LiDAR is an acronym for Light Detection and Ranging.
Sonar uses sound waves to work out the distance between things.
Radar uses radio waves in the same way.
LiDAR uses light from a laser to measure the distance, elevation and contours of an area.
How is the LiDAR data recorded?
LiDAR can be recorded from the ground, from an airplane or from a satellite in space. OSi gathers its data from regular airplane flights.
There are four components to a LiDAR system
- A LiDAR unit that contains the laser onboard the plane. This is used to scan the ground from above.
- A GPS navigation system is used to marry coordinates to the area scanned by the laser. It also records the altitude of the plane.
- An Inertial Measurement Unit is used to adjust the readings for the yaw, pitch and roll of the plane they may occur during turbulence.
- A computer application to record the data
How does it work?
Pulse and Return
As the plane flies over an area a laser pulse is emitted from the LiDAR unit. The system uses speed of light calculations to measure the time the pulse took to bounce back up to the plane from the ground. This gives us the distance between the plane and the ground.
Calculating Ground Elevation
The GPS will record the altitude of the plane in the air over the area being scanned. By subtracting the distance measured by the Pulse and Return from the altitude of the plane, this gives the ground elevation of the area scanned.
Correcting for turbulence
As the plane moves through the air, the flight path will be affected by:
Yaw – the vertical axis; a movement of the plane that changes the direction to the left or right of its motion.
Pitch – the lateral axis; a movement that affects the elevation of the plane
Roll – longitudinal axis; this type of movement causes the plane to bank
This where the Inertial Measurement Unit (IMU) comes into play.
The IMU will adjust for any distortion caused by yaw, pitch and roll but also the angle at which the laser pulse is emitted and returned to the plane.
Adjusting for tree cover
Once a pulse is emitted it may yield more than one return.
The type of light emitted from a laser a is either green light or near infra-red light because these can detect and make a return from vegetation.
One pulse can make multiple returns as it travels through the foliage. This allows us to record tree height and the density of the vegetation.
What types of application does LiDAR have?
LiDAR can be used to create elevation maps to show the sunlight exposure and water drainage for an area. This helps identify the areas which require more water or fertilizer and will help farmers to save time and money.
Some green light LiDAR pulses can penetrate water and therefore help understand what is happening at the bottom of a lake or the movement of a riverbed. This underwater information can help in monitoring floodplains and help model for future erosion of an area.
Monitoring Air Quality
Some near infrared LiDAR pulses can be used to create images of the particles in the air above us. LiDAR can detect the particles of pollutants like carbon dioxide, sulphur dioxide and methane. City planners can use this information to monitor emissions and act to reduce pollution in affected urban areas.
As we’ve heard above, LiDAR can penetrate vegetation and therefore detect micro topography that is hidden from view by bushes and trees. This digital information can be used to create a 3-D map of a surface and allow better planning and smarter allocation of resources.
This is one for the future. Tesla, Google and Uber have been testing self-driving cars powered by LiDAR scanners. There is still a lot of work to perfect the vehicle, but LiDAR will eventually enable self-driving cars to view its surroundings and travel with incredible accuracy.
How can OSi help you?
If you would like to hear more about how we use LiDAR in our aerial mapping process, follow this link Ordnance Survey Ireland mapping services.
If you would like to talk to one of our team about how mapping services can help you make better decisions for your business, please contact us directly via the form below or email Kevin Brady or Stuart Doherty directly.