Many of the following definitions have been extracted from the ASTM Designation: E2544-10; these have been cited (ASTM) after the definition. It should be noted that ASTM has gathered standard terminology from sources including: ASTM Standard E456, ASMI Standard B89.4.19, ISO Standard 11146-1 and VIM, and NIST/SEMATECH Standard. In addition, ASTM E2544-10 provides greater detail and discussion about the following terminology. The Seach box can be used to find words of interest.
| Term: | Definition: |
|---|---|
| 2D | Two-dimensional. Typically referring to data that has been mapped to a plane such as a map, plan, or profile view. |
| 2.5D | Two-and-a-half dimensional. This typically refers to the situation where a horizontal coordinate system and vertical coordinate system are separated. Generally, in this case, there is one elevation value for a given XY coordinate. Generally most DTMs are 2.5D rather than 3D. |
| 3D | Three-dimensional. In a 3D Cartesian coordinate system (XYZ), there can be multiple Z values at any given XY coordinate. |
| 3D imaging system | A non-contact measurement instrument used to produce a 3D representation (for example, a point cloud) of an object or a site. (ASTM) |
| 3D reconstruction | A process of creating a 3D model from data that is not 3D. For example, a series of 2D photographs of an object can be combined to produce a 3D model. |
| Absolute Accuracy | The level of accuracy that can be obtained in a global coordinate system. |
| Accuracy of measurement | See Figure G-1. Accuracy can be defined as how close and how often a measured value(s) is to the "true" value. Note that the "true" value is never actually known (all measurements have some level of error) and should be determined from a dataset of higher accuracy than the dataset to be verified. An accuracy statement should always include a confidence interval. For example, if one states that a dataset is accurate to ±3 cm, the end user does not know how often that statement is true for the dataset. However, when stated per FGDC 1998 data standards as "Tested 0.03 meters horizontal accuracy at 95% confidence level" then the end user knows that 95% of the values would be expected to be accurate to ±3 cm horizontally and only 5% would exceed that threshold. "Closeness of the agreement between the result of a measurement and a true value of the measurand." (ASTM) |
| Adjustment | A correction applied to the data, typically through a least-squares analysis, to correct for positioning errors. See also geometric correction. |
| Alignment | The process of aligning adjoining scans to each other, see registration. |
| ALS (aerial laser scan) | Laser scans that are captured from and aerial platform such as an airplane or helicopter. |
| Artifacts | Erroneous points in a scan that do not accurately depict the objects intended to be measured. |
| As-built | Refers to a survey to document a project after construction. |
| ASCII | American standard code for information interchange. A code used to store and transfer information between computers consisting of 128 characters. The characters in text files on a computer are ASCII text. |
| ASTM | Formerly, American Society for Testing Materials. Agency which provides a consensus of terminology and/or specifications for testing through international volunteers. |
| Azimuth | An angular measurement of the scanner's facing direction to north. |
| Beam divergence | The increase in beam width as the distance from scan origin increases. |
| Beam width | The extent of the irradiance distribution in a cross section of a laser beam (in a direction orthogonal to its propagation path) at a distance away from the origin. (ASTM) |
| Birds | Refers to actual birds captured in a scan or artificial points in sky. See artifacts. These can sometimes be created by moisture in the air. |
| Boresight | In MLS systems this term refers to the rotation of the laser scanner frame to align with the body frame of the IMU. |
| CAD\CADD | Computer aided design and drafting. The use of computer technology to design, draw, model, and analyze real-world objects. |
| Calibration | "Set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by standards." (ASTM) A system calibration corrects for manufacturing errors (e.g. lever arm offsets, orientations) and produces a set of parameters that remain constant as long as the hardware is not modified or disturbed. (The frequency of re-calibration depends on the desired system accuracy and capabilities and is typically done by the manufacturer). It should not be confused with a geometric correction or adjustment (sometimes called a site calibration or local transformation), which corrects for errors in the GNSS and IMU positioning\trajectory by adjusting the scan data to control or between adjacent passes (e.g., strip adjustment). This correction is applied uniquely to each pass within each project. |
| Change detection | Use of remote sensing data to analyze how the attributes of a region change over a period of time. |
| Checksum | A fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage (from Wikipedia) |
| Classification | The assignment of a point to a single predefined category, such as ground, vegetation, noise, or water. |
| Confusion Matrix | A table to show the effectiveness of an algorithm comparing the algorithms predicted results to actual results. Typically used for a classification accuracy assessment. |
| Consolidated vertical accuracy | A verification of vertical accuracy for several types of ground cover, which consists of bare, open ground, and other types of land cover. |
| Control network | A collection of identifiable points (visible or inferable), with stated coordinate uncertainties, in a single coordinate system. (ASTM) |
| Control point | An identifiable point which is a member of a control network. (ASTM) |
| CORS | Continuously operating reference station. Satellite receivers that are continuously operating in a fixed location to provide highly accurate positional location for use in applications such as RTK GPS. |
| Cross section | A 2D planar slice of the 3D point cloud. |
| Decimation | A method of lowering point density in a point cloud. |
| DEM (Digital Elevation Model) | A DTM focused on elevation values only. |
| Density | The number of points per unit area; can also be expressed as the average distance between points in a point cloud. |
| Detail Scan | A scan, or portion of scan, that is performed at higher resolution. Often a detail scan of targets will be used for better alignment. |
| DGPS (differential GPS/GNSS) | Use of ground-based reference stations to correct for pseudo range ambiguities in GPS/GNSS signals |
| DHM (digital height model) | A DEM that utilizes ground surface as a zero elevation to gain height values above ground level, commonly used for tree heights in forestry applications. |
| Discrete pulse | A method by which a scanner records returning pulses as a series of discrete values. |
| DMI (distance measuring instrument) | A device that physically measures distance traveled along the ground surface. |
| DSM (digital surface model) | A DEM that has not had surface features removed, vegetation and structures are preserved. |
| DTM (Digital Terrain Model) | A digital representation of ground surface topography, usually consisting of a grid and triangulated irregular network (TIN). |
| E57 | A binary file format that has been specifically developed by the ASTM to improve efficiency and compatibility of working with 3D imaging data, including LIDAR data. |
| Echoes | Used to describe all reflected returns to the scanner from an emitted laser pulse, see first return, last return, multiple returns. |
| EDM (electronic distance measurement) | Devices that use infra-red or laser light to accurately measure distance by measuring the time-of-flight of the light. |
| Ephemeris | The flight path that a satellite takes through space. |
| Error (of measurement) | See Figure G-1. Result of a measurement minus a true value of the measurand. (ASTM) |
| Fast static GPS | See Rapid Static GPS. |
| Field of View (FOV) | The angular extent within which objects are measurable by a device such as an optical instrument without user intervention. (ASTM) |
| Filtering | The removal of points from a point cloud, often to reduce the density. Common filters include range, XYZ coordinates, minimum separation, isolated points, and intensity values. |
| First Return | For a given emitted pulse, it is the first reflected signal that is detected by a 3D imaging system, time-of-flight (TOF) type, for a given sampling position, that is, azimuth and elevation angle. (ASTM) |
| Flash LIDAR | A new LIDAR technology that operates by illuminating an entire field of view simultaneously, similar to taking a picture, compared to traditional systems which fire pulses one by one through incrementing angles. |
| Footprint | See beam width. |
| Full waveform | A method of recording the full returning waveform of a laser scan to permit more advanced processing than in a discrete pulse method. |
| FVA (Fundamental vertical accuracy) | A verification of vertical accuracy using only ground control check points in a location on bare, open ground with a high probability of LIDAR sensor detection. |
| Geomatics | The discipline of gathering, storing, processing, delivering, and analyzing geographic or spatially referenced information (modified from Wikipedia). Geomatics is a broad field encompassing surveying, GIS, geographic science, geospatial intelligence, all of which interface with a variety of other disciplines relying on such information. |
| Geometric Correction | A geometric correction or adjustment is done to correct for errors in the GNSS and IMU positioning information by adjusting the scan data to control or between adjacent passes. This correction would be applied uniquely for each project. Not the same as a calibration. |
| GDOP (geometric dilution of precision) | See PDOP (positional dilution of precision). |
| Geo-reference | The process of assigning a coordinate system and location information to a point or points in space. See registration. |
| GIS (geographic information system) | A computing program designed to analyze spatial data. |
| GNSS (global navigation satellite system) | A satellite system with global coverage that provides autonomous geo-spatial positioning. Includes the United States' GPS system, Russia's GLONASS, and will include China's COMPASS and Europe's Galileo. |
| GPS (global positioning system) | A GNSS system put into use by the United States. |
| Grazing Angle | The angle between the laser beam and the surface (90 Degree incidence angle). Low grazing angles mean the laser beam is nearly parallel to the surface (oblique, poor data quality) while high grazing angles mean that the laser beam is perpendicular to the surface (orthogonal, good data quality). See Figure G-2. Also, see http://en.wikipedia.org/wiki/Angle_of_incidence |
| Grid | A point cloud that has been reduced by assigning points into equally distributed cells, typically used as a form of DEM generation. |
| Ground | Used to describe the physical ground surface with any occluding material removed such as vegetation and structures |
| HDOP (horizontal dilution of precision) | An indicator of how well a satellite receiver can be horizontally located in 3D space based on the geometry of over-head satellites. |
| HTDP (Horizontal Time-Dependent Positioning) | A utility provided by the NGS which enables one to transform coordinates across time and between spatial reference frames (https://www.ngs.noaa.gov/TOOLS/Htdp/Htdp.shtml) |
| ICP (iterative closest point) algorithm | A software algorithm commonly used to register adjoining point clouds by iteratively minimizing the distance between paired or corresponding points in the cloud. |
| Incidence Angle | See Figure G-2. The angle between the incoming laser pulse and the surface normal. Low incidence angles, meaning orthogonal, direct data acquisition, are preferred. Antonym of grazing angle. Also, see http://en.wikipedia.org/wiki/Angle_of_incidence |
| IMU (Inertial Measurement Unit) | A device which utilizes a combination of gyroscopes and accelerometers to provide velocity and orientation information. |
| INS (Inertial Navigation System) | Not applicable to mobile mapping, see IMU. |
| Instrument origin | Point from which all instrument measurements are referenced, that is, origin of the instrument coordinate reference frame (0, 0, 0). (ASTM) |
| Intensity | The quantity of laser energy measured at the scanner after light is reflected and returned from a surface. Typically scaled as a floating point from 0 to 1.0 or an integer from 0 to 255 or 0 to 65535. |
| LAS | A binary file format that has been specifically developed by the American Society for Photogrammetry and Remote Sensing (ASPRS) to improve efficiency and compatibility of working with LIDAR data between software packages. Current version: 1.4. This is the most common format used for mobile LIDAR data. |
| Last return | For a given emitted pulse, it is the last reflected signal that is detected by a 3D imaging system, time-of-flight (TOF) type, for a given sampling position, that is, azimuth and elevation angle. (ASTM) |
| Lever Arm | In MLS systems this term refers to the difference in origin of the laser scanner frame and the body frame of the IMU. See Figure G-3. |
| LIDAR | LIght Detection And Ranging, a method of measuring the flight time of a beam of light to calculate range to objects at predetermined angular increments, resulting in a point cloud |
| Line scan | Constraining the Z-rotation of a laser scanner so that vehicular or platform motion results in linear scan swaths through a corridor. |
| Local | A coordinate system that is referenced using the laser scanner location as the origin of the point cloud. |
| Local Accuracy | The local accuracy of a control point is a value that represents the uncertainty in the coordinates of the control point relative to the coordinates of other directly connected, adjacent control points at the 95-percent confidence level. The reported local accuracy is an approximate average of the individual local accuracy values between this control point and other observed control points used to establish the coordinates of the control point. (FGDC-STD-007). |
| MLS (mobile laser scanner) | A mobile system capable of collection of geo-referenced remotely sensed data, utilizing the use of at least one LIDAR scanner. |
| MMS (mobile mapping system) | A mobile system capable of collection of geo-referenced remotely sensed data, typically using imagery based sensors. A MMS may or may not include a MLS. |
| Multiple returns | The signals returned to a single detector element from simultaneously-illuminated multiple surfaces resulting from a single laser pulse. (ASTM) |
| Network Accuracy | The network accuracy of a control point is a value that represents the uncertainty in the coordinates of the control point with respect to the geodetic datum at the 95-percent confidence level. For NSRS network accuracy classification, the datum is considered to be best expressed by the geodetic values at the Continuously Operating Reference Stations (CORS) supported by NGS. By this definition, the local and network accuracy values at CORS sites are considered to be infinitesimal, i.e., to approach zero. (FGDC-STD-007). |
| Noise | See artifacts. |
| Oblique | When the view of the laser to the object is positioned such that the laser strikes the surface at an angle. This can degrade the data quality. See Figure G-2. |
| Occlusions | Areas within a point cloud that are void of measurements due to objects blocking the scanners line of sight. |
| On-the-fly | 1) A mode of mobile mapping that utilizes continuous movement of the mapping platform while collecting data. 2) Processing of scan data in real time. |
| OPUS (online positioning user service) | A service provided by the National Oceanic and Atmospheric Administration (NOAA), which allows GPS users to increase the accuracy of collected GPS point locations using through post processing. |
| Orthogonal | See Figure G-2. When the view of the laser to the object is positioned such that the laser strikes direct or perpendicular to the surface. |
| Overview Scan | A low resolution scan, may be used to select areas specific areas within a scan which need to be scanned at higher resolution. |
| Panoramic scan | Allowing the scanner head to rotate in the Z-axis up to 360¡. |
| Parallax | The apparent displacement of a distant object in relation to a nearer as viewed from different locations. |
| PDOP (positional dilution of precision) | An indicator of how well a GPS receiver can be located in 3D space based on the geometry of over-head satellites relative to the GPS receiver. |
| Phantom points | See artifacts |
| Phase-based | A method of measuring distance by observing the phase shift of a laser's sinusoidally modulated waveform and the reflected return from a surface. Used over smaller ranges with a higher data collection rate. |
| Photon LIDAR | A LIDAR technology under development that splits the laser pulse into individual photons to improve spatial resolution. It also reduces the spot size, improving accuracy. |
| Pitch | Rotation about the Y axis in a Cartesian coordinate system. |
| Pits | Refers to artificial points captured below the ground surface. See artifacts. |
| Point Cloud | A collection of data points in 3D space (frequently in the hundreds of thousands), for example as obtained using a 3D imaging system. (ASTM) |
| Polar coordinates | A coordinate system that locates points in space by defining an angle and a distance from a fixed reference pole. |
| Post spacing | Elevation or z-values at evenly spaced grid intervals in the horizontal or 'x' and 'y' direction. In airborne LIDAR, the nominal post spacing is defined as the typical separation distance between points. |
| PPK (post-processed kinematic) | A method of improving GPS receiver positioning by using a precisely calculated post-flight ephemeris instead of the pre-flight predicted ephemeris. |
| PPM | Parts per million. Often used to discuss the degradation with distance. For example, 1 ppm would mean a loss of accuracy of 1 ft per 1 million feet of distance. |
| Precision | See Figure G-1. Closeness of agreement between independent test results obtained under stipulated conditions. (ASTM). |
| RAID | RAID (redundant array of independent disks, originally redundant array of inexpensive disks) is a storage technology that combines multiple disk drive components into a logical unit (from Wikipedia). |
| Random error | Result of a measurement minus the mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions. (ASTM) |
| Range resolution | The distance, in units of length, between a point in space and an origin fixed to the 3D imaging system that is measuring that point. (ASTM) |
| Range | The distance, in units of length, between a point in space and an origin fixed to the 3D imaging system that is measuring that point. (ASTM) |
| Rapid static GPS | Collection of 15 minutes to 2 hours of GPS data over a point location which is then submitted to OPUS for accuracy enhancement via post processing. |
| Raw scan data | Unprocessed data as it is initially captured from the scanner.. |
| Reference frame | The coordinate system or location that is used to refer to an object or point location. |
| Reflectance | A measure of how much light is reflected off a surface compared to how much initially hit the surface. |
| Registration | The process of determining and applying to two or more datasets the transformations that locate each dataset in a common coordinate system so that the datasets are aligned relative to each other. (ASTM) |
| Relative Accuracy | The level of accuracy than can be obtained within a local coordinate system. |
| Repeatability (of results or measurements) | Closeness of the agreement between the results of successive measurements of the same measurand carried out under the same conditions of measurement. (ASTM) |
| Reproducibility | See Repeatability. However, repeatability typically refers to a single system\person's ability to produce a result, whereas reproducibility can refer to someone's ability to follow someone else's work. |
| Resolution | The degree of detail which can be seen. See density. |
| Rigid body transformation | Refers to the translation and rotation of a point cloud in which the point cloud is treated as a rigid body that has no deformation of the points with relation to other points in the cloud. |
| RINEX | Receiver INdependent EXchange Format. A common data interchange format for raw GPS data. |
| RMS(E) (root mean square (error)) | An indicator of precision by measuring the differences of an estimated or modeled object to the values of the physically observed object. |
| Roll | Refers to the rotation about the X axis in a Cartesian coordinate system. |
| Rotation matrix | A matrix that is used in linear algebra to rotate a point in 3D space. |
| RTK (real time kinematic) GPS | An enhancement to satellite navigation that utilized carrier phase measurements for better positioning; allows for GPS corrections in real time. |
| Scan | The result of a LIDAR scanner, often interchangeable with point cloud. |
| Spot size | Beam width on the target. |
| State Plane Coordinate System (SPS, SPCS) | A set of 124 geographic zones developed in the 1930's to minimize topographic distortion from map projections. Each state has one or more zone. A variety of map projections are used for each zone. |
| Static GPS | Collection of 2 hours to 48 hours of GPS observations over a point location which is then submitted to OPUS for accuracy enhancement via post processing. |
| Stop-and-go | A simplified mode of mobile mapping that utilizes non-continuous movement of the mobile mapping platform, data points are only collected while the platform is stationary. |
| Strip Adjustment | A process of registering two or more adjacent scan passes together to correct for errors in the trajectory. |
| Subsample | A lower density of points, or a small collection of points taken from a larger sample. |
| Supplemental vertical accuracy | A verification of vertical accuracy over ground cover that does not consist of bare, open ground. |
| Systematic error | Mean that would result from an infinite number of measurements of the same measurand carried out under repeatability conditions minus a true value of the measurand. (ASTM) |
| TIN (Triangulated Irregular Network) | A type of DTM created by generating triangles to connect points that are irregularly spaced. The three points that form each triangle are used to create a plane that is used for interpolation (typically for elevation) between the points. |
| Time-of-flight | A method of measuring distance by observing the time it takes for a laser beam to travel from the scanner, reflect off a surface, and return to the scanner. |
| TLS (terrestrial laser scan) | Laser scans that take place from a momentarily fixed platform, typically tripod based. |
| Transformation matrix | A matrix that is used in linear algebra to translate and rotate point(s) in 3D space. Depending on the type of transformation, the matrix can also include scaling and skewing parameters. |
| Uncertainty of measurement | Parameter, associated with the result of a measurement, which characterizes the dispersion of the values that could reasonably be attributed to the measurand. (ASTM) In other words, how well you can trust the measurement. |
| UTM | Universal Transverse Mercator. A coordinate system developed by the US Army Corps of Engineers for horizontal positioning that divides the Earth into sixty zones, representing six degrees of longitude and uses a secant transverse Mercator projection for each zone. . |
| Validation | Verification that data meets certain criteria. |
| VDOP (vertical dilution of precision) | An indicator of how well a satellite receiver can be vertically located in 3D space based on the geometry of over-head satellites. |
| Voids | Areas within a point cloud which were not well detailed, typically due to blocking of the scanner line of sight. |
| VRS (virtual reference station) | A method of assigning a virtual base station near the survey location to permit RTK corrections along short baselines. |
| Waveform | See Full Waveform. |
| XYZRGBI | Any combination of these letters may be used to define a scanner file format, represented by X, Y, and Z point coordinates, (R)ed, (G)reen, (B)lue color values assigned to the point, and (I)ntensity value assigned to the point. |
| Yaw | Rotation about the Z axis in a Cartesian coordinate system. |