News New massive Mayan site discovered in Guatemala

[Greg Bernhardt]

I love when this happens!

Lasers Reveal a Maya Civilization So Dense It Blew Experts’ Minds
https://www.nytimes.com/2018/02/03/world/americas/mayan-city-discovery-laser.html

They were hidden there, all this time, under the cover of tree canopies in the jungles of northern Guatemala: tens of thousands of structures built by the Maya over a millennium ago.

Not far from the sites tourists already know, like the towering temples of the ancient city of Tikal, laser technology has uncovered about 60,000 homes, palaces, tombs and even highways in the humid lowlands.

The findings suggested an ancient society of such density and interconnectedness that even the most experienced archaeologists were surprised.

 

[fresh_42]

Yes, I've read this, too, today. What a giant site and they estimate that 11,000,000 people had lived in the area, supposedly three times as many as previously thought. Just WOW!

If I think about such discoveries, or about Angkor Wat, it always makes me humble considered the size of such places and what we already had lost in our short lifetime as a species.

 

[BillTre]

Here is an update on this from the NY Times.
Very nice pictures showing pictures of jungle compared with highly detailed lidar images of the same place.
Paths can be seen in the dirt!

They report there was a large areas of suburbs, apparently cultivation of some nearby wetlands.
During the Late Classic Period (~650 A.D. to ~800 A.D), they figure there was a population 7 to 11 million, much larger than was previously thought.
The wetlands agriculture is thought to support the larger population.

I wonder if they are doing anything to protect sites that get identified this way from being looted.
I suppose they try to keep the exact locations of sites not commonly known secrete. ??

 

[berkeman]

Pretty cool. How does LIDAR penetrate the foliage? I wonder what wavelength of laser they are using for this (didn't see it mentioned in the articles).

 

[BillTre]

Its my understanding that it is supposed to penetrate the foliage and show the ground underneath.
I think the choice of wavelength of laser light used is important for this.

 

[berkeman]

How does LIDAR penetrate the foliage?

Hah, the answer appears to be pretty non-obvious. And it seems to be a recent invention...

https://www.osa.org/en-us/about_osa..._the_forest_through_the_trees_with_a_new_lid/

Until now, LiDAR measurements of surfaces hidden behind foliage have been difficult to acquire. A majority of the original light in these cases gets thrown away, as far as the camera detecting light from the ground is concerned, since the light hitting the leaves never reaches the ground in the first place. Moreover, the light blocked, and therefore reflected, before getting to the ground often overpowers the signal hitting the camera and hides the fainter signal that does make it to the ground and back.

“We have been working with a process called optical phase conjugation for quite some time and it dawned on us that we might be able to use that process to essentially project a laser beam through the openings of the leaves and be able to see through a partial obscuration,” Lebow said. “It was something that until maybe the last five years was not viable just because the technology wasn’t really there. The stuff we had done about 20 years ago involved using a nonlinear optical material and was a difficult process. Now everything can be done using digital holography and computer generated holograms, which is what we do.”

This new system uses a specially designed laser that alone took a year and a half to develop, but was a necessary component according to Lebow and his colleague, Abbie Watnik, who is also at the Naval Research Laboratory and another of the work’s authors.

“The real key to making our system work is the interference between two laser beams on the sensor. We send one laser beam out to the target and then it returns, and at the exact same time that return [beam] hits the detector, we interfere it locally with another laser beam,” Watnik said. “We need complete coherence between those beams such that they interfere with one another, so we had to have a specially designed laser system to ensure that we would get that coherence when they interfere on the camera.”

Using a pulsed laser with pulse widths of several nanoseconds, and gated measurements with similar time resolution, the holographic system selectively blocks the earliest-to-arrive light reflecting off obscurations. The camera then only measures light coming back from the partially hidden surface below.

Very Cool