Glossary

This page contains a list of common terms used in the field of airborne laser scanning (ALS). The terminology was defined by the Institute of Photogrammmetry and Remote Sensing of the Vienna University of Technology (I.P.F.) and was developed with the help of the former online-dictionary of the German Society for Photogrammetry, Remote Sensing and Geoinformation (DGPF), the Technical Note 1297 (Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results) of the National Institute of Standards and Technology (NIST, United States Department of Commerce Technology Administration) and the online-encyclopedia Wikipedia. Some of the definitions include some emphasised specific statements that are regarded to be useful for current ALS flight missions, but may be adapted to individual applications. Some definitions of terms which are here preconditioned (e.g. laser, coordinate system, etc.) can be found in the previously mentioned resources.


Accuracy/Precision

General term for the assessment of the convergence of a value to a reference value. In practice the quality of this convergence is described by the mean quadratic difference. Within the description of the convergence quality one has to distinguish between absolute and relative accuracy. If not specified explicitly, the term accuracy is equalised with the absolute accuracy, while for the relative accuracy the term precision is used.
Relative/inner accuracy - precision The term precision describes the variation of independent acquired values that can be achieved by multiple realisations of the process under given conditions. Therefore the reference value for the assessment is the mean value.
Absolute/outer accuracy - accuracy Describes the convergence in respect to an externally estimated reference value. This accuracy value is the result of varying the external conditions or parameters that are present during the measurement process. Next to the precision a systematic error portion is included in this accuracy measure.


Airborne laser scanning (ALS)

Dynamic measurement method that allows to acquire data with a laser scanner from an airborne platform.


Analysis unit (AU)

Planar area that is used for the analysis of the acquired laser scanner data within the analysis unit. Typically a planar quadratic area that nominally contains 1000 points. In the case of a demanded point density of 4 points/m^2 this quadratic area has a size of 250m^2.


Aperture angle

Angle across the flight direction within the laser scanner allows to record echoes.


Beam divergence

(Whole) Angle of the widening of the laser beam. Due to the typically varying energy distribution within the laser beam the angle is limited to the region where the energy decrease from the maximum follows the ratio of 1/e^2.


Crosswise (cross) harp

Harp where the main direction does significantly differ from (is tilted in respect to) the general flight direction (has to be defined) within one project area.


Crosswise (cross) strip

Flight strip where the main direction does significantly differ from (is tilted in respect to) the general flight direction (has to be defined) within one project area (may be part of a crosswise harp).


DSM- resp. DTM-Accuracy

Describes the accuracy of the representation of the earth surface. One differentiates planar and height accuracy, but the latter is typically of higher interest. The accuracy depends, next to the single point accuracy and the local support point distance, on the model discretisation of the surface.


Data hole

Area within the project that was not covered by data (e.g. in the case of ALS: too low backscatter signal within the footprint). Extensive data holes (bigger than the analysis unit) that result from an imperfect strip overlap can be seen as an imperfect ALS data acquisition.


Digital model (geometrical)

Organised geometry objects (points, raster, grid, lines, etc.) and their geometric relations that allow a representation of an object (with a certain accuracy) stored in a file or database.


Digital surface model (DSM)

Digital model for the description of the earth surface similar to the DTM, but including the description of artificial and natural objects (buildings, trees, etc.).


Digital terrain model (DTM)

Digital model for the description of the earth surface neglecting all artificial objects (e.g. buildings, bridges) and the natural vegetation cover (e.g. trees, bushes). The DTM may consist of regular grid points and important feature lines (e.g. break lines). Water surfaces are - if not stated explicitly - part of the DTM. A DTM that includes the water bed surface (DTM of a watercourse (DTM-W)) instead of the water surface has to be specified explicitly.


Echo

Temporally connected part of the whole backscattered laser power that was detected by the receiver and that can be allocated to a certain reflecting surface element.


Echo duration/echo width

Temporal length of a detected echo. Due to the fact that the echo will not have an exact rectangular form, the duration is - if not stated differently - the temporal length where the power of the echo is above the half maximum (full width at half maximum (FWHM) or the full duration at half maximum (FDHM)).


Flight path/trajectory

Recording of the attitude (position, orientation and timestamp (at least 7 parameters)) of the sensor coordinate system (SCKS). The flight path is the basis for the georeferencing of all synchronised observations of sensors on-board the aircraft in respect to a common coordinate system.


Flight strip

Recoded sequence of echo information (coordinates, echo amplitudes, etc.) without stopping the acquisition process of the laser scanner.


Footprint

Illuminated area(s) within the divergence of the laser beam.


Full-waveform

Detected and digitised backscattered laser energy of the laser scanner receiving unit that can be allocated to one observation direction. In the case of multiple reflecting surface elements within the footprint the recorded waveform is composed of multiple echoes.


Georeferencing

This term describes the transformation of different local measurements or coordinates into one common geodetic reference frame.


Global coordinate system (GCS)

A global coordinate system is a three-dimensional Cartesian geodetic reference system.
For Europe the European sub-commission (EUREF) of the International Association of Geodesy (IAG) defined the European Terrestrial Reference System 1989 (ETRS89) as one GCS.


Grid/Raster

In computer graphics, a raster graphics image or bitmap is a data structure representing a generally rectangular grid of pixels, or points of color. Grid/Raster images are stored in image files with varying formats. In the context of OPALS, a grid is understood as a rectangular mesh (wireframe). The reference point of a grid is strictly the center of the pixel, thus, four points are necessary to span a single grid cell. This interpretation is also referred to as 'pixel-is-point'. By contrast, a raster is understood as a step-model, i.e. the raster value is valid for the entire cell area and not only for the center of the pixel. This interpretation is referred to as 'pixel-is-area'.


Half width

The term half width of a non-negative function with one single maximum is the difference between two values that have a function value that descents to the half of the maximum value. Typically for this half width value the term FWHM (full width at half maximum) is used. If the function value depends on time the abbreviation FDHM (full duration at half maximum) is often used. Remark: In the case of a normal distribution with a standard deviation (sigma) and an expectation value of zero the FWHM can be calculated by $ FWHM = 2*\sqrt{2*\ln(2)}*\sigma =2.35*\sigma$.


Harp

Assembly of flight strips that are approximately parallel in a planar view.


Intensity

Characterisation of the backscattered signal strength of an echo. Often this specification is not accurately defined. The term intensity can e.g. express the maximum as well as the total energy of one echo.


Laser beam

Continuously emitted connected light resp. energy packet with a very small wavelength range.


Laser scanner (LS)

A laser scanner is a measurement device recording by active illumination of object surfaces. Range and angular observations of reflecting objects in the sensor coordinate system are made, while the observation direction varies continuously within the field of view with the help of a deflection unit. In general, for the range measurement two different methods are in use: the time-of-flight of a laser pulse, where the travelling time between an emitted pulse and received echo is determined, and the continuous wave ranging, where the phase difference between the amplitude modulated continuously emitted laser beam and its received echo is estimated. In both methods time differences are determined that have to be converted to metric range values. Next to the observation of distances and ranges further observations (e.g. maximal echo amplitude) can be recorded by the laser scanner system.


Lengthwise (along) harp

Harp where the main direction does not significantly differ from the general flight direction (has to be defined) within one project area.


Lengthwise (along) strip

Flight strip where the main direction does not significantly differ from the general flight direction (has to be defined) within one project area (in general part of a lengthwise harp).


Light Detection And Ranging (LiDAR)

LIDAR is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target.


Measurement rate

Number of emitted pulses per second that illuminate the surface. Remark: Depending on the system design the measurement rate can differ from the frequency of laser pulse generation (not every generated pulse may be emitted to the surface direction).


Model

Simplified visual or mathematical description of structures, functionality or form, e.g. the atom model, but also the surface or terrain model.


Point cloud

Points within one specific coordinate system. Next to geometric information further point attributes like the intensity information, time stamp, classification results, etc. can be added to the singular points. The main difference to raw data is that the sequence of points may not follow the measurement process (may be arbitrary) and furthermore the point cloud must not provide all observations or attributes.


Point density

Average number of recorded points per square meter within an analysis unit (in the case of multiple echoes per pulse only the points defined by the last echo have to be considered). If the strip overlap is smaller than 50% or in the case of crosswise strips that do not cover the whole analysis unit, the point density has to be estimated with the help of the points of the singular cover. Areas with data holes have to be excluded from the computation and have to be considered in a different way.


Pulse duration/pulse width

Temporal length of a pulse. Due to the fact that the pulse will not have an exact rectangular form, the duration is - if not stated differently - the temporal length where the power of the echo is above the half maximum (full width at half maximum (FWHM) or the full duration at half maximum (FDHM)).


Pulse/Laserpulse

A connected short light resp. energy packet emitted by a laser scanner within a small wavelength range.


Raw data

Strip wise organised point sequence of all echoes within a specified coordinate system with all acquired information (e.g. intensity information, acquisition time) stored in the sequence of the data acquisition. In order to allow the synchronisation with the flight path a synchronised time stamp has to be stored.


Scan line

Point sequence resulting from one sequence of single deflections of the laser beam within the whole selected aperture angle of the laser scanner system across the flight direction.


Sensor coordinate system (SCS)

Coordinate system that is connected to a certain sensor in which respective sensor observations are performed. Within the ALS data acquisition, if not specified explicitly, the abbreviation SCS is typically used for the SCS of the laser scanner.


Strip difference

Height difference of two digital surface models that are determined from the last echo point cloud of geo-referenced overlapping flight strips.


Strip difference model

Surface model generated from the local strip differences.


Strip overlap

Proportional portion of a scan line of the common strip region of two neighbouring and overlapping flight strips in respect to the width of the respective scan line. For each strip a left and right strip overlap can be determined. Local strip overlap
Average strip overlap for a limited region. For ALS flight missions the determination within a flight length of 100m is considered as meaningful. Average strip overlap
Median of the maximal (with respect to all neighbouring strips to the left and right of the flight path) local strip overlap per flight strip. Based on this definition, a left and right average strip overlap per strip exists.


Target coordinate system (TCS)

The target coordinate system (TCS) is the geodetic coordinates system in which all results have to be delivered.
For ALS flight campaigns in Austria this is typically the Austrian countrywide coordinate system with the datum defined by the Militärgeographischen Institutes (MGI) mapped according to Gauß-Krüger and related to the respected reference meridian (M28, M31 and M34) which has to be fixed in case of doubt with the customer. The coordinate system is realised with the help of the coordinate frame of the Austrian reference-point network. The height reference system utilises orthometric heights in respect to the reference-points of the Austrian precision levelling. Present net distortions and the Geoid have to be considered.