The most powerful aid for jungle archaeology - LiDAR

Human beings are small but tenaciously multiplying on the earth. How did our ancestors live on this vast land? They planted food, built houses, formed tribes, and established cities, leaving traces of their existence.

Through the exploration of ancient monuments, we can infer the lifestyle, social culture and technological development level of the ancients. However, after thousands of years, it is not an easy task to find the relics left by the ancients.

Because of the vicissitudes of life, the original buildings may have been buried deep underground. Whether it is vast vegetation or dense forests, they can easily block our exploration of the underground world. However, the emergence of LiDAR technology can help us unveil the "veil" on the land and let us see the most original "face" of the earth.

LiDAR, a powerful tool for exploring ruins

In the early 20th century, British explorer Percy Fawceet went deep into the Amazon region many times to search for the ancient civilization that once existed in that land, but he got lost in the dense forest of the Amazon while searching for relics. As a result, the historical relics in the Amazon region disappeared from people's sight along with that expedition team.

However, a paper recently published in Nature magazine said that scientists used LiDAR to finally get a picture of the ancient city ruins in the Amazon forest. Let's first take a look at what the ruins look like:

(Image source: Reference 1)

The buildings shown in the picture were built by the Kasarbe community between 500 and 1400 AD, and are traces of previous man-made things such as houses, terraces, and walls. This picture was obtained by flying a helicopter equipped with laser radar over the forest and then post-processing the image.

But you may find that this picture seems a little different from usual aerial photos. In fact, the biggest difference is that the trees are gone.

Left: Forest image taken by an aerial camera

Right: Image obtained by LiDAR detection at the same location

(Image source: Reference 1)

Without the cover of the trees, the traces of the ancient buildings are clearly visible. Archaeological work has become much smoother. Before the advent of LiDAR, in order to explore the remains in the forest, you had to learn how to survive in the wild, and you had to jump up and down in the tropical forest during field surveys (archaeologists: It's too hard for me).

Now with LiDAR, archaeologists can avoid the interference of trees and observe the ancient architectural sites more clearly. So, how are topographic maps that eliminate the interference of trees drawn?

Active imaging illuminates dark corners

In the dense forest, leaves block most of the sunlight, and very little sunlight can reach the ground under the trees. When observing from the sky, ordinary cameras can hardly obtain images of the ground, and the gaps between the leaves are mostly dark shadows.

A camera is a passive imaging device that can only take photos after receiving light emitted or reflected by external objects; lidar is an active detector that emits lasers to detect the shape of the outside world.

For a complex topography like a forest, there are still a lot of laser beams that can reach the ground under the trees. The point cloud data obtained by the lidar detection will be sent to the computer. The computer will identify which data points are from the forest and which are from the ground, and then filter out the point cloud data of the forest. What remains is the ground topography data.

Raw point cloud data obtained by LiDAR

(Image source: answers.usgs.gov)

The computer classifies the point cloud (black points are vegetation data points, and colored points are surface data points)

(Image source: Reference 2)

The bare soil image of the terrain surface obtained after filtering out the vegetation point cloud is also called a digital elevation model (DEM).

(Image source: answers.usgs.gov)

The "superpower" of LiDAR in terrain mapping has made it popular among archaeologists all over the world. In 2016, the Pacaunam Project conducted a large-scale LiDAR survey of the Maya Biosphere Reserve in Guatemala, discovering more than 60,000 ancient buildings and more than 100 kilometers of causeways. This result shows that there was a Maya civilization with a population of millions in the area.

This technology was also used in the survey and investigation of the Liangzhu ancient city. The laser radar scanned the Liangzhu ancient city ruins, and after filtering out the vegetation on the ground, the ancient city's water conservancy facilities, outer walls and other structures were revealed.

Not only can it discover the past, but it can also warn of the future

In addition to using LiDAR to detect relics, LiDAR's clear presentation of topography also allows it to play a role in preventing landslides.

Landslides are one of the most common geological disasters. Every year, people lose property or even their lives due to landslides. Understanding the mechanism of landslides, observing the precursors of landslides, and establishing a corresponding landslide prediction system are important means to reduce the hazards of landslides.

The high-precision digital elevation model (HRDEM) obtained by lidar mapping is a powerful aid for geologists to analyze the mechanism of landslides and establish early warning models.

To study landslides, a large amount of landslide data is needed as research material. Scientists have established a landslide inventory map using remote sensing satellites, lidar, aerial cameras and other observation methods.

The map shows the location of past landslides in various areas, the morphology of landslides, and also contains information such as rainfall in the area, soil type, earthquake shaking level, etc. Using the landslide inventory map, people can calculate the probability of landslides in various areas.

The landslide inventory map obtained based on lidar mapping. Areas of different colors represent different types of landslides. The red blocks in the small map in the lower right corner represent deep landslides, and the blue blocks represent shallow landslides.

(Image source: Reference 3)

Before a landslide occurs, a certain height of bulge will form at the bottom of the landslide, and cracks will appear on the slope. Due to the coverage of vegetation, it is impossible to observe these signs with the naked eye or camera, but with the help of LiDAR, this problem can be solved well.

Deploy swarms of drones equipped with lidar in areas prone to landslides to conduct uninterrupted scanning and observation of the surface morphology of the slopes. The observation data will effectively improve our ability and accuracy in predicting landslides.

It is not easy to explore the remains of past activities on a piece of land. As time goes by, the traces left by human activities and crustal changes will gradually be buried by nature. However, LiDAR gives us the opportunity to re-understand the "past" and find inspiration for the future from the remains of the "past".

Editor: Wang Tingting

References:

[1] Erickson CL. An artificial landscape-scale fishery in the Bolivian Amazon.[J]. Nature, 2000, 408(6809):190-3.

[2] Vanvalkenburgh P , Cushman KC . Lasers Without Lost Cities: Using Drone Lidar to Capture Architectural Complexity at Kuelap, Amazonas, Peru[J]. Journal of Field Archaeology, 2020, 45(supplement 1).

[3]Gorum, Tolga. Landslide recognition and mapping in a mixed forest environment from airborne LiDAR data.

[4]M, J, Lato, et al. Reducing Landslide Risk Using Airborne Lidar Scanning Data[J]. Journal of Geotechnical and Geoenvironmental Engineering, 2019, 145(9).

[5]https://www.usgs.gov/faqs/what-difference-between-lidar-data-and-digital-elevation-model-dem

[6]http://m.app.cctv.com/video/detail/aa9c36ee6d92b0a7a7a98d2f86eb447c/index.shtml

Produced by: Science Popularization China

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