# Drone LiDAR Glossary

Absolute Accuracy - A measure that accounts for all systematic and random errors in a dataset. Absolute accuracy is stated with respect to a defined datum or reference system.

Accuracy - The closeness of an estimated value (measured or computed) to a standard or accepted (true) value of a particular quantity. Related to the source data and DEM products quality.

Active sensor - Sensor that generates the energy that it uses to perform the sensing.

Altitude – The AGL is the aircraft altitude above ground level at Nadir position. In this context, the altitude is defined as a height measured with respect to the underlying ground surface, meaning above mean sea level.

Bare earth (bare-earth) - Digital elevation data of the terrain, free from vegetation, buildings, and other man-made structures. Elevations of the ground.

Beam divergence - The beam divergence of an electromagnetic beam (for example, the laser used in LiDAR) is an angular measure of the increase in beam diameter or radius with distance from the optical aperture or antenna aperture from which the electromagnetic beam emerges.

Boresight: Calibration of a LiDAR sensor system equipped with an Inertial Measurement Unit (IMU) and Global Positioning System (GPS) to determine or establish the accurate Position of the instrument (x, y, z) with respect to the GPS antenna and Orientation (roll, pitch, heading) of the LiDAR instrument with respect to straight and level flight[8].

Bias – A systematic error inherent in measurements due to some deficiency in the measurement process or subsequent processing.

Calibration - Process of quantitatively defining a system's responses to known, controlled signal inputs.

Checkpoint - A surveyed point used to estimate the positional accuracy of a geospatial dataset against an independent source of greater accuracy. Checkpoints are independent of, and may never be used as, control points on the same project.

Classification (of LiDAR) - The classification of LiDAR point clouds returns in accordance with a classification scheme to identify the type of target from which each LiDAR return is reflected. The process allows future differentiation between bare-earth terrain points, water, noise, vegetation, buildings, other manmade features and objects of interest.

Confidence level - The percentage of points within a dataset that is estimated to meet the stated accuracy. For example, accuracy reported at the 95% confidence level means that 95% of the positions in the dataset will have an error on true ground position that is equal to or smaller than the reported accuracy value.

Control point (calibration point) - A surveyed point used to adjust a dataset geometrically to establish its positional accuracy relative to the real world. Control points are independent of, and may never be used as, checkpoints on the same project.

Coordinates – A group of 3D numbers that define a point in 3D space. Traditionally, a vertical coordinate would be defined as a 3D coordinate, that is, an x/y coordinate with an associated z-value.

Correction - Compensation for an estimated systematic effect.

Data product specification - Detailed description of a dataset or dataset series together with additional information that will enable it to be created, supplied to and used by another party.

Dataset - Identifiable collection of data.

Datum - A set of reference points on the Earth’s surface from which position measurements are made and (usually) an associated model of the shape of the Earth (reference ellipsoid) to define a geographic coordinate system. Horizontal datums are used for describing a point on the Earth’s surface, in latitude and longitude or another coordinate system. Vertical datums are used to measure elevations or depths.

Digital Elevation Model (DEM) - A 3D computer generated representation of a terrain's surface. DEMs are used often in geographic information systems, and are the most common basis for digitally produced relief maps.

Design Ground Point Density – The planned/required ground point density; calculated on hard surfaces (rooftops, roads, man-made structures of planar character).

Digital Surface Model (DSM) - A 3D computer generated representation of the earth's surface and includes all objects on it.

Digital Terrain Model (DTM) - A 3D computer generated representation of the bare ground surface without any objects like plants and buildings

Easting - Distance in a coordinate system, eastwards (positive) or westwards (negative) from a north-south reference line.

Elevation - The distance measured upward along a plumb line between a point and the geoid. The elevation of a point is normally the same as its orthometric height, defined as H in the equation: H = h – N where h is equal to the ellipsoid height and N is equal to the geoid height

Error - Measured quantity value minus a reference quantity value.

First return - First reflected signal that is detected by a 3D imaging system, for a given sampling position and a given emitted pulse.

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.

Geographic Information System (GIS) - A system of spatially referenced information, including computer programs that acquire, store, manipulate, analyse, and display spatial data.

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.

Georeferencing - Geopositioning an object using a Correspondence Model derived from a set of points for which both ground and image coordinates are known.

Geospatial data - Information that identifies the geographic location and characteristics of natural or constructed features and boundaries of the earth. This information may be derived from remote sensing, mapping, and surveying technologies. Geospatial data generally are considered to be synonymous with spatial data; however, geospatial data always are associated with geographic or Cartesian coordinates linked to a horizontal or vertical datum, whereas spatial data.

Global Positioning System (GPS) - 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 location through trilateration (determining position with respect to two other points by measuring the distance between all three points).

Horizontal accuracy - Positional accuracy of a dataset with respect to a horizontal datum.

Inertial Measurement Unit (IMU) - The combination of a 3-axis accelerometer combined with a 3-axis gyro. An onboard processor, memory, and temperature sensor may be included to provide a digital interface, unit conversion and to apply a sensor calibration model. The IMU by itself does not provide any kind of navigation solution (position, velocity, attitude). It only actuates as a sensor, in opposition to the INS (Inertial Navigation System), which integrate the measurements of its internal IMU to provide a navigation solution. For instance, an Inertial Navigation System (INS) uses an IMU to form a selfcontained navigation system which uses measurements provided by the IMU to track the position, velocity, and orientation of an object relative to a starting point, orientation, and velocity.

Inertial Navigation System (INS) – A self-contained navigation system, comprised of several subsystems: IMU, navigation computer, power supply, interface, etc. Uses measured accelerations and rotations to estimate velocity, position and orientation. An unaided INS loses accuracy over time, due to gyro drift.

Intensity (LiDAR) - For discrete-return LiDAR instruments, the intensity is the recorded amplitude of the reflected LiDAR pulse at the moment the reflection is captured as a return by the LiDAR instrument. LiDAR intensity values can be affected by many factors such as the instantaneous setting of the instrument’s automatic gain control, and angle of incidence and cannot be equated to a true measure of energy. LiDAR intensity data make it possible to map variable textures in the form of a gray-scale image. Intensity return data enable automatic identification and extraction of objects such as buildings and impervious surfaces and can aid in LiDAR point classification. Interpolation – Procedure used to estimate the z-values at a point with x/y coordinates at locations lacking sampled points and is based on the principles of spatial autocorrelation, which assumes that closer points are more similar compared to farther ones.

Last return - Last reflected signal that is detected by a 3D imaging system, for a given sampling position and a given emitted pulse.

Light Detection And Ranging (LiDAR) - An instrument that measures the distance to a reflecting object by emitting timed pulses of light and measuring the time difference between the emission of a laser pulse and the reception of the pulse’s reflection(s). The measured time interval for each reflection is converted to distance. This distance conversion, combined with position and attitude information from GPS, INS and the instrument itself, allows the derivation of the 3D point location of the reflecting target’s location.

Mass points – Irregularly spaced points, each with an x/y location and a zvalue, used to form a DTM. When generated manually, mass points are ideally chosen to depict the most significant variations in the slope or aspect of the terrain. However, when generated by automated methods, for example, by LIDAR or InSAR scanners, mass point spacing and pattern depend on characteristics of the technologies used to acquire the data. Mass points are most often used to make a TIN or derive a gridded DEM by interpolation.

Measurement accuracy - Closeness of agreement between a test result or measurement result and the true value.

Measurement error - Measured quantity value minus a reference quantity value.

Measurement precision - Closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions.

Metadata - Any information that is descriptive or supportive of a geospatial dataset, including formally structured and formatted metadata files. For example, eXtensible Markup Language (XML)-formatted Federal Geographic Data Committee (FGDC) metadata), reports (collection, processing, Quality Assurance/Quality Control (QA/QC)), and other supporting data (e.g., survey points, shapefiles).

Model - Abstraction of some aspects of reality.

Nominal Point Density (NPD) - A common measure of the density of a LiDAR dataset; NPD is the typical or average number of points occurring in a specified areal unit. The NPD is typically expressed as points per square metre (pts/m2). This value can be predicted in mission planning and empirically calculated from the collected data, using only the first (or last) return points as surrogates for pulses. Assuming meters are being used in both expressions, NPD can be calculated from NPS using the formula: 𝑁𝑃𝐷 = 1/𝑁𝑃𝑆2

Nominal Point Spacing (NPS) - As a common measure of the density of a LiDAR dataset, NPS is the typical or average lateral distance between points, typically expressed in meters and most simply calculated as the square root of the average area per first return points. This value can be predicted in mission planning and empirically calculated from the collected data, using only the first (or last) return points as surrogates for pulses. Assuming meters are being used in both expressions, NPS can be calculated from Nominal Point Density (NPD) using the formula: 𝑁𝑃𝑆 = 1/√𝑁𝑃𝐷

Noise - Unwanted signal which can corrupt the measurement or irrelevant or meaningless cells that exist due to poor scanning or imperfections in the original source document.

Northing - Distance in a coordinate system, northwards (positive) or southwards (negative) from an east-west reference line.

Orthometric Height – the height of point above the geoid as measured along the plumbline between the geoid and a point on the Earth’s surface, taken positive upward from the geoid.

Pitch - Vehicles that are free to operate in three dimensions, such as an aircraft, can change their attitude

and rotation about the three orthogonal axes centered on the vehicle’s centre of gravity — the longitudinal, vertical, and horizontal axes. Motion about the lateral axis is called pitch and it is a measure of how far an airplane’s nose is tilted up or down.

Plumbline - A line that corresponds to the direction of gravity at a point on the earth's surface; the line along which an object will fall when dropped.

Positional accuracy - The accuracy of the position of features, including horizontal and vertical positions, with respect to horizontal and vertical datums.

Positioning system - System of instrumental and computational components for determining position.

Platform - Structure which supports a sensor, or sensors.

Precision - Measure of the repeatability of a set of measurements. The closeness with which measurements agree with each other, even though they may all contain a systematic bias. Related to the source data and DEM products quality.

Projected coordinate reference 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).

Quality - Degree to which a set of inherent characteristics fulfils requirements. Accuracy (exactitude) and precision (repeatability) are the means used to evaluate the quality of the source data and DEM products.

Quality Assurance (QA) - Set of activities for ensuring quality in the processes by which products are developed. In particular, the measures taken to ensure the quality of the source data, before and during acquisition of the data.

Quality Control (QC) - Set of activities for ensuring quality in products. The activities focus on identifying defects in the actual products produced. The verification of the quality of the deliverables is part of the QC.

Raster - Set of regularly spaced, continuous cells with, in the case here, bare-earth elevation values attached to the centre of each cell and the value for a cell is assumed to be valid for the whole cell area.

Remote sensing - Collection and interpretation of information about an object without being in physical contact with the object.

Roll - Vehicles that are free to operate in three dimensions, such as an aircraft, can change their attitude and rotation about the three orthogonal axes centred on the vehicle’s centre of gravity — the longitudinal, vertical, and horizontal axes. Motion about the longitudinal axis is called roll and it determines how much the wings of the aircraft are banked.

Root Mean Square Error (RMSE) - The square root of the average of the set of squared differences between dataset coordinate values and coordinate values from an independent source of higher accuracy for identical points. The RMSE is used to estimate the absolute accuracy of both horizontal (RMSEx and RMESy) and vertical (RMSEz) coordinates where standard or accepted values are known, as with GPS-surveyed checkpoints of higher accuracy than the data being tested.

Sensor - Element of a measuring system that is directly affected by a phenomenon, body, or substance carrying a quantity to be measured.

Vertical accuracy - The measure of the positional accuracy of a dataset with respect to a specified vertical datum, at a specified confidence level or percentile. Vertical accuracy is an indicator of quality for geospatial products.

Triangulated Irregular Networks (TINs) – A set of adjacent, nonoverlapping triangles computed from irregularly spaced points with x/y coordinates and zvalues. The TIN model stores the topological relationship between triangles and their adjacent neighbours. The TIN data structure allows for the efficient generation of surface models for the analysis and display of terrain and other types of surfaces. TINs are able to capture critical points that define terrain discontinuities and are topologically encoded so that adjacency and proximity analyses can be performed.

Yaw - Vehicles that are free to operate in three dimensions, such as an aircraft, can change their attitude and rotation about the three orthogonal axes centred on the vehicle’s centre of gravity — the longitudinal, vertical, and horizontal axes. Motion about the perpendicular axis is called yaw, and it determines which way the nose of the aircraft is pointed. (Note: Aircraft do not necessarily fly in the same direction as the nose is pointed if there are significant winds.)

Read more about drone LiDAR performance on our blog or visit our drone LiDAR page.

References:

Specifications for Airborne LiDAR for the Province of British Columbia .

Wikipedia Digital Elevation Model