carr, Eric Weaver, Liwy Grazioso, Brian Lane, John G. Jones, Palma Buttles, Fred Valdez, and David L. Lentz
2012    Water and sustainable land use at the ancient tropical city of Tikal, Guatemala. Proceedings of the National Academy of Sciences (published online first)

LIDAR!

The central portion of Angamuco

The first paper is a brief description of recent LIDAR mapping at the Maya city of Caracol and the western Mexican city of Angamuco. Most readers will probably already have seen some LIDAR maps; if not, check out this article and some of the other publications. These maps are absolutely incredible. Arlen and Diane Chase (and their crew) spent decades instrument mapping at Caracol, and they still had only covered a portion of the site. Now with one application, the LIDAR map covers the entire (huge) urban area, with high resolution and great accuracy. Arlen first showed me the maps two years ago and I was blown away. It is hard to express just how much of a leap forward this is for archaeological mapping, particularly in highly vegetated areas like the Maya lowlands. The PNAS paper only has a couple of images (see above); see some of the other publications, cited in that paper, for more images.

The Angamuco map, done for the project directed by Chris Fisher, is also pretty amazing (above). It is one of a series of west Mexican urban settlements built on lava flows. Some French teams have been working on other sites of this type, which have the potential to greatly illuminate our understanding of urban form (since many house foundations and other features can be mapped).

So what are the larger research goals that can be addressed with these and other LIDAR-mapped sites? Leaving aside the obvious goals of providing more details about individual archaeological sites, I am excited about this work because of the potential to advance our understanding of urban morphology in ancient cities. It is going to take some time to reach this goal, since we presently lack the methods to translate good maps (whether made with LIDAR or with old non-electronic instrument mapping, or with a compass and tape) into rigorous results about city layout and planning. It is striking to see high-tech spatial methods (LIDAR, various prospecting methods like ground-penetrating radar, NASA satellite imagery) used to make visually arresting maps, which are then interpreted in a subjective and impressionistic manner.

Perhaps the situation is analogous to provenience studies of artifacts. For many years, even decades, we have had good data on the places of origins of lots of artifacts, but few models or concepts on how to translate those data into reliable economic inferences. I have complained about this for many years, in various review articles and such. Methods and data often far out-run our interpretive approaches. Now, finally, archaeologists are working out methods for reconstructing things like market systems from artifact sourcing studies (see, for example, Garraty and Stark, eds, 2010, Archaeological Approaches to Market Exchange in Ancient Societies, Univ Press of Colorado). So, I hope that archaeologists and others will made the kinds of advances in studying urban form that are needed to really take advantage of the great maps produced by LIDAR (and other methods).

MAYA RESERVOIRS AND SUSTAINABILITY

Reservoirs in central Tikal

The second PNAS paper is a nice study of the construction and use of reservoirs at the Maya city of Tikal. Over many years, Vincent Scarborough has led an effort to show how the ancient Maya managed water resources, and the social context of water. His excellent study (The Flow of Power: Ancient Water Systems and Landscapes, 2003, SAR Press) helps put the Maya case into a broader comparative framework. The reservoirs at Tikal (see image above) have been known for a long time, but now Scarborough and his colleagues have learned how and when they were built and how they were used.

I am a bit skeptical about the sustainability argument of this paper. There isn't much of an explicit argument here. The implied argument seems to be that the schemes for water control and use identified by fieldwork were a form of sustainable land use, thereby permitting the city of Tikal to flourish for many centuries. Perhaps. That seems a reasonable notion, but how can it be confirmed or falsified? This is a causal argument (these practices caused -- or at least allowed and stimulated -- a long occupation). But to confirm this hypothesis, more cases and better interpretive models are needed. What would non-sustainable practices look like? Are there some cities that used major water control methods and lasted for many centuries, while other similar cities did not use the water technology and did not last as long?

It is difficult, perhaps impossible, to make a convincing causal argument from a single case. One minimally needs to consider the counterfactual case -- suppose that the rulers or builders of Tikal had NOT designed such clever water control features. What would have been the consequences? Perhaps an argument can be made that the city would not have lasted so long, or would not have grown so large, without these features. But even though this kind of explicit counterfactual argument can suggest a causal model, any real conclusions about ancient sustainable practices require a much larger sample of cases. I develop this argument in my 2010 paper in CAJ: my point is that archaeologists have data to address issues like urban sustainability, but we have yet to assemble rigorous samples and perform the necessary analyses to produce reliable results.

Research like that described in the paper by Scarborough et al in PNAS is important for understanding Tikal and for building knowledge about ancient systems of water control. It could also be important for generating findings about ancient sustainable practices, but to do this, it needs to be joined by many more studies to build a reliable base of information. This paper is an excellent step in that direction, but I think it is premature to make any claims about sustainability from single studies like this.