LiDAR Sensors Information

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LiDAR LiDAR is a technology that uses non-visible electromagnetic radiation (infrared or ultraviolet are examples) to locate objects and measure distance to them. LiDAR, which stands for light detection and ranging, is similar to Sonar (sound navigation and ranging) and Radar(radio detection and ranging). Sonar measures distance by emitting a powerful sound pulse and measuring the time it takes for the pulse to return after impinging the object. By frequently doing this and moving the Sonar to point to different locations, we can “map” an entire environment. Radar uses the same principle, but instead of sound, it emits a radio (electromagnetic) wave with a much higher frequency than the frequency used in Sonar. Both systems, however, give poor image resolution.

LiDAR produces high-resolution images by using lasers to measure distance in a similar way. The system is designed to represent every distance measurement by one pixel. By taking millions of measurements, millions of pixels can be rendered on a screen resulting in an image rich in details, similar to black-and-white photographs. Each pixel carries certain light intensity, and all the pixels together form the image.

LIDAR sensors emit focused beams of light pointed at an object and measures the time it takes for the reflections to be detected by the sensor. The time of the reflected ray is used by a sensor to compute ranges, or distances, to objects. When laser ranges are combined with position and orientation data generated from integrated GPS and inertial measurement unit systems, scan angles, and calibration data, the result is a dense, detail-rich group of elevation points, called a "point cloud."

Point clouds are specific points on the Earth's surface from where the beam has been reflected because these points collect the values for the three dimensions — latitude, longitude, and height — that specify the point. Mapping all the points on an object allows the LiDAR sensor to generated accurate 3D images of geo-spatial objects such as building models, contours, digital elevation models, and others.

LiDARS are remote sensing devices. There are two classes of remote sensing technologies that are differentiated by the source of energy used to detect a target: passive systems and active systems. Passive systems use external sources — like the sun — to generate and collect data using the source radiation. Active systems generate and direct energy toward a target and subsequently detect the radiation. LiDAR systems are active systems because they emit pulses of laser beams and detect the reflected light. This characteristic allows LiDAR data to be collected at night when the air is usually clearer and the sky contains less air traffic than in the daytime. In fact, most LiDAR data are collected at night. Unlike radar, LiDAR cannot penetrate clouds, rain, or dense haze and must be flown during fair weather.

LiDAR instruments can rapidly map objects by sending pulses at sampling rates greater than 150 kilohertz. The resulting product is a densely spaced network of highly accurate point clouds. Many LiDAR systems operate in the near-infrared region of the electromagnetic spectrum, others operate in the green band to penetrate water and detect bottom features.

Applications

Key applications for LiDAR sensors include remote sensing, agriculture, military, security, forestry, automotive, robotics, industrial, and unmanned aerial vehicle (UAV) applications. They can be used for speed, 3D imaging, speed detection, and collision-avoidance. Some specific applications where LiDAR has played a basic role include:

Forest planning and management
Forest fire management
Flooded area maps
Coastal mapping
Agriculture
Oil and gas exploration
Mining
Archeology
Architecture
Accident scene

Types

There are several types of LiDAR, most of which are based on application.

Profiling LiDAR is the first type created in the 1980s.
Small footprint LiDAR are the typical systems used nowadays. The scan angle of this type is about 20 degrees.
Large footprint LiDAR uses full waveforms and returns are 20 m footprints.
Ground-based LiDAR is used to scan objects such as buildings and sits on a tripod.

Specifications

Being a data collection device, many of the specifications have a common base with these types of general data collection and storage tools. Following recommendations by NOAA and other federal agencies, the basic parameters to specify a LiDAR system are as follows:

LAS, or laser file format — The LAS file format is a public file format for the interchange of a three-dimensional point cloud data between data users. Although developed primarily for the exchange of LiDAR point cloud data, this format supports the exchange of any three-dimensional x, y, z tuple. LAS is a binary file format that maintains information specific to the LiDAR nature of the data, while not being overly complex.
RMSE, or root mean square error — A measure of the accuracy of the data similar to the measure of standard deviation if there is no bias in the data.
Fundamental vertical accuracy (FVA) — A measure of the accuracy of the data in open areas at a high level of confidence (95%); calculated from the RMSE using the formula RMSE x 1.96 = FVA.
Classification — Data that have been processed to define the type of object that the pulses have reflected off; can be as simple as unclassified (i.e., object not defined) to buildings and high vegetation. The most common method is to classify the data set for points that are considered “bare earth” and those that are not (unclassified).
Return number (first/last returns) — Many LiDAR systems are capable of capturing the first, second, third, and ultimately the “last” return from a single laser pulse. The return number can be used to help determine what the reflected pulse is from (e.g., ground, tree, understory).
Point spacing — How close the laser points are to each other, analogous to the pixel size of an aerial image; also called “posting density” or “nominal point spacing.” The point spacing determines the resolution of derived gridded products.
Pulse rate — the number of discrete laser “shots” per second that the LiDAR instrument is firing. Systems used in 2012 were capable of up to 300,000 pulses per second. More commonly, the data are captured at approximately 50,000 to 150,000 pulses per second.
Intensity data — When the laser return is recorded, the strength of the return is also recorded. The values represent how well the object reflected the wavelength of light used by the laser system (e.g., 1,064 nanometers for most commercial topography sensors in the U.S.). These data resemble a black and white photo, but cannot be interpreted in exactly the same manner.
RTK GPS (real time kinematic GPS) — Satellite navigation that uses the carrier phase (a waveform) that transmits (carries) the Global Positioning System (GPS) signal instead of the GPS signal itself. The actual GPS signal has a frequency of about 1 megahertz, whereas the carrier wave has a frequency of 1,500 megahertz, so a difference in signal arrival time is more precise. The carrier phase is more difficult to use (i.e., the equipment is more costly); however, once it has been resolved, it produces a more accurate position in relation to the higher frequency.
DEM, or digital elevation model — A surface created from elevation point data to represent the topography. Often, a DEM is more easily used in a geographic information system (GIS) or computer-aided design (CAD) applications than the raw point data it is constructed from.

Components

The general structure of airborne LiDAR systems consist of four main parts, totally integrated to produce high-resolution images.

Sensors are used to scan objects and they operate normally in the green or near-infrared bands of the spectrum.
GPS receivers track altitude and location of the LiDAR unit, normally in an airplane, drone, or other airborne vehicles. The collected values are very important to achieve accurate results.
Inertial measurement units (IMU) track the tilt of the airborne vehicle.
Computers or data recorders record all data collected as the LiDAR scans objects.