He discovered the Mayan "Lost City" in the rainforest, relying on a free map?

Archaeologist Takeshi Inomata from the University of Arizona is very excited recently. He is studying the ancient Mayan civilization and recently discovered a long-lost Mayan ruin, which is a major breakthrough in the study of the entire Mayan civilization.

The treasure map that led him to find this "lost city" was actually a completely free online map.

Takeshi Noda uses free maps to discover a Mayan ruin | uanews.arizona.edu

Mayan ruins in free lidar maps

The story begins in the 1980s. At the time, Takeshi Noda was a doctoral student. Although mainstream research at the time focused on the heyday of Mayan civilization's politics and economy (between 250 and 900 AD), Takeshi Noda was more interested in the origins of Mayan culture. When he was able to conduct research independently, he couldn't wait to start this project.

It made sense that no one was studying the origins—there was a lack of archaeological evidence for the origins of civilization, and no large-scale remains, so everything had to be started from scratch. It wasn't until 2005 that he and his wife, Daniela Triadan, an anthropologist at the University of Arizona, began excavating the ancient city of Ceibal in the Peten rain forest in Guatemala. There, they discovered some of the earliest known Mayan buildings.

Dr. Noda Takeshi and Dr. Cui Yadan believe that the earliest Mayan civilization was influenced by the Olmecs from the very beginning, and the relationship between the two can be traced back to the origin of Mesoamerican civilization. In order to further explore the relationship between the two cultures, it is necessary to study the area between the ancient city of Cebal and the Olmec cultural center.

The problem is that this area is too vast, stretching from Veracruz, Mexico in the north to Guatemala in the south, with a straight-line distance of more than 800 kilometers, which is beyond the reach of most archaeological teams. And most of this area is untouched tropical rainforest, and archaeological excavation in these places is a very difficult challenge, requiring heavy-duty measuring equipment and arduous treks through the jungle. It takes several years to find a single relic, which is a very time-consuming and labor-intensive task.

Noda Takeshi started thinking about whether it is possible to use technology to analyze existing map information. Most people might think of GPS satellite maps. Indeed, today's satellite maps are accurate to 1 meter, and even cars on the ground can be seen clearly. But for archaeologists, the problem with satellite maps is that they are too "flat". Most of the ruins are hidden in the rainforest and cannot be identified at all.

There is another type of map that is more powerful and more accurate than GPS: Lidar. This map can provide extremely detailed topographic information, but the only problem is that it is too expensive. A commercial Lidar map of less than 100 square kilometers costs more than 60,000 US dollars, which is not something that the research funds of the archaeology department can afford.

However, for Noda Takeshi, things suddenly turned around: he actually found free resources on the Internet! It turned out that the Mexican government had already surveyed this area, and everything was publicly available.

When Takeshi Noda received a lidar map of all of southern Mexico from Rodrigo Liendo, an archaeologist at the National Autonomous University of Mexico, he was so excited that he didn't know what to say. The map, released in 2011 by the National Institute of Statistics and Geography, covers 11,500 square kilometers of the Mexican states of Tabasco and Chiapas. Tabasco is home to the Olmec site of San Lorenzo.

Dr. Noda clearly sees the outlines of countless archaeological sites on the map. So far, he has used it to identify the sites of 27 previously unknown Maya ceremonial centers that contain a type of architecture that archaeologists have never seen before. Some of these ruins have platforms as long as 1.6 kilometers.

"If you walk on the platform, you don't realize you are walking on the remains of an ancient city because it is so big that it looks like part of the natural landscape, but now we can better see what the entire society looked like," said Dr. Takeshi Noda.

Mayan ruins La Carmelita discovered by Takeshi Noda | Nacional Center for Airborne Laser Mapping

The ruins look like this in reality, and there is no difference from other places | Noda Takeshi

These sites may help us understand the origins of Mayan civilization, which is exactly what Noda is most interested in. Currently, Noda's team is excavating the largest religious ruins center found on the lidar map, hoping to learn more about the earliest religious rituals of the ancient Maya there. The Mexican government is constantly expanding this free map to cover new areas.

Lidar, a good friend of archaeologists

The application of LiDAR in the field of archaeology began in the early 21st century. Especially for relics hidden under dense vegetation, LiDAR technology saves archaeologists a lot of hard treks and surveying in the jungle. Now, archaeologists use LiDAR technology to find the target, fly there comfortably, and complete the surveying work in a few days.

Arlen Chase, an archaeologist at Pomona College, was an early adopter of lidar, using it to map the Maya city of Caracol in Belize in 2009. His son, Adrian Chase, is currently working on a doctorate at Arizona State University, using lidar to compare the living space of more than 4,000 homes at Caracol to infer the extent of social inequality at the time (as now, wealthier residents had larger homes).

Before the advent of LiDAR, such batch analysis was almost impossible. In 2013, archaeologist Damian Evans of the University of Sydney used LiDAR technology to discover the ancient city of Mahendraparvata in Cambodia, which had been silent for 1,200 years.

What is LiDAR?

Lidar is short for "Light Detection and Ranging", which is a remote sensing method that uses light in the form of pulsed lasers to measure variable distances. Lidar instruments are mainly composed of lasers, scanners and dedicated Global Positioning System (GPS) receivers, and many of them are now carried by aircraft and helicopters.

In 2015, with the help of staff from the National Center for Airborne Laser Mapping (NCALM) at the University of Houston, Takeshi Noda's team completed the mapping and lidar data collection of an area of ​​470 square kilometers around the ancient city of Sebar in just six days.

They placed the lidar equipment on a small plane and hovered at an altitude of 700 meters above the rainforest. When the laser of a specific wavelength is emitted from the airborne transmitter, some of the laser penetrates the dense canopy of the rainforest and reaches the target. The light beam is reflected by the surface it encounters and returns to the receiving sensor. The time recorded in the meantime is used to calculate the distance between the target and the laser transmitter, and based on this, the three-dimensional shape of the ground is recorded. This is similar to a junior high school physics problem - knowing the speed of light and the round-trip time from the light source to a certain point, calculate the distance between the light source and the target position.

For areas with different vegetation densities and altitudes, staff also have to adjust the flight altitude and the frequency of laser emission accordingly. For the central area of ​​the city of Sebar, a "combination punch" of pulses of different frequencies are fired from altitudes of 700 meters, 600 meters, and 400 meters respectively, penetrating layers of woods and providing extremely subtle archaeological features for this place.

The dense data cannot intuitively show the shape of the ruins. At this time, the Digital Elevation Model (DEM) is needed to help.

DEM is a digital simulation of the ground terrain through terrain elevation data, that is, data obtained by laser scanning placed on an aircraft. After the data was run, the research team obtained a model with a horizontal resolution of 0.5 meters and without vegetation. A lost ancient ruins appeared on the screen.

Bare earth model after Lidar data passes through DEM

Lidar's ultimate gift to mankind

LiDAR technology not only provides archaeologists with new ways to analyze the ancient world, but also allows humans to look at the changes in their perspectives in the present and future - can we use laser scanning to draw detailed maps of all the land on the earth?

Chris Fisher, an archaeologist at Colorado State University, is pushing for this to happen, creating an "Earth Archive," a project inspired by the nonprofit CyArk, which is using lidar technology to record cultural relics and sites in 3D.

However, lidar can record much more than just these.

The application range of lidar is very flexible. Depending on the different purposes, the optical parameters of laser ranging will also be adjusted. For example, when measuring terrain, near-infrared lasers with wavelengths of 1047 nm, 1064 nm, 1550 nm, etc. are usually used; and when measuring the altitude of the seabed and riverbed, blue-green laser bands with better water permeability, such as 532 nm, are used.

Therefore, LiDAR can not only scan land, but also easily obtain geospatial information of water environments such as ice sheets, coastal areas, and deep seas. Building a 3D map of the entire earth is no longer a dream. Moreover, the resolution of the data is already amazingly high. "We can see something about 20 centimeters on the ground, about the size of a building brick."

LiDAR works well on land and on ice sheets, which has important practical implications for coastal areas threatened by rising sea levels, as well as for recording the Amazon River basin.

In addition, LiDAR can reveal the age and complexity of forests, reconstruct landscapes, and track changes over the years. For ecosystems with rich layers and differential distributions (such as forests), LiDAR technology can replace traditional time-consuming and labor-intensive field surveys to estimate the aboveground biomass of forests. For the white polar regions, LiDAR can monitor the thickness of the ice sheet and quantitatively analyze the accumulation of snow on polar glaciers, thereby obtaining the relationship between glacier melting and global temperature changes.

Professor Fisher's purpose in promoting this project is to respond to the climate crisis. Before the threatened cultural heritage - archaeological sites, landscapes, and ecological heritage - plants, animals, and the entire landscape, geology, hydrology, etc. of these places undergo fundamental changes, everything will be recorded as the ultimate gift to future generations.

For us, LiDAR is like an eye that can see the ancient times and record the future; it can take into account the vast land with fine precision and build a bridge between the past and the future. This is the meaning of science.

References

[1] https://www.nytimes.com/2019/10/08/science/archaeology-lidar-maya.html

[2] https://www.theguardian.com/science/2019/oct/11/ultimate-gift-to-future-generations-plan-to-laser-map-all-land-on-earth

[3] https://oceanservice.noaa.gov/facts/lidar.html

[4] https://doi.org/10.1371/journal.pone.0221943

[5] https://academic.oup.com/bioscience/article-pdf/52/1/19/26891841/52-1-19.pdf

[6] https://www.pnas.org/content/pnas/110/31/12595.full.pdf

[7] http://cdmd.cnki.com.cn/Article/CDMD-85402-1015543026.htm

[8] http://www.cnki.com.cn/Article/CJFDTOTAL-ZNTB201826009.htm

Author: Vivian

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