What You Should Know About LiDAR

Imagine flying in a small plane (some of you are probably starting to feel sick), looking out the window to the Earth below. You are flying over dense jungle with a thick canopy. The sea of green is a beautiful vision but you are missing the pearls which lay below, out of sight. Or imagine mapping the surface of a planet. What is the best technology for this? What would NASA use? Light Detection and Ranging technology, better know as LiDAR, is rapidly becoming an extremely useful tool for a range of situations just like these.

So what is LiDAR and what can it do?

LiDAR was invented soon after lasers in the 1960’s. It is a detection system that determines the distance to an object using light pulses. LiDAR can map the surfaces of planets, detect the environment surrounding a (driverless) car, or find ancient cities previously hidden by dense jungle. However, due to the extremely precise nature of the measurement techniques used for LiDAR, it has rarely been used in application – until now.

But how?

LiDAR is the sister of sonar which is used in submarines and by bats. Instead of pulsing sound waves, LiDAR uses beams of light. These pulses reflect off of objects in its path and return to the receiver. As a pulse is sent, a computer measures the time for that pulse to reach the receiver. The distance the beam has travelled can then be calculated using the observed time and the constant speed of light.

To help clarify how LiDAR works and to see the very simple mathematics involved, here is a cartoon made by me!

Explaining LiDAR simply
Explaining LiDAR Simply

Lets say as an example that the pulse takes 0.2 milliseconds to reflect off the person and reach the receiver box. Therefore,

d = 3,000,000 x 0.0002 = 600 m

So the distance to the person is 300 m, simply half the distance travelled by the light pulse.

Why haven’t I heard of LiDAR before?

LiDAR has been in the shadows because it requires very accurate components. Computational power, location accuracy, receiver accuracy, and data storage have all posed problems in the past. However, LiDAR has finally reached a point in which it can successfully survey extensive areas with a high degree of precision due to the gradual improvements in its individual components.

LiDAR has been used in the past, but only in rare cases. The most notable being in the Apollo 15 mission when NASA used LiDAR to map the surface of the moon.

A single grain of sand on the beach

In 2015, Australian archaeologist Dr. Damien Evans surveyed almost 2000 square kilometres of dense Cambodian jungle using LiDAR technology. Evans’ archaeological team were hoping to find some clues about Cambodia’s history. Instead they uncovered numerous ancient cities and elaborate waterway technology which was believed not to have existed at this time in history. Evans’ discoveries have forced historians to reevaluate Cambodia’s history leading up to the 12th century. But without LiDAR, none of these discoveries could have been made.

LiDAR is the only imaging technique that could see under the jungle canopy from above. If you were to stand in a Cambodian jungle, sun rays can sneak through tiny gaps in the canopy. This means that light from a laser perched on an airplane can reach the ground too. A lot of the light pulses reflect off the canopy, but Evans’ team gathered enough reliable data to take a snapshot of the jungle floor. The LiDAR technicians deleted any sudden jumps in height which were likely trees or other large obstacles. This improved the accuracy of their data.

LiDAR was a blessing for Evans’ archaeological team and his results were incredible. However, Evans was not the first archaeologist to utilize LiDAR to uncover ancient secrets and there is no doubt in my mind he will be the last.

Frengineer 2017
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