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Photonics News No 61

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Secrets in the Rainforest

Photonics News No 61

New Insights into Ancient Cultures


UK61-0410

Impenetrable Rainforest

Everyone has seen pictures of the temples and pyramids of Tikal. But if you have not visited it yet, it is hard to imagine how vast the ruins really are. The national park in which they are located covers an area of 575 km². Most of it is covered in dense rainforest. Compared to the surrounding Mayan biosphere reserve, however, this is only a small blob. This huge nature reserve in the northern part of the province of Petén covers 21,000 km² - an impenetrable and ­protected rainforest the size of Wales.

It has long been suspected that the dense vegetation hides further remains of Mayan civilisation, but the search for them has proven difficult. In the days of Percy Fawcett, when researchers still struggled through the thicket with a machete, it was often pure luck when they came across old buildings. Many a ruin may have gone undiscovered because an expedition missed it by just a few hundred metres. Of course, more state-of-the-art methods have been available for a long time; for example, aerial photos. However, even photos of the region do not reveal much more than a dense ceiling of treetops. It was not until the application of laser technology that it was possible to look through the trees.

Complex Aerial Measurements

Light detection and ranging (LiDAR) uses laser light to measure distances. When the laser pulse hits an obstacle, the reflected light is detected by a detector. The exact distance to the obstacle can be calculated from the time between the emission of the pulse and the arrival of the returning light, which is referred to as time of flight (ToF) in technical jargon. This principle is familiar to every DIY enthusiast who has measured his or her home with a laser rangefinder before. This technology is also used in obstacle detection during autonomous driving or with self-piloting drones (Photonics News #PN60, pp. 8 – 9). One of the great advantages of LiDAR is its high ­resolution: Compared to other technologies, laser-based systems operate with very short wavelengths and can, therefore, record considerably more details.

To create a digital elevation profile, the laser scans the landscape from an aero­plane or helicopter. Several ­thousand pulses are sent every second. In addition to LiDAR, two other technologies are used to determine the exact elevation profile: A satellite-supported GPS constantly records the exact geographical position of the aircraft so that the LiDAR measurements can be located later on the map. This happens in all three dimensions because the exact flight altitude naturally has a crucial influence on the ToF result. In addition, an inertial measurement unit (IMU) – essentially a gyroscope – measures the various angles of inclination of the aircraft since these directly influence the path length of the reflected laser beam.

Trees Are Essentially Removed

Within trees and other plants, an effect occurs that is particularly useful in laser cartography. Unlike buildings or rocks, leaves do not reflect all light. One part penetrates the leaves and continues to move towards the ground until it hits the next “obstacle” and so on. Thus, it can happen that the same light pulse is reflected several times – each time with lower intensity and of course with continuously increasing ToF. All these reflected signals can be assigned later to the original pulse.

The result is a three-­dimensional image of the tree – or even of an entire forest.
Using complex algorithms, a computer can virtually remove vegetation from the landscape profile identified. What remains is a detailed model of the bare floor. The Maya researchers were surprised at how much new knowledge they were able to gain from the LiDAR data. The surface structures showed that hundreds of years ago houses, high roads, and fields once existed where the rainforest now grows. Until recently, it has been assumed that the ­hinterland of the Mayan cities was sparsely ­populated. Now the archaeologists know better: The metropolises were closely interlinked.

New Discoveries Everywhere

Not only in Central America does laser technology provide new insights into the past. In Lancashire, Cumbria and Northumberland, LiDAR investigations revealed ancient Roman roads. Archeologists had always expected them to be there, but no trace of them was visible on the aereal photographs they had worked with, up to now. At least, they know what they found. Their ­colleagues in Guatemala are far from that. They must first evaluate and analyse all data. It, therefore, remains very exciting.


Avalanche Photodiodes and Pulsed Laser Diodes for LiDAR Measurements

Pulsed laser diodes in the near IR range are commonly used in LiDAR cartography. Avalanche photodiodes (APDs) detect the reflected laser pulses. At our facilities in Canada and the USA, we manufacture both components in order to equip LiDAR systems for a wide variety of applications – not just archaeology.


Datasheet:

Further product information:
Silicon APD Arrays
QuickSwitch® Pulsed Laser Diodes

Manufacturer:
LASER COMPONENTS Detector Group

Contact:

Contact Person:    Chris Varney
Company:    LASER COMPONENTS (UK) Ltd.
Address:    Goldlay House 114 Parkway
Postcode / City:    CM2 7PR Chelmsford Essex
Phone:    +44 1245 491499
Fax:    +44 1245 491801
Email:    cvarney@lasercomponents.co.uk
 

LiDAR Measurement


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