Lidar Explained

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Lidar (also spelled LiDAR and LIDAR) is an acronym for Light Detection And Ranging. Lidar is a technology developed to detect features and to use reflected light off of objects as a kind of visual sonar in order to measure the depth and height of those features.

What is Lidar?

Lidar systems are a way to remotely sense variation in and on the Earth’s surface. Light beams are pulsed from a laser that reflect off of features in a geographic area. The speed of the return of the light beam is used to calculate the distance between the laser scanner and the ground.

The laser ranges are combined with location and orientation data provided by integrated GPS and Inertial Measurement Unit systems, scan angles, and calibration data. The result is three-dimensional collection of geographic coordinates (latitude, longitude, and height) known as a point cloud.

A laser, a specific GPS receiver, and a scanner are the basic components of LiDAR devices, which are often mounted on an airplane or helicopter for use over a large region. Lidar can also be collected using ground-based lidar systems.

Lidar Returns

A graphic showing a plane emitting lidar laser pulses over a forested area.
In order to acquire information on the height of features, pulses emitted by a LiDAR system are reflected back to the system. Image: Jason Stoker, USGS, public domain.

The LiDAR system sends a pulse of light down to the Earth’s surface, and the return pulses convey information about the feature’s location and distance from the LiDAR system. Depending on the order in which those pulses are returned, different return pulses can convey information about the geographical terrain.

Lidar data is categorized into the order in which the light is reflected back to the system. The first return is measured as the highest point in the landscape. This can be the top of a hill, mountain, building, or the canopy of a forest. The last return is recorded as the lowest point in a landscape and is known as the “bare earth”.

Point Clouds Versus Bare Earth

The group of individual lidar points reflected off everything on the surface, including structures and vegetation, is known as a point cloud.

This oblique view of Washington D.C. is point cloud lidar data. The street layout and buildings are easily identified. The heights of the buildings are shown with a color ramp. The highest buildings are red and the lowest elevations, such as the level of the streets are blue.

A lidar image of Washington DC show elevations of the buildings and streets.
An oblique view of a lidar point cloud of Washingon, D.C. Image: Sources/Usage Public Domain. Photographer Jason Stoker, USGS.

“Bare Earth” lidar data is data that has vegetation and man-made structures stripped away. Only the ground topography is shown.

In this side by side comparison, the left image shows a lidar point cloud dataset of a hill. Dense forest covers the hill, make it hard to see the underlying ground. The bare earth digital elevation model shows the same hill with the vegetation stripped away, revealing areas where landslides have occurred in the past.

Landslides, particularly historic landslides, may be invisible on aerial photos and difficult to spot from the ground amid dense forest canopies. 

The left image is a lidar point cloud of a forested hill.  The right is a bare earth digital elevation model of the same hill.
A lidar point cloud vs. bare earth DEM. Image: USGS, public domain.

Types of Lidar Data

Topographic and bathymetric data are two types of LiDAR data.

For collecting 3-dimensional data of land surfaces, near-infrared laser is used. After collecting topography data with topographic LiDAR, digital elevation models can be created by interpolating the data (DEM).

Using water-penetrating green light, bathymetric lidar measures seafloor and riverbed heights. Lidar bathymetry can be used to map out detailed submerged features, including coral reefs, trenches, and sea mounts.

A color ramped lidar bathymetry of an island.  Lowest elevations are pink, sea level is blue, and above sea level is green to red.
Perspective view of coastal bathymetry looking onshore of St. Thomas, US Virgin Islands. Image: Xan Fredericks, USGS, public domain.

Researchers in the domains of geography, archeology, geology, seismology, atmospheric sciences, laser studies, and many others use LiDAR systems to create accurate and high resolution maps and data.

When was Lidar First Developed?

The first lidar-like system was first conceived in December of 1958 and introduced in 1961 by the Hughes Aircraft Company. This early system was called “Colidar” an acronym for “coherent light detecting and ranging”.

The earliest applications of lidar was in meteorology where it was used to measure clouds and pollution.

In 1971, astronauts used a laser altimeter to scan the moon’s surface with Lidar during the Apollo 15 mission.

Uses of Lidar

Since then, lidar’s applications have expanded to multiple industries and uses. Lidar is use to study landslides and ground movement.

Lidar in Archaeology

Lidar has been used to detect areas of archaeological significance. Lidar has been used to uncover the underlying history of a landscape.

Researchers have utilized lidar to reveal ancient Maya structures, roadways, and other features, as well as generate a three-dimensional picture of a Maya settlement in Belize. Lidarhas also been used to create high-resolution models of Renaissance buildings, such as Florence’s Salone dei Cinquecento. Lidar is being utilized in England to find additional sites in the Stonehenge plains.

Lidar in Wildfire Management

Lidar has been using in rapidly collecting before, during, and after three-dimensional data about wildfires.

These before and after lidar images of the 2014 King Fire in El Dorado National Forest show researchers the devastation of the fire. Ground level is blue and with higher measurements ranging from green to red.

Before-and-After LIDAR Images from 2014 King Fire in El Dorado National Forest.
Before (left) and after (right) lidar images from 2014 King Fire in El Dorado National Forest. Images: USFS.

Lidar in Infrastructure

Lidar data is also used for the planning, permitting, and construction of infrastructure. For example, landslides and fault lines can be identified and evaluated using lidar data to determine the safest locations for energy infrastructure by analyzing terrain features and evaluating geologic risks (for example, landslides and fault lines).

Use of Lidar in Agriculture

By using lidar to collect high resolution terrain information, precision agriculture can adjust the application of seed, fertilizer, lime, pesticides, and water based on the landscape. Crop yields can be improved by understanding key topographical characteristics such as soil type, soil wetness, drainage, and topographic changes within farm fields (slope, aspect, and curvature).

Read next: Mapping the Entire Surface of the Earth with Lidar

References

Abshire, J. B. (2010, October). NASA’s space lidar measurements of earth and planetary surfaces. In Frontiers in Optics (p. SMB1). Optical Society of America. https://opg.optica.org/abstract.cfm?URI=FiO-2010-SMB1

Goyer, G. G., & Watson, R. (1963). The laser and its application to meteorology. Bulletin of the American Meteorological Society44(9), 564-570. https://doi.org/10.1175/1520-0477-44.9.564

Johnson, K. M., & Ouimet, W. B. (2014). Rediscovering the lost archaeological landscape of southern New England using airborne light detection and ranging (LiDAR). Journal of Archaeological Science43, 9-20. https://doi.org/10.1016/j.jas.2013.12.004

Laser imaging helps archaeologists dig up history. (2017). NASA Spinoff. https://spinoff.nasa.gov/Spinoff2017/it_1.html

Sugarbaker, L. J., & Carswell, Jr., W. J. (2016, December). The 3D Elevation Program—Precision Agriculture and Other Farm Practices. U.S. Geological Survey. https://pubs.usgs.gov/fs/2016/3088/fs20163088.pdf

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