| Term | Explanation |
|---|---|
| Absorption | The process of extinction by which electromagnetic radiation is admitted by an object and converted into other forms of energy. |
| Absorptance | The amount of radiation absorbed by a body per unit of area as a proportion of the total irradiation. |
| Active sensor | Sensor with a built in source of radiation so that both transmission and reception occur (e.g., radar, lidar). |
| AD conversion | Conversion of the values of an analogue flow of signals into digital values. See also Quantization. |
| Additive colours | Blue, green and red. Beams of these colours yield white light by addition. |
| Albedo | The fraction of solar irradiation reflected by natural objects in all directions (hemispherical reflectance) and considered over the entire reflective region of the electromagnetic spectrum. |
| Angle of incidence | See Incidence angle. |
| Angular aperture | See IFOV. |
| Antenna (in the radar principle) | The part of the radar source spatially distributing the radiation excited by a transmitter, or receiving the radiation reflected by an object. |
| Antenna gain pattern | Angular gain function describing the relative spatial distribution of transmitted and received energy. |
| Antenna theory | Theory proving the fixed relationship between the gain in the transmission mode and the receptive area in the receiving mode, for antennae being used for both transmission and reception. |
| Anti-vignetting filter | Special filter of which the transmittance gradually increases according to the distance to the centre in order to correct the influence of light fall-off and vignetting. |
| Aperture | The dimension(s) of the radiation-collecting device of a sensor (lens, parabolic mirror or antenna); it may also concern the dimensions of the slit or the circular aperture of a densitometer. |
| Area object | Coherent region in a remote sensing (RS) image, where pixels show corresponding radiation values; or terrain object that by the object definition may be considered as an area in the RS image, noting particularly the image scale and the contrast of radiation with the surroundings (not pertinent to the area) with respect to the spatial and radiometric resolution of the RS recording. |
| Arid regions | Land areas characterized by drought. Classification of these areas on earth depend on climatological data or on the conditions of the soil, vegetation and drainage. |
| Array scanner | See Pushbroom scanner. |
| Backscatter coefficient | See Differential scattering cross section. |
| Band-pass filter | Filter in optical instrumentation pervious to electromagnetic radiation within a certain interval of wavelengths only. A specific example is the interference filter. |
| BIL (Band interleaved by line) | Way of organizing digital remote sensing values on a magnetic tape, where for each scanline the pixel values of all spectral bands are placed in sequential order of scanning before the next scanline follows. |
| BIP (Band interleaved by pixel) | Way of organizing digital remote sensing values on a magnetic tape, where for each pixel the pixel values of all spectral bands are placed in succession before the next pixel follows. |
| Bit slicing | The analysis or representation of the successive bit levels of each numerical value within a remote sensing image. For instance, when digital pixel values range from zero to 255 (eight bits) the zeroth, first, second, third, ..., seventh bit individually could be reproduced as a binary image. |
| Blackbody | A hypothetical, ideal perfect radiator, which absorbs and emits fully and completely all incident radiation of all wavelengths and at all temperatures. |
| Brightness | Concept used in the microwave window for radiance. See also Radiance. |
| BSQ (band sequential) | Way of organizing digital remote sensing values on a magnetic tape, where for each spectral band all pixels of the image are placed in sequential order of scanning before the next spectral band follows. |
| Calibration | See Internal calibration and External calibration. |
| Camera flare | Radiant energy impinging on a film after internal reflections within the camera, not directly emanating from an object photographed. |
| CCD | See Charge-coupled device. |
| Characteristic curve | DlogH curve or DlogQ curve; the graphical representation of the relationship between the logarithm of the radiant exposure and the measured density of the film. |
| Charge-coupled device (CCD) | Semi-conducting detector used for the electro-optical image formation, usually arranged in an array. |
| Chirp principle | A radar modulation technique in which the frequency of the transmitted radar pulse is not a constant but is linearly changed in a positive sense (up-chirp) or in a negative sense (down-chirp). |
| Classification | The arrangement of individual pixels or groups of pixels into classes and the assignment of a label (class name) to each of these pixels; part of pattern recognition. |
| Cluster | The distribution of points around a certain characteristic point of a specific class in the feature space. |
| Coherent | Electromagnetic waves generated by one (or more) source(s) are coherent when they have the same frequency and phase, both in time as well as in space. |
| Colour additive viewer | Instrument assigning coloured light-beams to three multispectral black-and-white photographs, projecting these as overlays in order to obtain colour composites which can be interpreted more easily. |
| Colour brightness | The grey tone of the colour (also called value). |
| Colour film | Normal colour film (true colour film, TC), built up of three separate emulsion layers, with the red imaging layer sensitive to red radiation, the green one to green radiation and the blue one to blue radiation. |
| Colour hue | Colour tone; "colour" in colloquial language. |
| Colour infrared film | Form of colour film where the red imaging layer is sensitive to near-infrared radiation, the green one sensitive to red radiation and the blue one sensitive to green radiation; also called false colour film (FC). |
| Colour saturation | Amount of saturation, colour density, the number of coloured points per unit of area (also called chroma). |
| Contrast | The ratio of a certain quantity of radiation between the brightest and the darkest part of an image or between two arbitrary places in the image, where the contrast is to be determined. |
| Contrast enhancement | See Rescaling and Eberhard effect. |
| Contrast ratio | The relative difference of grey tones between two adjacent parts of an image or between two pixels, referring to different objects in the image. |
| Control points | Imaginary or physically visible points in the terrain recognizable in an image; also used for imaginary or physically visible points in one image corresponding with recognizable similar points in the other image (image control points). |
| Convolution filter | Linear filter type as used in digital image processing, of which the window operation has a mathematically linear character (weighted summation). Examples are low-pass filter, high-pass filter, gradient filters, Laplacian filters, etc. |
| Corner reflector | A set-up where three flat surfaces of metal plates are mounted at right angles to one another. They are used as references for radar measurements having a known radar cross section. |
| Cross-over point (refers to radar) | Concerning the curves showing the dependency of the radar backscatter on the incidence angle of the radar irradiation on to surfaces of differing roughness, the incidence angle where the diffuse part changes into the specular part of the curves; or the incidence angle where the effect of soil roughness vanishes and the radar backscatter value is determined exclusively by the presence of moisture. |
| Data | In the context of this book remote sensing values in various spectral bands either in analogue or in numerical (digital) form, from which information can be derived for the user by data processing and image interpretation. |
| Database | Data bank; structured collection of digital data files with a high degree of organizing relationships and coherence with respect to the contents. |
| Data processing | Changing data from one form into another by means of a computer system. |
| dB (decibel) | Measure to indicate the power ratio (10.logQ2/Q1) or voltage ratio (20.logV2/V1). |
| Density | A measure of opacity of a transparent medium, particularly the darkening of a film negative or film positive (density = -log [transmittance]). |
| Density curve | See Characteristic curve. |
| Densitometer | A device for measuring density. |
| Densitometry | The measurement of the opacity of a transparent medium. |
| Detector | Part of a remote sensing system to detect electromagnetic radiation. It converts the amount of radiation received into a signal value. |
| Dielectric constant | Electrical property of material partly determining the radar backscatter. It is also called complex dielectrical constant. |
| Difference operator | See Edge detection. |
| Differential scattering cross section | The radar cross section per unit of area (of the region illuminated by radar). See also Radar cross section. |
| Diffuse reflection | See Reflection. |
| Digital elevation model | Description of the elevation of the terrain with a regular x,y grid of coordinates. |
| Distributed target | Compound object; with respect to earth observation with radar each resolution cell may be considered as a collection of point targets with a certain distribution, where the object may even be larger than the region covered by the antenna. |
| DlogH curve | See Characteristic curve. |
| DlogQ curve | See Characteristic curve. |
| DN | Digital number. See Pixel value. |
| Dynamic range (of a sensor) | The ratio of the maximum observable power (Qmax) and the minimum still useful power (noise level, Qmin) expressed in decibels (See dB). |
| Eberhard effect | A local increase in density in photography, due to lateral effects of the developer at a sharp transition between a high and a low exposure; also called chemical contrast enhancement. |
| Edge detection | Form of segmentation, where edges between adjacent homogeneous areas are detected by means of difference operators, i.e., window operations to determine the partial derivatives and to test whether these values are sufficiently large to decide if the point concerned is an edge point indeed. |
| Electromagnetic | See EM wave and EM spectrum. |
| Electronic detection | The determination of the position and the radiant intensity in microwave systems by means of an electronic comparison of the physical condition of the transmitted microwaves (active system) with the physical condition of the echo coming from the object. Passive systems apply a comparison with an internal reference signal. |
| Electro-optical detection | Photo-electrical recording of radiation by means of an interaction of radiation either with a photo-electrical surface functioning as a radiation detector (image tube system), with a photon detector where electrons are excited, or with silicon diodes generating a quantity of charge current which is read out in an integrated circuit of metal oxide semi-conductors. In all cases an electrical signal is excited proportional to the irradiance. |
| Emission | The object itself emits electromagnetic radiation as a consequence of its kinetic temperature. |
| Emissive infrared | The infrared wavelength region from 3.5 µm until approximately 20 µm where the emission of objects on earth is effective. |
| Emissivity | The ratio of the radiant energy emitted by an object at a certain temperature and wavelength with regard to the radiant energy emitted by a blackbody at the same temperature and wavelength (a blackbody has an emissivity equal to one, other objects between zero and one). |
| Emittance | See Emissivity. |
| EM spectrum | Electromagnetic spectrum the total wavelength range of electromagnetic waves. |
| EM wave | Electromagnetic wave. |
| Expert system | A digital processing system where expert knowledge of interpretation is stored, and whereby (in the form of question and answer) the system can converse with itself and with the user, and may try to solve the interpretation problem iteratively. |
| Exposure meter | See Photometer or Radiometer. |
| Exterior orientation | See Orientation. |
| External calibration | Calibration to correct disturbing or undesirable influences from outside the sensor, such as the atmosphere or the level of irradiation. The calibration may be carried out by employing reference panels, reference terrain objects or reference measurements in the field. |
| False colour film (FC) | See Colour infrared film. |
| FC | See False colour film. |
| Feature space | The mathematical space describing the radiation values (features) of pixels (e.g., in the two-dimensional case the radiation values of two spectral bands can be plotted against one another). |
| Feature space plot | Two-dimensional graphical data representation in the feature space. |
| Field of view (FOV) | The total range of viewing of a sensor. See, however, IFOV with respect to the instantaneous field of view of the sensor. |
| Film sensitivity | See Sensitivity. |
| Filter (in optical instrumentation) | A material that - by absorption or by reflection - selectively modifies the radiation transmitted through an optical system. See Low-pass, high-pass and band-pass filter, respectively. |
| Filter (in digital image processing) | See Window operation. |
| Fluorescence | The phenomenon occurring in chemical systems that after absorption of light, emission of light quanta at a longer wavelength happens. Plants re-emit a small part of the absorbed radiant energy in the visible region into the red and near-infrared (0.65-0.75 µm). |
| Gamma | Scattering cross section; radar cross section per unit of area projected in the direction of transmission; sometimes also denoted simply as radar cross section. |
| Geocoding | Registration of images to the reference geometry of a map; also called georeferencing. |
| Geographical information system (GIS) | Well-considered combination of mutually referring data sets of various kinds of position-bound thematic data (data- base) and the necessary software to visualize this database, to manipulate it interactively and to analyse it in order to attain meaningful results. The data may be in raster or vector format or occur as individual points. The data sets should be registered. |
| Geometric resolution (or spatial resolution) | A theoretical measure of the smallest detail that can be detected (or the smallest spatial distance between two objects, usually expressed in radians or metres). In photography it is expressed as the number of optical line-pairs per millimetre. The geometric resolution determines also the practical choice of the instantaneous field of view (IFOV). |
| GIS | See Geographical information system. |
| Grain density | The number of silver halide crystals per unit of area of film. |
| Grain distribution | The way in which silver halide crystals are distributed on the film (e.g., random, systematic, irregular). The grain distribution depends on the Grain size distribution and Grain density. |
| Graininess | Subjective visual impression of the granular texture of the film image after a uniform exposure and after development (e.g., coarse or fine graininess). |
| Grain size distribution | The distribution (histogram) of the sizes of crystals in a film. |
| Granularity | The objectively measured irregularity of the silver grain distribution of a uniformly exposed photographic emulsion after development. It is equivalent to noise. |
| Granularity factor | Quantity referring to the densitometric determination of the granularity. |
| Graph | Representation of the topological neighbour relations of pixels, where a pixel is denoted by a node and the connectivity relationship by an arc connecting two such nodes with the same label. |
| Grazing angle | Angle of viewing relative to the horizontal on the spot of the object considered. |
| Grey level thresholding | Form of image segmentation by which a bivalent plan has been set up of an image on the basis of a certain threshold. |
| Ground control points | See Control points. |
| Ground truth | Field data; more generally reference data; data about the physical condition of the earth's surface obtained from other sources than the primary remote sensing (RS) data sources, and used in support of the analysis of the RS data. |
| High-pass filter (in optical instrumentation) | Absorption filter pervious to electromagnetic radiation above a certain wavelength only. |
| High-pass filter (in digital image processing) | Linear window operation that emphasizes the value of the central pixel relative to the values of the surrounding pixels within the window. Only when all pixels within the window have equal value, the resulting filter output value is zero. In general, differences in grey levels (step edges) will be shown in an enhanced way. |
| Histogram | Graphical representation of a data set showing the frequency of occurrence (along the vertical axis) of individual measurements or of successive intervals of pixel values (along the horizontal axis). |
| Histogram equalization | Form of image visualization (see also Image enhancement) where the histogram is changed by a non-linear rearrangement of the successive intervals of pixel values so as to become as flat as possible. Subsequently, on behalf of the picture the available number of grey levels or colour steps is attributed to these new intervals. |
| Ideal transmitting antenna | Antenna able to distribute the transmission power so as to obtain an equal power density in every point of a differential area - subtended by a limited solid angle - of a sphere with the radiator in the centre. The power density in all other points of the sphere is equal to zero at the same time. |
| IFOV | Instantaneous field of view of the sensor. It is the angular aperture within which the sensor is sensitive to electromagnetic radiation (expressed in degrees or radians); see also Resolution cell. This angular aperture is fixed by its construction, relative to the theoretical spatial and radiometric resolutions of the sensor. |
| Image | A (usually latent) two-dimensional representation (in a formal sense) of the spatial structure of an object with respect to its spectral features, also called image raster (grid) or image matrix as distinct from the picture which represents the visual representation on a screen, paper or film according to the wish of the user. |
| Image analysis | Analysis of the contents of an image supported by a digital image processing system in view of the extraction of information for the user. Also called digital image processing or digital interactive image analysis technique. |
| Image element | See Pixel. |
| Image encoding | special sampling of an image through data compression or reduction for efficient and reliable transmission or storage of remote sensing images. |
| Image enhancement | The improvement of the displaying (picturing) of images to facilitate visual interpretation or further digital processing. |
| Image interpretation | The recognition and identification in the image of objects in the terrain. Part of pattern recognition. The image interpretation can be carried out visually or digitally. |
| Image matrix | See Image. |
| Image motion | Blurring of a photograph because of the movements of the sensor platform (aeroplane or satellite), especially in the forward direction. |
| Image point | See Pixel. |
| Image processing (digital) | Digital techniques, consisting of a number of operations and modules as expedients to elucidate the information contents of images to the utmost. An interactive approach of the data analysis and image interpretation is common practice. |
| Image processing (human organic) | The organic processing by the human brains of images perceived with the senses. Cognitive and associative functions play a part in this image interpretation. |
| Image raster | See Image. |
| Image restoration | The recovery of the original image from a version disturbed by noise and deformation. |
| Image segmentation | The subdivision of an image in coherent parts or components, each being more or less homogeneous in a particular property. In addition, the neighbouring areas should be mutually different. |
| Image space | The mathematical space describing the position (coordinates) of pixels in the image (e.g., the i- and j-coordinates in the image matrix or on the screen). |
| Incidence angle | Angle of viewing relative to the vertical on the spot of the object considered. |
| Instantaneous field of view | See IFOV. |
| Interference filter | Filter reflecting radiation selectively in a very narrow spectral band. |
| Interior orientation | See Orientation. |
| Internal calibration | Calibration to correct disturbances or irregularities in the detection part of a sensor, usually carried out by checking the signal coming from a standard. |
| IRLS | Infrared line scanner; line scanner of the opto-mechanical type, exclusively for remote sensing detection in thermal infrared (TIR) channels. The name TIR scanner is preferred. |
| Irradiance | The radiant flux (power) incident on a surface, expressed per unit of area (Wm-2). |
| Irradiation | Electromagnetic radiation impinging upon a surface. |
| Isotropic radiator | Radiator able to distribute the transmission power equally over a spherical surface with the radiator in the centre. |
| Kirchhoff's law of radiation | Radiation law stating that, at a given absolute temperature and wavelength, the emission and absorption by an arbitrary body (object) are equal. |
| LAI | Leaf area index; the total one-sided green surface of foliage per unit of soil area. |
| Lambertian radiator | Object having the property of the emitted or reflected radiance being independent of the viewing angle by the sensor. With respect to reflection the object can be considered to be a diffusely reflecting surface. |
| Lambertian surface | See Lambertian radiator. |
| Land evaluation | The definition of the optimal land use for a particular land unit by comparison of land use requirements and land qualities. |
| Laplacian filter | Linear window operation (digital filter) concerning second derivatives of the pixel values within a window, either uni-directional or bi-directional (orthogonal). |
| Latent image (as opposite to a real image) | Invisible image generated by photographical or electro-optical detection. It is visualized after photographical film development or may be displayed by means of an image processing system. |
| Layover | See Radar layover. |
| Leaf area index | See LAI. |
| Lidar | Light detection and ranging; technique by which a coherent laser emits radiation (0.3-11 µm) and the atmospheric reflection or absorption is determined. |
| Light fall-off | See Radiation fall-off. |
| Light meter | Exposure meter. See Photometer or Radiometer. |
| Line object | Pixels with similar radiation values forming a line; or an object in the terrain, by the object definition, to be considered as a line (curved or not) without a significant width in the remote sensing (RS) image, taking into account the image scale and the radiation contrast with the surroundings with respect to the spatial and radiometric resolution of the RS recording. |
| Line scanner | Line scanning sensor system of the opto-mechanical type. See Opto-mechanical scanner. |
| Long-wave | Wavelengths longer than 3.5 µm. |
| Look angle | Angle of viewing referring to the vertical (nadir) in the sensor. |
| Low-pass filter (in optical instrumentation) | Absorption filter pervious to electromagnetic radiaton below a certain wavelength only. |
| Low-pass filter (in digital image processing) | Linear window operation that smoothes the image pixel values by arithmetical averaging of the pixel values within the window and assigning the filter output value to the central pixel of the window concerned. |
| Matching | See Registration. |
| Median filter (in digital image processing) | Non-linear window operation that replaces the value of the central pixel by the median value when all pixel values within the window are placed sequentially in order of magnitude. |
| Microwaves | Electromagnetic radiation in the microwave window. |
| Microwave window | The window in the electromagnetic spectrum referring to radiation of wavelengths ranging from 0.1 to 70 cm. |
| Middle-infrared | Wavelengths ranging from 1.3 to 2.5-3.0 µm; part of the reflective infrared region. |
| MIR | See Middle-infrared. |
| Mixel | Mixed pixel; pixel with a numerical radiometric value which is a mixture of various objects within the spatial resolution of the remote sensing system. |
| Modulation | The variation of frequency, phase or amplitude of an electromagnetic wave. The principle often applied to radar systems reads that the sinusoidal vibration generated in the transmitter is emitted at short intervals. The signal then consists of a periodical set of pulses. Another type of modulation is the so-called chirp principle. |
| Monitoring | Surveillance; guarding, supervising, checking, following of changes. |
| Monotemporal | Concerning one instant of time only. |
| MSP | See Multispectral photography. |
| MSS | Multispectral scanning of the opto-mechanical detection type. |
| Multilook technique | The technique of averaging a number of independent samples per pixel, applied to radar in order to reduce speckle. |
| Multispectral | Containing two or more spectral bands. |
| Multispectral classification | Classification, based on pixel values in two or more spectral bands. See Classification. |
| Multispectral photography (MSP) | Type of photography, recording the radiation on black-and-white films in separate small spectral bands by a selection of films and filters. |
| Multispectral scanning | Remote sensing survey type, whereby the sensor records digitally the same area simultaneously in several spectral bands through scanning. |
| Multitemporal | Concerning two or more instants of time. |
| Near-infrared (NIR) | Wavelengths ranging from 0.7 to 1.3 µm; part of the reflective infrared region. |
| NEP | See Noise equivalent power. |
| NIR | See Near-infrared. |
| Noise | Undesirable pattern of disturbing effects of a magnitude of the radiometric resolution with a random or repetitive nature, reducing the information contents of data. |
| Noise equivalent power (NEP) | the smallest detectable radiation unit. |
| Objects | In a general sense characterized in the terrain by their thematics and by their geometrical position, shape and size. The thematics, represented by the spectral signature of the object, may lead to a labelled arrangement into an object class, while the geometry may lead to the assignment of an object type (area, line or point object). |
| Object parameter | Variable quantity with respect to the object, influencing the actual remote sensing observation as there are (where applicable) moisture content, temperature, surface roughness, tilling, plant direction, spatial geometrical structure and position relative to the sensor, volume, density, soil cover, number of leaf layers, variation in time, etc. |
| Opacity | Amount of imperviousness to light. |
| Optical window | The window in the electromagnetic spectrum from ultraviolet (UV) to the thermal infrared (TIR) inclusive (wavelengths ranging from 0.30 to 14 µm). |
| Opto-mechanical detection | Form of electro-optical detection, employing an oscillating or rotating mirror applied to a line scanning whiskbroom-scanner. |
| Opto-mechanical scanner | Sensor system of the opto-mechanical detection type, employing an oscillating or rotating mirror as well as the forward movement of the platform itself to scan the earth's surface point-wise and line-wise. It operates either in one spectral band only (e.g., a thermal infrared band; see IRLS), or in various spectral bands (multispectral scanning); also called whiskbroom scanner. |
| Orientation | Interior orientation: the relative position of the detecting and
imaging components of a sensor. Exterior orientation: the position and attitude of the sensor platform in the object space. |
| Panchromatic | A wide spectral band covering the visible part of the electromagnetic spectrum. |
| Panchromatic film | Black-and-white film sensitive to the entire visible part (0.4-0.7 µm) of the electromagnetic spectrum. |
| Panoramic distortion | Distortion of the image occurring with whiskbroom scanners. The image at right angles to the flight direction (scanning direction) is formed at a constant angular velocity, proportional to the viewing angle. The scanning over the earth's surface in that direction is, however, proportional to the tangens of the viewing angle. The result of the imaging makes the terrain looking compressed towards the sides. |
| Parallax | Mutual shift of objects being in our field of view. |
| Passive sensor | Sensor without its own source of radiation (it measures, e.g., the reflection of solar irradiation by objects on the earth's surface). |
| Pattern (spectral) | The vector of pixel values involving the spectral values of a pixel in the various spectral bands. |
| Pattern (spatial) | The arrangement and positional distribution of colour or grey tone differences in the image space, together showing a more or less fixed relationship. One may distinguish between noise patterns (with a random spatial distribution) and structural patterns (with an identified spatial distribution). |
| Pattern recognition | The measurement of the characteristic pattern features, and the classification (possibly by comparison with a known pattern of training areas). See also Classification. |
| Phenomenological model | Model where formal, theoretical reasoning comes in the place of physical definitions and explanations, and where human observation, action and decision making play an important part. |
| Phenomenological resolution | The extent of the surroundings of a detail necessary for recognition. |
| Photograph | Analogue real image, obtained by means of photography. |
| Photographical detection | Photo-chemical recording of radiation by means of the interaction of radiation (light) with silver halide crystals in the film emulsion. |
| Photogrammetry | The determination of the dimensions of an object from a photograph; the science and techniques of the terrain survey and the manufacturing of databases and/or maps from aerial photographs, especially in relation to the geometrically correct representation of the position of the terrain objects to be surveyed. |
| Photometer | Exposure meter with a spectral sensitivity according to the photopic curve. |
| Photopic curve | The curve of the spectral sensitivity of the eye; an international standard curve approximating the monochromatic spectral sensitivity of the human visual system to daylight. |
| Photosynthesis | The synthesis of carbohydrates by green plants with carbon dioxide from the air and water from the soil under the influence of light. |
| Physiographic image interpretation | Image interpretation aimed at the distinction of land types and the subdivision into land units. |
| Picture | Visual representation of a latent image according to the option of displaying as wished by the user. The production of pictures is part of the digital interactive image analysis technique. |
| Pixel | Acronym for picture element, the position of which is determined by means of the position in the image raster, and the appertaining numerical value is taken artificially from complete or partial resolution cells. Sometimes also called image point. The size of a pixel in a remote sensing image of the earth's surface is usually related to the particular terrain dimension, e.g., 30 m by 30 m. |
| Pixel value | The digital radiation value of a pixel, expressed as a digital number (DN) or digital count (DC), radiance value, reflectance or other radiation value. |
| Planck's radiation law | Radiation law stating that the temperature of a blackbody is related to the emitted radiance as a function of the wavelength (or the frequency). |
| Point object | Terrain object that by the object definition may be considered as a point in the remote sensing (RS) image, noting the image scale, its relative small shape and size and the radiation contrast with the surroundings with respect to the spatial and radiometric resolution of the RS recording. |
| Point operation | Processing or analysis of the radiation values of each pixel separately, as opposite to window operations; or the determination of a new value (output) of a particular pixel in a remote sensing image based on the value of this individual pixel (input) in the original image. |
| Point target (radar) | Object without dimensions (in practice very small size compared to the area covered by the antenna). |
| Polarization direction | The plane of vibration of the electric field of an electromagnetic wave. |
| Power | The term used in the microwave window for the radiant flux. See Radiant flux. |
| Power density | The term used in the microwave window for irradiance. See Irradiance. |
| Principal components analysis | Principal components transformation; orthogonal, linear transformation of N original correlated variables (e.g., observed radiation values in various spectral bands) into N new uncorrelated variables (called principal components). |
| Pulse repetition frequency | Frequency of transmission of a radar pulse. |
| Pulse width | Duration of transmission of each separate pulse. |
| Pushbroom scanner | line scanning system, where all points of an image line are recorded simultaneously by an array of charge-coupled device elements. |
| Quantization | The arrangement of the range of measured values into a finite number of discrete levels (digital sampling). |
| Radar | Radio detection and ranging; technique of transmitting microwaves and the subsequent recording of these waves reflected by an object. |
| Radar backscatter | Radar echo; the process of scattering of microwave energy by an object in the direction of the radar antenna, after actively being irradiated by the radar source. |
| Radar cross section | A hypothetical area of an object of such an extent that if the power intercepted by this area is distributed isotropically over the space, it renders the same power density at the receiving antenna as the power density brought about in reality by the presence of the object. Usually the radar cross section concerning compound objects (distributed targets) is normalized either as a radar cross section per unit of area (differential scattering cross section or backscatter coefficient) or as a radar cross section per unit of area projected in the direction of transmission (gamma or scattering cross section). |
| Radar echo | See Radar backscatter. |
| Radar equation | Equation describing the formal relationship between the power received by the radar and the radar cross section of an object. |
| Radar image | The two-dimensional image recorded by means of an imaging radar system. |
| Radar layover | The phenomenon by sideways-looking radar that the top of a high object is imaged earlier than the foot, as the top is nearer to the antenna than the foot (or its fictitious vertical projection on to the ground reference plane). The layover in radar images thus is directed towards the nadir point. |
| Radar principle | The principle of radar the object to be observed is actively irradiated by a source of microwaves. Subsequently, the radiation scattered back by the object in the direction of that source (the radar) is measured (active sensor system). |
| Radar receiver | The part of the radar detecting the radiation received by an antenna. |
| Radar scattering | See Radar backscatter. |
| Radar scattering cross section | See Gamma. |
| Radar shadow | The phenomenon that the terrain behind high objects cannot be observed by sideways-looking radar, so that no echo returns. |
| Radar signature | The radar response (expressed by differential radar cross section values or by gamma values) of a particular material or object as a function of the frequency, grazing angle, polarization or time. |
| Radiance | The radiance of an observed point on a surface at a given angle to the normal of the surface, is the radiant flux leaving an element of the surface surrounding the point, and propagating in the direction defined by an elementary cone containing the given direction, divided by the product of the solid angle of the cone and the area of the orthogonal projection of the element of the surface on to a plane perpendicular to the given direction. |
| Radiant energy | Energy in the form of electromagnetic waves or photons. |
| Radiant exitance | The radiant flux emitted from a surface per unit of area. |
| Radiant exposure | The total irradiance integrated over the integration or exposure time of the sensor. |
| Radiant flux | The quantity of radiant energy moving from one point to another per unit of time (also called power). |
| Radiant intensity | The radiant flux proceeding from a source per unit of a solid angle in the direction considered. |
| Radiant power | See Radiant flux. |
| Radiation fall-off (light fall-off) | Reduction of the irradiance toward the edges of a photograph, depending on the incidence angle with respect to the optical axis of a camera. |
| Radiometer | Radiation measurement device, with a uniform spectral sensitivity within the spectral bandwidth it is constructed for. |
| Radiometric resolution | The smallest observable difference in radiant energy with respect to reflection, emission, temperature differences, intensity differences, power differences, etc. |
| Radiometry | Measurement of radiation. |
| Raster format | The reproduction of point, line and area objects in the image space in the form of an image matrix, as a raster (grid) of pixels; cf. vector format. |
| Rasterizing | The conversion of data (e.g., line maps) into a raster format. |
| Raster-overlay-structure | Method of presentation of data in raster format, where the dimensions of the raster of different data are equal. Hence, the thematic aspects of various types of data can be compared. |
| RBV | See Return beam vidicon. |
| Reference panel | Artificial object with known reflective characteristics used in optical remote sensing (RS) as a reference for the external calibration of recordings. These panels are imaged and recorded under the same observational conditions as the other objects in the remote sensing scene. Concerning aerial photography, the internal calibration is also involved in the calibration procedure, integrated with the external calibration. |
| Reflectance | The radiance of an object in a certain direction under certain conditions of irradiation, relative to the radiance of an ideal white diffuse surface in the same direction and the same irradiation conditions (depending on the wavelength). |
| Reflectance factor | See Reflectance. |
| Reflectance model | Model describing the reflectance of an object as a function of sensor and object parameters. |
| Reflectance percentage | The reflectance expressed as a percentage. Sometimes also the percentage of irradiation reflected by an object (often defined in the direction of a sensor and in a defined wavelength band). |
| Reflection | The process of scattering of electromagnetic radiation by an object. The diffuse reflection causes the radiance of the reflected radiation to be equal in all directions (e.g., reflection from a rough surface). Specular reflection has a direction of preference (e.g., the reflection of a smooth surface). The use of the terms "smooth" and "rough" is dependent on the wavelength. |
| Reflective infrared | The region of the near-infrared and middle-infrared wavelengths; 0.7 to 2.5-3.0 µm. |
| Refractive index (of a medium) | Inverse ratio of the wavelength (or velocity) of electromagnetic radiation in the medium to that in the vacuum. It is equal to the square root of the complex dielectric constant of a homogeneous and isotropic medium. |
| Registration | A geometric rearrangement of the pixels for superimposing images (matching); often to the reference geometry of a map (geocoding). |
| Relief displacement | A shift of the position of a remotely sensed object in the image plane (focal plane), due to the local elevation of the object. This shift depends on the specific imaging properties of the sensor system concerned. |
| Remote sensing (RS) | The instrumentation, techniques and methods to observe the earth's surface at a distance and to interpret the images or numerical values obtained in order to acquire meaningful information of particular objects on earth. |
| Resampling | The rearrangement of the resolution cells of each scanned line into geometrically equal terrain elements (geometric rearrangement) by creating artificial pixels of which the spectral radiation data are computed from the original values proportional to the area coverage by the new pixels with respect to the resolution cells. In general a resampling follows after a geometric rearrangement (registration) of the pixels because of the matching of two different images of the same region by means of a mathematical transformation. The resampling then concerns the assignment of artificial pixel values to the newly formed pixels according to a selected sampling algorithm. |
| Rescaling | Application of a change of scale to observed values, where usually a linear mathematical function is employed. In case of visualization of (latent) images by making pictures (see there), rescaling is a tool for image enhancement with the purpose of enhancing contrasts in the image or elucidating specific groups of objects. The available amount of successive grey tones and colour hues are then assigned to a different interval of numbers than the range of the original pixel values; for instance, to the interval situated between the minimum and maximum pixel values instead of the range from zero to 255; or to the specific interval concerning some type of land use. |
| Resolution | Resolving power, in particular of a sensor; used for spatial, radiometric, spectral and temporal resolution. |
| Resolution cell | The smallest ground area possible to be observed by a sensor under conditions in force. This area is determined by the instantaneous field of view in the case of a scanner, and by the height of the sensor above the earth's surface. |
| Resolution element | See Resolution cell. |
| Resolving power | See Resolution. |
| Return beam vidicon | Panchromatic image tube system for electro-optical detection; used in the first Landsat satellites. |
| RMS granularity | Root mean square granularity; standard deviation of the granularity of a film, sometimes denoted in parts per thousand. |
| Roughness | Ruggedness or coarseness of a surface; a concept dependent on the wavelength of the irradiation. It may happen that a surface interacting with short-wave irradiation is considered as rough, but at the same time as smooth as an interaction with long-wave irradiation. It may comprise macro-roughness (like local coarse terrain elevation differences, presence of plough ridges after tilling, crop rows, stubble direction of harvested areas or tree row spacing in forestry) or micro-roughness, indicating sub-local height inequalities of bare soil, or the canopy structure of a vegetation cover of the earth's surface. Micro-roughness is usually expressed in terms of root mean square heights. |
| RS | See Remote sensing. |
| SAR | See Synthetic aperture radar. |
| Scale | The ratio of a distance in an image and the corresponding distance in the terrain. |
| Scanner | A sensor system scanning the earth's surface in the optical window; either an opto-mechanical scanner or a pushbroom scanner. |
| Scattering cross section | See Gamma. |
| Scatterometer | In general, a non-imaging research instrument to measure either the gamma or the differential scattering cross section. |
| Segmentation | See Image segmentation. |
| Semantics | Concerning the significance of an object. |
| Sensitivity | A measure of a sensor's sensitivity to electromagnetic radiation. |
| Sensitometry | The checking and calibration of the process of film development in photography by means of a characteristic curve, necessary for a quantitative radiometrical analysis of film material. |
| Sensitometric curve | See Characteristic curve. |
| Sensitometric sensitivity | The reciprocal of the radiant exposure H necessary to attain a certain density D, based on the position of the DlogH curve with regard to the calibrated logH axis. |
| Sensor | Instrument sensitive to a particular physical quantity (radiation). It is able to transpose this quantity into a photographical or electrical value. |
| Sensor parameter | Variable quantity concerning sensors, with which the user may influence the remote sensing observational process, such as detector type, spectral band, viewing angle, polarization, diaphragm, focal distance, recording instant, recording duration (integration time), etc. Sensor parameters may also be thought of as the instrumental selection with regard to the desired sensor resolutions. |
| Short-wave | The electromagnetic wavelength region up to 3.5 µm. |
| Sideways-looking airborne radar (SLAR) | Sideways-looking radar applied from an aeroplane. The resolution in the flight direction is determined by the resolution of the antenna (azimuth resolution). |
| Sideways-looking radar (SLR) | The principle of the antenna subtending a fixed angle with the earth's surface and scanning a drawn out sector of the terrain at right angles to the flight direction with its transmitting fan beam (synthetic aperture radar or sideways-looking airborne radar system). |
| Signal-to-noise ratio | The ratio of the amplitude level of the radiant flux of the information containing signal, and that of the noise; also denoted S/N ratio. It may be expressed in dB: 20.log(S/N). See also Dynamic range. |
| Signature | See Radar signature, Spectral signature, Temporal signature. |
| SLAR | See Sideways-looking airborne radar. |
| SLR | See Sideways-looking radar. |
| Spatial resolution | The practically effective measure for the (theoretical) geometric resolving power; corresponding to a resolution cell. It may also concern the size of the pixels after preprocessing by the producer of the magnetic tape comprising the remote sensing data. See also Geometric resolution. |
| Speckle | The phenomenon of a strong variation of echo signals from one resolution cell to another occurring in radar, because the echo received consists of the summation of the contributions of a collection of point targets in each resolution cell, in continuously changing combinations (interference). |
| Spectral band | A well-defined continuous range (interval) of wavelengths in the electromagnetic spectrum; wavelength band, frequency band, channel. |
| Spectral resolution | A measure of the width of a wavelength band (also called spectral band or channel), regarding its location in the electromagnetic spectrum. |
| Spectral sensitivity | The sensitivity of a sensor to incoming electromagnetic radiation as a function of the wavelength. |
| Spectral signature | The remote sensing response (reflection) of a particular object (material, species, class, etc.) as a function of wavelength. |
| Spectroradiometer | Radiometer for carrying out measurements in a large number of small wavelength bands. |
| Specular reflection | See Reflection. |
| Speed (of a film) | See Sensitometric sensitivity. |
| Stefan-Boltzmann's radiation law | Law stating that the entire quantity of energy emitted by a blackbody is proportional to the fourth power of the absolute temperature of that body. |
| Stereoscopy | The spatial three-dimensional observation of related two-dimensional images, showing the same objects under different viewing angles. |
| Structural element | Element for structural pattern recognition, built up with pixels or resolution cells. These can be obtained, for instance, through the grey tone thresholding within a 3x3 window, which renders binary (bit) patterns to be grouped into a limited number of classes. These are mainly determined topologically. |
| Subtractive colours | Yellow, magenta and cyan. They are obtained by subtracting an additive colour from white light. |
| Sun-synchronous | With respect to a satellite system by which the remote sensing data are always collected at the same local time of day over the same region. Concerns an earth observation satellite of which the orbital plane is near-polar and has an attitude such that within the cycle of a year this plane rotates at the same rate as the mean rate of the earth rotation about the sun. As a consequence the satellite passes over areas along the same geographical latitude at the same local sun time each day the satellite passes, whether in the south-bound ("descending node") direction or in the north-bound ("ascending node") direction. By sun-synchronous satellite sensing the irradiance conditions are approximately equal in successive passes over the same region and over all areas on earth having the same latitude. |
| SWIR | Short-wave infrared wavelength region, an alternative to middle-infrared. |
| Synergy | The total is more powerful than the summation of the parts. It is used in the context of the well-thought-of acquisition and combination of the observational data of optical and microwave sensors of operational earth observation systems, also in connection with prior knowledge and other sources of spatial data. Hence, an increasing chance to score may be expected than would have been possible with separate earth observations in the individual windows of the electromagnetic spectrum. |
| Synopsis | Review of a large part of the earth's surface by remote sensing, so that the coherence and connectivity of the imaged objects or phenomena finds an expression in a wider context. |
| Synthetic aperture radar (SAR) | radar system with a high spatial resolution in the flight direction (azimuth resolution) as every target area (terrain element) is observed in the flight direction for a longer time. Each return signal (echo) is recorded in amplitude and phase for a slight displacement of the antenna in the flight direction. The effect of a considerable larger antenna is obtained synthetically by storing and combined processing of these multiple echoes. The SAR method operates only when the radar radiation is coherent. |
| TC | See True colour. |
| Temporal resolution | The frequency (in time) of remote sensing imaging possible with a particular sensor. |
| Temporal signature | The remote sensing response (reflection or emission) of an object (material, species, class, etc.) as a function of time. |
| Texture | Property of an image with a local character, connected with a certain area or extent. It can be described as a tissue built up with basic elements within the image segment. The image texture can serve as a spatial pattern of the distribution of grey tones in that segment, showing the character either of noise or of a structure pattern. The image texture can be denoted qualitatively with terms as coarse, fine, regular, irregular, fibrous, smooth. It can be described quantitatively by texture measures. |
| Texture analysis | Investigation of the texture properties of an image. |
| Texture measure | Function assigning a numerical value to the original image texture. |
| Thematics | (Thematic information) concerns the contents, nature or properties of an object or object class. |
| Thematic decision rule | With the aid of the spectral signature (thematics) of pixels their pertinent object class can be indicated. Cf. Topological decision rule. |
| Thermal image | The imaging of the thermal infrared radiation emitted by the earth's surface. |
| Thermal infrared (TIR) | Wavelengths ranging from 3.5 to 14 µm; part of the emissive infrared region. |
| Thermal noise | The detector signal (of a thermal infrared scanner) caused by the thermal energy of the detector itself, occurring when the system is cooled insufficiently. |
| Thermography | Part of remote sensing, concerning the thermal infrared (thermal images). |
| Thresholding | See Grey level thresholding. |
| TIR | See Thermal infrared. |
| TIR scanner | Thermal infrared line scanning system. See also IRLS. |
| Topological | Concerning the formal positional relationship (connectivity) between objects or pixels. |
| Topological decision rule | With the aid of the neighbourhood of pixels (topology) their pertinent object can be indicated (not only object class but also object type); cf. Thematic decision rule. |
| Training pixels | Pixels of a known object class. In a classification, one determines their corresponding cluster in the feature space and then it is decided that all other pixels represented in that cluster also belong to that object class. |
| Training set | A collection of training pixels or training data. |
| Transmission | The process of transmitting electromagnetic radiation. |
| Transmission coefficient | See Transmittance. |
| Transmittance | The ratio of the radiation transmitted by a body (object) per unit of area and the total irradiation. |
| Transmittance factor | See Transmittance. |
| Transmitter (concerns the radar principle) | Part of the radar exciting, modulating and transmitting the radiation of a desired frequency. |
| True colour (TC) film. | See Colour film. |
| UV | Ultraviolet. |
| Vector format | The representation of area objects in the image space by their circumference; line objects are represented by the mutual connection of their subsequent points defining the course of the line (straight or curved); point objects are represented by their coordinates defining their position in a given geometric reference system; cf. Raster format. |
| Vector-overlay-structure | Method of processing data in vector format in a common coordinate system in such a way as to compare the thematic aspects of different kinds of data. |
| Vegetation index | A mathematical function of reflection values (reflectances, digital pixel values) in different spectral bands, used to estimate vegetation characteristics. Such a function also serves to correct undesirable influences, for instance differences of soil reflectance or atmospheric conditions. |
| Viewing angle | Angle of sensor observation referring to the vertical (nadir) in the sensor. In current literature synonyms are look angle; illumination angle; pointing angle; off-nadir angle. |
| Vignetting | Reduction of the area of an oblique bundle of light passing through the lens in case of large diaphragms, because of partly screening by the casing of the camera and the mount of the lens. |
| VIS | See Visible light. |
| Visible light | Wavelengths ranging from 0.4 to 0.7 µm. |
| Water absorption bands | Wavelength bands where water - free or bound - absorbs radiation to a high degree. |
| Whiskbroom scanner | Line scanning opto-mechanical sensor system. See Opto-mechanical scanner. |
| Wien's displacement law | Radiation law indicating that the wavelength, at which the emitted amount of energy by a blackbody is maximal, is inversely proportional to the absolute temperature of that body. |
| Window (Electromagnetic) | Wavelength region in which the atmosphere is (largely) transparent for electromagnetic radiation. See Optical window and Microwave window. |
| Window (in image processing) | Annotation of the size of the surroundings of a pixel, which is used for some processing of that pixel (e.g., 3x3 window, 5x5 window). |
| Window operation | Processing of the (radiation) values of pixels within a predefined window, mostly limited to one spectral band; also called filter. Examples are convolution filters, median filters, variance filters, etc. The filter output is assigned to the central pixel of the window. |
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