LiDAR Surveys

How lidar works. Measuring the world with light.

Lidar means light detection and ranging. It fires a laser pulse and times how long it takes to bounce back.

Distance is the speed of light times that time, halved, because the pulse goes there and back.

It does that hundreds of thousands of times a second.

A distance alone isn't a map point. The system must also know which way the laser was pointing, and where the drone was.

The scanner measures range and angle. G N S S, with R T K or P P K, gives position. The I M U gives attitude.

On site, an R T K base station sends corrections for centimetre-level positioning.

Here's the clever part. The beam spreads as it travels, so one pulse can return from the canopy, a branch, and the ground.

Lidar finds the ground through gaps in the leaves. Photos can't.

Software classifies the points. Top returns make the surface model, the D S M. Ground points make the terrain model, the D T M.

Density is measured in points per square metre. Fly lower, slower, or with more overlap, and it goes up.

An R G B camera then colours each point.

Choose lidar for vegetation, power lines, low light and low texture. Choose photogrammetry for colour detail and lower cost.

Quick check. You need ground levels under thick bush. Which sensor?

Lidar. Its pulses reach the ground through the gaps.

That's why lidar maps mine stockpiles, forests, power line corridors, flood plains and construction sites.

On a corridor job, the route follows the terrain at a constant height above ground, so density stays even.

Before scanning, calibrate the I M U with the manoeuvres your manufacturer specifies.

Check the laser class. Many are Class 1, eye-safe.

Keep the boresight calibration current, and survey ground check points to prove your accuracy.

Recap. Time of flight gives distance. G N S S and I M U place each point. Multiple returns find the ground.

Visual descriptions

The original narration is accompanied by soft instrumental music.

The animation shows laser pulses leaving a drone, striking objects and returning. Distance equals the speed of light times the round-trip time, divided by two. A worked example uses a 667-nanosecond return for approximately 100 metres.

Illustrated DJI Zenmuse L2 examples show up to 240,000 points per second for a single return, up to five returns per pulse, and 5-centimetre horizontal and 4-centimetre vertical system accuracy at 150 metres. These figures are manufacturer examples under specified test conditions, not a promised project result.

A pulse reaches the canopy, a branch and ground through gaps. The point cloud separates surface points for a DSM from ground points for a DTM. Density and accuracy are different measures.

The short check asks which sensor can obtain ground observations under thick bush. The choices are a camera, LiDAR, or either. The animation selects LiDAR because pulses can reach the ground through gaps.

The corridor example follows the terrain. The final cards show time of flight, GNSS and IMU, multiple returns, and planning, calibration and independent ground checks.