Skip to Content

LiDAR Glossary

For a more in depth look at LiDAR processes used at AGRG, see the WIKI.

Airborne Laser Scanning (ALS)

Airborne Laser Scanning (ALS)
Airborne Laser Scanning. Also referred to as laser altimetry or LiDAR; an active remote sensing technology that measures topography of the earth's surface, and landscape feature heights and intensity. Comprised of the Position Orientation System (POS) to accurately determine the position and orientation of the airplane (the sensor platform), and the Laser Scanning System (LSS) to emit laser pulses and receive pulse-returns using the laser scanner and LiDAR mechanisms.
Airborne Laser Terrain Mapper (ALTM)
Optech's Airborne Laser Terrain Mapping commercial LiDAR system used to accurately and efficiently acquire terrestrial elevation data.
Attitude (pitch, roll, yaw)
Pitch: vertical rotation of the plane (nose up, nose down); Roll: rotation of the plane around the flight vector (wing up, wing down); Yaw: horizontal rotation of the plane (nose left, nose right).
Avalanche Photodiode (APD)
At the time of pulse transmission and reception, the APD converts the light signal into an electric voltage pulse.
Backscatter
Electromagnetic energy that is reflected back toward its source by terrain or particles in the atmosphere.
Beam Divergence
Divergence of a light beam is the increase in beam diameter with distance from the aperture. Typically measured in mrad (milliradians). Higher frequency beams generally have lower divergence.
Canopy Height Model (CHM)
A Canopy Height Model is the representation of the difference between the top canopy surface and the underlying ground topography. Derived by filtering LiDAR point clouds to separate ground and canopy hits.
Collimated
Light that does not disperse and therefore has low beam divergence is said to be collimated. LiDAR laser beams are highly collimated. It is sometimes said that collimated light rays are parallel and focussed at infinity.
Constant Fraction Discriminator (CFD)
An electronic signal processing device, receiving electric voltage pulse input from the Avalanche Photodiode (APD) and used to determine the precise timing of pulse transmission and reception to determine the range of a feature. Since pulse peak heights and shapes vary depending on temperature at transmission (e.g., sensor temperature) and reception (e.g., ground or feature temperature), the CFD finds the maximum of the pulse curve (where slope=0) to ensure that a consistent point on the curve is chosen among pulses.
Echo
Each of multiple returns from an emitted laser pulse in a multiple-pulse-return Laser Scanning System (LSS)(e.g., first, intermediate…last).
Fuselage
The aircraft's main body section which holds passengers or cargo.
Inertial Measurement Unit (IMU)
Inertial Measurement Unit. Monitors angular accelerations (using 'accelerometers') and rotations (using 'gyroscopes') of the aircraft (sensor) with respect to three primary axes: x=long axis (roll); y=horizontally perpendicular to x (pitch); and z=vertical axis perpendicular to x (yaw or heading). Integrating these measurements with time allow the precise orientation of the sensor platform for each increment of time. Orientation measurements typically recorded at rates of 50Hz-200 Hz.
Laser Scanning System (LSS)
Laser Scanning System. Comprised of the LiDAR (or laser ranging unit) to transmit laser pulses and receive pulse returns; and the laser scanner to distribute these pulses in the "across track" direction (i.e., perpendicular to the flight axis). (The "along track" distribution of pulses is achieved by the forward motion of the airplane).
Lever arm offsets
The lever arm is the perpendicular distance from the axis of rotation to the line of action of the force. Lever arm offsets between system components are measured precisely when setting up the ALS to enable the integration of information from all subcomponents of the system during post-processing.
Light Detection and Ranging (LiDAR)
LIght Detection And Ranging. An active remote sensing system that uses a laser light beam to measure vertical distance from the features of interest. Also see ALS
Near Infra-red (NIR)
Near Infra-red wavelengths fall between 800nm and 2500nm of the electromagnetic spectrum. The majority of commercial ALS sensors use NIR wavelengths of 1047nm, 1064nm, and 1550nm. This is mainly due to the availability of stable and efficient lasing materials at these wavelengths, since natural surfaces are sufficiently reflective at these wavelengths, and since NIR has low signal-to-noise ratio in sunlight and is more eye-safe than other visible wavelengths.
Point cloud
The cluster of points that comprise LiDAR data, achieved from reflection of the laser beam off various landscape features, thereby representing feature height and the 3D spatial relationships between features.
Point dropout
Laser pulses for which no energy was returned to the sensor. These can occur because the aircraft is too high, the surface material is absorbing the radiation or because the ground level energy is reflected away from the sensor.
Point spacing
The average ground distance between successive pulse returns. All things being equal, low point spacing can allow for higher resolution descriptions of the landscape.
Positioning Orientation System (POS)
Position Orientation System. Accurately determines the position and orientation of the laser sensor platform in space at the time of laser pulse transmission and reception. Comprised of a DGPS system to determine the geographic position of the platform to within 5cm of the true trajectory; and the Inertial Measurement Unit (IMU) to measure the orientation of the platform through time based on changes in the plane's attitude.
Pulse footprint
The area of ground intersected by the laser pulse. It is a function of range, angle of incidence and beam divergence.
Pulse footprint smearing
Smearing/Elongation of pulse footprint caused when the laser beam encounters sloped terrain, leading to increased horizontal position uncertainty.
Pulse Repetition Frequency (PRF)
The frequency of transmitted laser pulses. High PRF enables dense point-spacing on the ground providing higher-resolution descriptions of the landscape. However since PRF is inversely related to pulse energy, high PRF might reduce the probability of foliage penetration in densely vegetated areas.
Pulse Return
Laser pulses reflected off surfaces encountered below the sensor and received by the LiDAR sensor. First pulse returns measure the range to the first surface encountered (e.g., vegetation, canopy); last pulse returns measure the range to the last surface encountered (e.g., ground).
Pulse return intensity
The intensity of pulse returns can be measured in some LiDAR systems, which can potentially allow improved discrimination and classification of scanned features. Intensity is a function of pulse range, pulse footprint size, angle of incidence at the point of return and the spectral characteristics of the surface encountered. At NIR wavelengths, metal roofs are highly reflective whereas moist surfaces and wet asphalt generally absorb much of the laser energy. Returns received at nadir angles are generally higher intensity than those received at scan edges because of increased specular reflectance away from the sensor.
Range
The distance between the laser aperture and the detected object or surface.
Scan angle
Half the angle of the full sweep of the laser scanner. Large scan angles are generally not chosen at high altitudes because of high dropout rates, increased error and obstruction shadowing at edges of the scan. Scan angles often do not exceed 30°.
Scan rate
The frequency of cross-track sweeps of the scanner in Hz.
Specular reflection
Perfect reflection of light off a surface, where the angle of incidence = the angle of reflection.
Swath width
The width of the survey area covered by a complete sweep of the scanner. This is related to flying height, and scanner half angle.
Time Interval Meter (TIM)
Used to time-stamp the pulse transmission and reception points, to determine the time between pulse transmission and reception and therefore the range of the reflective feature.

Geomatics

Benchmark
A brass or bronze disk set in a concrete base or other permanent structure, inscribed with a mark showing its elevation above or below an adopted vertical datum.
Datum
In geomatics, a mathematical model used to approximate the size and shape of the earth. Different datums might be more appropriate (better approximations) for different locations on the earth. WGS84 is the datum currently used by GPS satellites to determine their position. NAD83 is the most commonly used datum in North America.
Digital Elevation Model (DEM)
Digital Elevation Model. The representation of continuous elevation values over a topographic surface by a regular array of z-values, referenced to a common datum; typically used to represent terrain relief.
Draping
A 2D image superimposed on a 3D surface. For example, an aerial photograph might be draped over a DEM to create a realistic terrain visualisation.
Geocentric datum
A horizontal geodetic datum based on an ellipsoid that has its origin at the earth's centre of mass (e.g., WGS84, NAD83)
Geodetic datum
A datum that is the basis for calculating positions on the earth's surface or heights above and below the earth's surface.
Geographic Coordinate System (GCS)
A 2D coordinate system defined by latitude and longitude, based on a reference ellipsoid approximation of the earth. Latitude and longitude are based on the angle from the equator and prime meridian respectively.
Geoid
The equipotential surface that coincides with the mean ocean surface of the earth. A smooth but highly irregular surface, known by gravitational measurements, to which the force of gravity is everywhere perpendicular.
Interpolation
The estimation of surface values at un-sampled points base on known surface values of surrounding points.
Inverse Distance Weighted (IDW) interpolation
An interpolation technique that estimates cell values in a raster from a set of sample points that have been weighted so that the farther a sampled point is from the cell being evaluated, the less weight it has in the calculation of the cell's value.
NAD83
North American Datum 1983. A horizontal datum based on different spheroid definitions (mathematical model) than WGS84. Although WGS84 and NAD83 often give similar location estimates, NAD83 is a better approximation of true locations in North America.
Nadir
The point that falls directly below (-90°) an observer; a line drawn directly through an observer to the centre of the earth
Photogrammetry
One of the earliest remote sensing techniques, which derives information about the earth from aerial photographs. 3D coordinates of earth features can be determined via stereoscopy (triangulation from two offset images of a similar location).
Projected Coordinate System
A method used to represent the curved, 3D surface of the earth on a 2D plane. Essentially, the conversion of location data from a sphere approximation to a planar surface (e.g., UTM).
Reference Ellipsoid
Since the earth is not a perfect sphere, this is a 3D ellipse used to approximate the shape of the earth. GPS satellites currently reference their position (height) to the reference ellipsoid WGS84. One result of using a reference ellipsoid is that at sea level elevation might not always be recorded as 0.
Registration
To align two or more maps so that equivalent geographic coordinates coincide. Or, to link map coordinates to ground control points.
Survey monument
An object, such as a metal disk, permanently mounted in the landscape to denote a survey station.
Universal Transverse Mercator (UTM)
Universal Transverse Mercator. A projected coordinate system which defines a location on earth relative to a 60-zone grid system. Locations are defined relative to the WGS84 reference ellipsoid. With a couple of exceptions, zones cover 6° longitude and 8° latitude. Within a given zone in the Northern hemisphere, location is defined in terms of the distance (m) east of the meridian over which the zone is centered (Easting), and distance (m) north of the equator (Northings). Because UTM coordinates are approximated differently for each zone (depending on it's shape), conversion from Lat/Long to UTM requires knowledge of the UTM zone.
WGS84
World Geodetic System 1984. The most commonly used geocentric datum and geographic coordinate system today. GPS measurements are based on the WGS84 reference ellipsoid.
Zenith
The point that falls directly above an observer; opposite the nadir and pointing directly away from the centre of the earth.

Global Positioning System (GPS)

Base station
A GPS receiver at a known location that broadcasts and collects correction information for roving GPS receivers.
Carrier Phase and Code Phase
GPS receivers determine their position by simultaneously calculating their distance from a number of GPS satellites. In Code phase, the receiver determines its distance to a satellite by lining up the C/A code (the pseudo-random number; PRN, transmitted within the L1 signal) sent by the satellite with an internally generated copy (generated within the receiver) of that code. The code sent by the satellite is delayed, relative to the GPS receiver copy, because of the distance required to travel to the receiver. The internally generated code is then time-lagged until the two codes line up and this indicates the time difference between the signals and thereby the distance between satellite and receiver. Since the C/A code has a phase of 1.023 MHz, and receivers are able to line-up the codes to about 1% accuracy, positional accuracy is about 3-6m. In Carrier Phase positioning, the receiver uses the L1 and L2 carrier signals themselves which are transmitted at about 1000 times the frequency (L1-1575.42 MHz; L2-1227.60 MHz), and therefore position accuracy approaches the cm-level.
Coarse/Acquisition Code (C/A code)
The standard positioning signal the GPS satellite transmits to the civilian user. It contains the information the GPS receiver uses to fix its position and time, and is accurate to 100m or better.
Differential GPS (DGPS)
Or differential correction. GPS rover-receiver location estimates are prone to error from a number of sources which lead to horizontal and vertical translations from the true location. Improved accuracy can be achieved by comparing these readings to readings recorded from a fixed GPS receiver at a known location, called a base station. At a base station precise location has been previously determined using GPS surveying methods. Base station data typically have no gaps (e.g., from multipath errors) and are reliable since they are often collected 24/7. If the rover and base station receivers are relatively close (within 50Km), they can be assumed to track the same satellites and experience similar ionospheric interference, and therefore, the difference between readings from the base station and the known location can be used to correct rover-receiver readings. Correction can be performed in real-time or on-the-fly.
GPS
Global Positioning System. A system of radio-emitting and -receiving satellites used to determine positions on the earth. Orbiting satellites transmit signals that allow a GPS receiver to calculate its own location through trilateration (determining position with respect to two other points by measuring the distance between all three points).
Kinematic survey
Time series of coordinates determined as receiver moves along a trajectory.
L1/L2 Frequency
Two carrier radio frequencies are transmitted by GPS satellites. L1 carries the C/A code, P-code (private or precision code) and the navigation message, and is transmitted at a frequency of 1575.42 MHz. L2 carries only the P-code and is transmitted on a frequency of 1227.6 MHz.
Multipath interference
GPS signals reach receivers via multiple pathways because of interference by atmosphere, or reflection of terrestrial features (e.g., mountains, buildings). This can cause phantom targets to appear and confounds the positioning of the location.
Navigation message
Message transmitted by each GPS satellite containing system time, clock correction parameters, ionospheric delay model parameters, and the satellite's ephemeris data and health. This info is used to process GPS signals to give the user time, position and velocity values.
NAVSTAR
NAVigation Satellite Timing And Ranging. The name given to GPS satellites.
Phase centre
The apparent center of signal reception at the GPS antenna. It is not constant but dependent on observation angle and signal frequency.
Post-processed differential GPS
During a survey the base and roving receivers have no active data link between them. Each receiver records satellite observations that will allow differential correction at a later time.
Precise Positioning Service (PPS)
The most accurate positioning possible with GPS based on the dual frequency P-code.
Pseudo-Random Number (PRN)
The signature signal transmitted by each GPS satellite and mirrored by the GPS receiver in order to separate and retrieve the signal from the background noise.
Rapid-Static Survey
Static GPS survey with short observation times. These can be achieved if the GPS receiver has fast ambiguity resolution times.
Real-time differential GPS
A base station transmits corrections usually through some sort of data link (e.g., VHF radio or cellular phone) with each new GPS observation. The roving receiver requires data-link-receiving equipment to receive the transmitted GPS corrections so they can be applied to receiver positions.
Real-time Kinematic (RTK) survey
DGPS procedure whereby carrier phase corrections are transmitted in real-time from a reference station to the kinematic (or moving) rover receiver.
RMS errors
Root Mean Square Error. A measure of the difference between locations that are known and locations that have been interpolated or digitized.
Rover receiver
A portable GPS receiver used to collect data in the field.
Static survey
Determination of location when the receiver's antenna is stationary, allowing the use of averaging techniques to greatly improve position accuracy.
Stop-and-Go Kinematic Survey
Kinematic survey where ambiguities are resolved at the first location (i.e., initialization), then the receiver moves between locations without losing satellite lock. Achieving survey accuracy at this new point is quicker than if lock is lost between sites.

References

Canadian Centre for Remote Sensing
“Glossary of Remote Sensing Terms”
http://ccrs.nrcan.gc.ca/glossary/index_e.php?term=All&choice=B
ESRI
Support Center “GIS Dictionary”
http://support.esri.com/index.cfm?fa=knowledgebase.gisDictionary.gateway
Garmin
“Glossary”
http://www8.garmin.com/aboutGPS/glossary.html
Hopkinson, C. An overview of Airborne Laser Scanning technology.
Trimble
“GPS Tutorial”
http://www.trimble.com/gps/index.shtml
Wikipedia
http://www.wikipedia.org
Tagged in: