## GLOSSARY

This Glossary was extracted directly from an Internet Site (address at end of Glossary) prepared by Jeff Weissel and others at the Lamont-Doherty Earth Observatory of Columbia University and is reproduced here with their permission. Their sources for the definitions come principally from:

1) S.A. Drury, `A Guide to Remote Sensing', (Oxford), 199 pp., 1990.

2) F.F. Sabins, `Remote Sensing Principles and Interpretation', 2nd Edition, (W.H. Freeman & Co.), 449 pp., 1987

# A

  absolute temperature-Temperature measured on the Kelvin scale, whose base is absolute zero, i.e. -273 °C; 0 °C is expressed as 273 °K.
  absorptance-A measure of the ability of a material to absorb EM energy at a specific wavelength.
  absorption band-Wavelength interval within which electromagnetic radiation is absorbed by the atmosphere or by other substances.
  absorptivity-Capacity of a material to absorb incident radiant energy.
  achromatic vision-The perception by the human eye of changes in brightness, often used to describe the perception of monochrome or black and white scenes.
  active remote sensing-Remote sensing methods that provide their own source of electromagnetic radiation to illuminate the terrain. Radar is one example.
  acuity-A measure of human ability to perceive spatial variations in a scene. It varies with the spatial frequency, shape, and contrast of the variations, and depends on whether the scene is coloured or monochrome.
  additive primary colors-Blue, green, and red. Filters of these colors transmit the primary color of the filter and absorb the other two colors.
  adiabatic cooling-Refers to decrease in temperature with increasing altitude.
  advanced very high resolution radiometer (AVHRR)-Crosstrack multispectral scanner on a NOAA polar-orbiting satellite that acquires five spectral bands of data (0.55 to 12.50 µm) with a ground resolution cell of 1.1 by 1.1 km.
  aerial magnetic survey-Survey that records variations in the earth's magnetic field.
  air base-Ground distance between optical centers of successive overlapping aerial photographs.
  airborne imaging spectrometer (AIS)-Along-track multispectral scanner with spectral bandwidth of 0.01 µm.
  airborne visible and infrared imaging spectrometer (AVIRIS)-Experimental airborne along-track multispectral scanner under development at JPL to acquire 224 images in the spectral region from 0.4 to 2.4 µm.
  AIS-Airborne imaging spectrometer.
  albedo (A)-Ratio of the amount of electromagnetic energy reflected by a surface to the amount of energy incident upon it.
  along-track scanner-Scanner with a linear array of detectors oriented normal to flight path. The IFOV of each detector sweeps a path parallel with the flight direction.
  alteration-Changes in color and mineralogy of rocks surrounding a mineral deposit that are caused by the solutions that formed the deposit. Suites of alteration minerals commonly occur in zones.
  amplitude-For waves, the vertical distance from crest to trough.
  Analog display-A form of data display in which values are shown in graphic form, such as curves. Differs from digital displays in which values are shown as arrays of numbers.
  analogue image-An image where the continuous variation in the property being sensed is represented by a continuos variation in image tone. In a photograph this is achieved directly by the grains of photosensitive chemicals in the film; in an electronic scanner, the response in, say, millivolts is transformed to a display on a cathode-ray tube where it may be photographed.
  angular beam width-In radar, the angle subtended in the horizontal plane by the radar beam.
  angular field of view-Angle subtended by lines from a remote sensing system to the outer margins of the strip of terrain that is viewed by the system.
  angular resolving power-Minimum separation between two resolvable targets, expressed as angular separation.
  anomaly-An area on an image that differs from the surrounding, normal area. For example, a concentration of vegetation within a desert scene constitutes an anomaly.
  antenna-Device that transmits and receives microwave and radio energy in radar systems.
  aperture-Opening in a remote sensing system that admits electromagnetic radiation to the film in radar systems.
  Apollo-U.S. lunar exploration program of satellites with crews of three astronauts.
  apparent thermal inertia (ATI)-An approximation of thermal inertia calculated as one minus albedo divided by the difference between daytime and nighttime radiant temperatures.
  artefact-A feature on an image which is produced by the optics of the system or by digital image processing, and sometimes masquerades as a real feature.
  ASA index-Index of the American Standards Association designating film speed, or sensitivity to light. Higher values indicate higher sensitivity. The ASA index has been replaced by the ISO index.
  ATI-Apparent thermal inertia.
  atmosphere-Layer of gases that surrounds some planets.
  atmospheric correction-Image-processing procedure that compensates for effects of selectivity scattered light in multispectral images.
  atmospheric shimmer-An effect produced by the movement of masses of air with different refractive indices, which is most easily seen in the twinkling of stars. Shimmer results in blurring on remotely sensed images, and is the ultimate control over the resolution of any system.
  atmospheric window-Wavelength interval within which the atmosphere readily transmits electromagnetic radiation.
  attitude-Angular orientation of remote sensing system with respect to a geographic reference system.
  AVHRR-Advanced Very High Resolution Radiometer, a multispectral imaging system carried by the TIROS-NOAA series of meteorological satellites.
  AVIRIS-Airborne visible and infrared imaging spectrometer.
  azimuth-Geographic orientation of a line given as an angle measured in degrees clockwise from north.
  azimuth direction-In radar images, the direction in which the aircraft is heading. Also called flight direction.
  azimuth resolution-In radar images, the spatial resolution in the azimuth direction.

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# B

  background-Area on an image or the terrain that surrounds an area of interest, or target.
  backscatter-In radar, the portion of the microwave energy scattered by the terrain surface directly back toward the antenna.
  backscatter coefficient-A quantitative measure of the intensity of energy returned to a radar antenna from the terrain.
  band-A wavelength interval in the electromagnetic spectrum. For example, in Landsat images the bands designate specific wavelength intervals at which images are acquired.
  base-height ratio-Air base divided by aircraft height. This ratio determines vertical exaggeration on stereo models.
  batch processing-Method of data processing in which data and programs are entered into a computer that carries out the entire processing operation with no further instructions.
  bathymetry-Configuration of the seafloor.
  beam-A focused pulse of energy.
  bin-One of a series of equal intervals in a range of data, most commonly employed to describe the divisions in a histogram.
  binary-Numerical system using the base 2.
  bit-Contraction of binary digit, which in digital computing represents an exponent of the base 2.
  blackbody-An ideal substance that absorbs all the radiant energy incident on it and emits radiant energy at the maximum possible rate per unit area at each wavelength for any given temperature. No actual substance is a true blackbody, although some substances, such as lampblack, approach its properties.
  blind spot-The point of the optic nerve to the retina where no radiation is detected by the eye.
  brightness-Magnitude of the response produced in the eye by light.
  brute-force radar-See real-aperture radar.
  byte-A group of eight bits of digital data.

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# C

  calibration-Process of comparing an instrument's measurements with a standard.
  calorie-Amount of heat required to raise the temperature of 1g of water by 1 °C.
  camouflage detection photographs-Another term for IR color photograph.
  cardinal point effect-In radar, very bright signatures caused by optimally oriented corner reflectors, such as buildings.
  cathode ray tube (CRT)-A vacuum tube with a phosphorescent screen on which images are displayed by an electron beam.
  CCD-Charge-coupled detector.
  CCT-Computer-compatible tape.
  cell assemblies-The linked receptors, retinal neurons, and neural cells in the visual cortex of the brain which enable interaction between perception and past experience.
  centerpoint-The optical center of a photograph.
  change-detection images-A difference image prepared by digitally comparing images acquired at different times. The gray tones or colors of each pixel record the amount of difference between the corresponding pixels of the original images.
  charge-coupled detector (CCD)-A device in which electron are stored at the surface of a semiconductor.
  chlorosis-Yellowing of plant leaves resulting from an imbalance in the iron metabolism caused by excess concentrations of copper, zinc, manganese, or other elements in the plant.
  chromatic vision-The perception by the human eye of changes in hue.
  circular scanner-Scanner in which a faceted mirror rotates about a vertical axis to sweep the detector IFOV in a series of circular scan lines on the terrain.
  classification-Process of assigning individual pixels of an image to categories, generally on the basis of spectral reflectance characteristics.
  coastal zone color scanner (CZCS)-A satellite-carried multi-spectral scanner designed to measure chlorophyll concentrations in the oceans.
  coherent radiation-Electromagnetic radiation whose waves are equal in length and are in phase, so that waves at different points in space act in unison, as in laser and synthetic aperture radar.
  color composite image-Color image prepared by projecting individual black-and-white multispectral images, each through a different color filter. When the projected images are superposed, a color composite image results.
  color ratio composite image-Color composite image prepared by combining individual ratio images for a scene using a different color for each ratio image.
  complementary colors-Two primary colors of light (one additive and the other subtractive) that produce white light when added together. Red and cyan are complimentary colors.
  computer-compatible tape (CCT)-The magnetic tape on which the digital data for Landsat MSS and TM images are distributed.
  conduction-Transfer of electromagnetic energy through a solid material by molecular interaction.
  cones-Receptors in the retina which are sensitive to colour. There are cones sensitive to the red, green, and blue components of light.
  contact print-A reproduction from a photographic negative in direct contact with photosensitive paper.
  context-The known environment of a particular feature on an image.
  contrast-The ratio between the energy emitted or reflected by an object and its immediate surroundings.
  contrast enhancement-Image-processing procedure that improves the contrast ratio of images. The original narrow range of digital values is expanded to utilize the full range of available digital values.
  contrast ratio-On an image, the ratio of reflectances between the brightest and darkest parts of an image.
  contrast stretching-Expanding a measured range of digital numbers in an image to a larger range, to improve the contrast of the image and its component parts.
  convection-Transfer of heat through the physical movement of heated matter.
  corner reflector-Cavity formed by two or three smooth planar surfaces intersecting at right angles. Electromagnetic waves entering a corner reflector are reflected directly back toward the source.
  COSMIC-Computer Software Management and Information Center, University of Georgia. This facility distributes computer programs developed by U.S. government-funded projects.
  cross-polarized-Describes a radar pulse in which the polarization direction of the return is normal to the polarization direction of the transmission. Cross-polarized images may be HV (horizontal transmit, vertical return) or VH (vertical transmit, horizontal return).
  cross-track scanner-Scanner in which a faceted mirror rotates about a horizontal axis to sweep the detector IFOV in a series of parallel scan lines oriented normal to the flight direction.
  CRT-Cathode ray tube.
  cut off-The digital number in the histogram of a digital image which is set to zero during contrast stretching. Usually this is a value below which atmospheric scattering makes a major contribution.
  cycle-One complete oscillation of a wave.
  CZCS-Coastal Zone color scanner.

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# D

  data collection system (DCS)-On Landsats 1 and 2, the system that acquired information from seismometers, flood gauges, and other measuring devices. These data were relayed to ground receiving stations.
  densitometer-Optical device for measuring the density of photographic transparencies.
  density, of images-Measure of the opacity, or darkness, of a negative or positive transparency.
  density, of materials (r)-Ratio of mass to volume of a material, typically expressed as grams per cubic centimeter.
  density slicing-Process of converting the continuous gray tones of an image into a series of density intervals, or slices, each corresponding to a specific digital range. The density slices are then displayed either as uniform gray tones or as colors.
  depolarized-Refers to a change in polarization of a transmitted radar pulse as a result of various interactions with the terrain surface.
  depression angle (y)-In radar, the angle between the imaginary horizontal plane passing through the antenna and the line connecting the antenna and the target.
  detectability-Measure of the smallest object that can be discerned on an image.
  detector-Component of a remote sensing system that converts electromagnetic radiation into a recorded signal.
  developing-Chemical processing of an exposed photographic emulsion to produce an image.
  dielectric constant-Electrical property of matter that influences radar returns. Also referred to as complex dielectric constant.
  difference image-Image prepared by subtracting the digital values of pixels in one image from those in a secon image to produce a third set of pixels. This third set is used to form the difference image.
  diffuse reflector-Surface that reflects incident radiation nearly equally in all directions.
  digital display-A form of data display in which values are shown as arrays of numbers.
  digital image-An image where the property being measured has been converted from a continuous range of analogue values to a range expressed by a finite number of integers, usually recorded as binary codes from 0 to 255, or as one byte.
  digital image processing-Computer manipulation of the digital-number values of an image.
  digital number (DN)-Value assigned to a pixel in a digital image.
  digitization-Process of converting an analog display into a digital display.
  digitizer-Device for scanning an image and converting it into numerical format.
  directional filter-Mathematical filter designed to enhance on an image those linear features oriented in a particular direction.
  distortion-On an image, changes in shape and position of objects with respect to their true shape and position.
  diurnal-Daily.
  Doppler principle-Describes the change in observed frequency that electromagnetic or other waves undergo as a result of the movement of the source of waves relative to the observer.
  Doppler shift-A change in the observed frequency of EM or other waves caused by the relative motion between source and detector. Used principally in the generation of synthetic-aperture radar images.
  dwell time-Time required for a detector IFOV to sweep across a ground resolution cell.

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# E

  EDC-EROS Data Center.
  edge-A boundary in an image between areas with different tones.
  edge enhancement-Image-processing technique that emphasizes the appearance of edges and lines.
  Ektachrome-A Kodak color positive film.
  electromagnetic radiation-Energy propagated in the form of and advancing interaction between electric and magnetic fields. All electromagnetic radiation moves at the speed of light.
  electromagnetic spectrum-Continuous sequence of electromagnetic energy arranged according to wavelength or frequency.
  emission-Process by which a body radiates electromagnetic energy. Emission is determined by kinetic temperature and emissivity.
  emissivity (e )- Ratio of radiant flux from a body to that from a blackbody at the same kinetic temperature and emissivity.
  emittance-A term for the radiant flux of energy per unit area emitted by a body. (Now obsolete).
  emulsion-Suspension of photosensitive silver halide grains in gelatin that constitutes the image-forming layer on photographic film.
  energy flux-Radiant flux.
  enhancement-Process of altering the appearance of an image so that the interpreter can extract more information.
  EOSAT-The commercial company that took over operations of the Landsat system in 1985.
  ERBSS-Earth Radiation Budget Sensor System, carried by NOAA satellites.
  EREP-Earth Resources Experiment Package, carried on Skylab and consisting of cameras and multispectral scanner.
  EROS-Earth Resources Observation System.
  EROS Data Center (EDC)-Facility of the U.S. Geological Survey at Sioux Falls, South Dakota, that archives, processes, and distributes images.
  ERTS-Earth Resource Technology Satellite, now called Landsat.
  ESA- European Space Agency, based in Paris. A consortium between several European states for the development of space science, including the launch of remote-sensing satellites.
  ETC-Earth-terrain camera.
  Evaporative cooling-Temperature drop caused by evaporation of water from a moist surface.

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# F

  false colour image-A colour image where parts of the non-visible EM spectrum are expressed as one or more of the red, green, and blue components, so that the colours produced by the Earth's surface do not correspond to normal visual experience. Also called a false-colour composite (FCC). The most commonly seen false-colour images display the very-near infrared as red, red as green, and green as blue.
  false color photograph-Another term for IR color photograph.
  far range-The portion of a radar image farthest from the aircraft or spacecraft flight path.
  film-Light-sensitive photographic emulsion and its base.
  film speed-Measure of the sensitivity of photographic film to light. Larger numbers indicate higher sensitivity.
  filter, digital-Mathematical procedure for modifying values of numerical data.
  filter, optical-A material that, by absorption or reflection, selectivity modifies the radiation transmitted through an optical system.
  flight path-Line on the ground directly beneath a remote sensing aircraft or space craft. Also called flight line.
  fluorescence-Emission of light from a substance following exposure to radiation from an external source.
  f-number-Representation of the speed of a lens determined by the focal length divided by diameter of the lens. Smaller numbers indicate faster lenses.
  focal length-In cameras, the distance from the optical center of the lens to the plane at which the image of a very distant object is brought into focus.
  foreshortening-A distortion in radar images causing the lengths of slopes facing the antenna to appear shorter on the image than on the ground. It is produced when radar wavefronts are steeper than the topographic slope.
  format-Size of an image.
  forward overlap-The percent of duplication by successive photographs along a flight line.
  fovea-The region around that point on the retina intersected by the eye's optic axis, where receptors are most densely packed. It is the most sensitive part of the retina.
  frequency (v )-The number of wave oscillations per unit time or the number of wavelengths that pass a point per unit time.
  f-stop-Focal length of a lens divided by the diameter of the len's adjustable diaphragm. Smaller numbers indicate larger openings, which admit more light to the film.

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# G

  GCP-Ground-control point.
  Gemini-U.S. program of two-man earth-orbiting spacecraft in 1965 and 1966.
  geographic information system (GIS)-A data-handling and analysis system based on sets of data distributed spatially in two dimensions. The data sets may be map oriented, when they comprise qualitative attributes of an area recorded as lines, points, and areas often in vector format, or image oriented, when the data are quantitative attributes referring to cells in a rectangular grid usually in raster format. It is also known as a geobased or geocoded information system.
  geometric correction-Image-processing procedure that corrects spatial distortions in an image.
  geostationary-Refers to satellites traveling at the angular velocity at which the earth rotates; as a result, they remain above the same point on earth at all times.
  Geostationary Operational Environmental Satellite-a NOAA satellite that acquires visible and thermal IR images for meteorologic purposes.
  geostationary orbit-An orbit at 41 000 km in the direction of the Earth's rotation, which matches speed so that a satellite remains over a fixed point on the Earth's surface.
  geothermal-Refers to heat from sources within the earth.
  Goddard Space Flight Center-The NASA facility at Greenbelt, Maryland, that is also a Landsat ground receiving station.
  GMT-Greenwich mean time. This international 24-h system is used to designate the time at which Landsat images are acquired.
  GOES-Geostationary Operational Environmental Satellite.
  gossan-Surface occurrence of iron oxide formed by the weathering of metallic sulfide ore minerals.
  granularity-Graininess of developed photographic film that is determined by the texture of the silver grains.
  gray scale-A sequence of gray tones ranging from black to white.
  grid format-The result of interpolation from values of a variable measured at irregularly distributed points, or along survey lines, to values referring to square cells in a rectangular array. It forms a step in the process of contouring data, but can also be used as the basis for a raster format to be displayed and analyzed digitally after the values have been rescaled to the 0-255 range.
  ground-control point-A geographic feature of known location that is recognizable on images and can be used to determine geometric corrections.
  ground range-On radar images, the distance from the ground track to an object.
  ground-range image-Radar image in which the scale in the range direction is constant.
  ground receiving station-Facility that records data transmitted by a satellite, such as Landsat.
  ground resolution cell-Area on the terrain that is covered by the IFOV of a detector.
  ground swath-Width of the strip of terrain that is imaged by a scanner system.
  GSFC-Goddard Space Flight Center

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# H

  harmonic-Refers to waves in which the component frequencies are whole- number multiples of the fundamental frequency.
  HCMM-Heat Capacity Mapping Mission, the NASA satellite launched in 1978 to observe thermal properties of rocks and soils. It remained in orbit for only a few months.
  heat capacity-(c ) Ratio of heat absorbed or released by a material to the corresponding temperature rise or fall. Expressed in calories per gram per degree centigrade. Also called thermal capacity.
  Heat Capacity Mapping Mission (HCMM)-NASA satellite orbited in 1978 to record daytime and nighttime visible and thermal IR images of large areas.
  highlights-Areas of bright tone on an image.
  high-pass filter-A spatial filter which selectively enhances contrast variations with high spatial frequencies in an image. It improves the sharpness of images and is a method of edge enhancement.
  HIRIS-High Resolution Imaging Spectrometer, possibly to be carried by the Space Shuttle.
  HIRS-High Resolution Infrared Spectrometer, carried by NOAA satellites.
  histogram-A means of expressing the frequency of occurrence of values in a data set within a series of equal ranges or bins, the height of each bin representing the frequency at which values in the data set fall within the chosen range. A cumulative histogram expresses the frequency of all values falling within a bin and lower in the range. A smooth curve derived mathematically from a histogram is termed the probability density function (PDF).
  hue-In the IHS system, represents the dominant wavelength of a color.

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# I

  IFOV-Instantaneous field of view.
  IHS-Intensity, hue, and saturation system of colors.
  image-pictorial representation of a scene recorded by a remote sensing system. Although image is a general term, it is commonly restricted to representations acquired by non-photographic methods.
  image dissection-The breaking down of a continuous scene into discrete spatial elements, either by the receptors on the retina, or in the process of capturing the image artificially.
  image striping-A defect produced in line scanner and pushbroom imaging devices produced by the non-uniform response of a single detector, or amongst a bank of detectors. In a line-scan image the stripes are perpendicular to flight direction, but parallel to it in a pushbroom image.
  image swath-See ground swath.
  incidence angle-In radar, the angle formed between an imaginary line normal to the surface and another connecting the antenna and the target.
  incident energy-Electromagnetic radiation impinging on a surface.
  index of refraction (n) -Ratio of the wavelength or velocity of electromagnetic radiation in a vacuum to that in a substance.
  instantaneous field of view (IFOV)- Solid angle through which a detector is sensitive to radiation. In a scanning system, the solid angle subtended by the detector when the scanning motion is stopped.
  intensity-In the IHS system, brightness ranging from black to white.
  interactive processing-Method of image processing in which the operator views preliminary results and can alter the instructions to the computer to achieve desired results.
  interpretation-The process in which a person extracts information from an image.
  interpretation key-Characteristic or combination of characteristics that enable an interpreter to identify an object on an image.
  IR-Infrared region of the electromagnetic spectrum that includes wavelengths from 0.7µm to 1 mm.
  IR color photograph-Color photograph in which the red-imaging layer is sensitive to photographic IR wavelengths, the green-imaging layer is sensitive to red light, and the blue-imaging layer is sensitive to green light. Also known as camouflage detection photographs and false-color photographs.
  ISO index- Index of the International Standards Organization, designating film speed in photography. Higher values indicate higher sensitivity.
  isotherm-Contour line connecting points of equal temperature. Isotherm maps are used to portray surface-temperature patterns of water bodies.

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# J

  Johnson Space Flight Center-A NASA facility in Houston, Texas.
  JPL-Jet Propulsion Laboratory, a NASA facility at Pasadena, California, operated under contract by the California Institute of Technology.

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# K

  Ka band-Radar wavelength region from 0.8 to 1.1 cm.
  kernel-Two-dimensional array of digital numbers used in digital filtering.
  kinetic energy-The ability of a moving body to do work by virtue of its motion. The molecular motion of matter is a form of kinetic energy.
  kinetic temperature-Internal temperature of an object determined by random molecular motion. Kinetic temperature is measured with a contact thermometer.
  Kodachrome-A Kodak color positive film.

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# L

  LACIE-Large Area Crop Inventory Experiment.
  Landsat-A series of unnamed earth-orbiting NASA satellites that acquire multispectral images in various visible and IR bands.
  Laplacian filter-A form of nondirectional digital filter.
  large-format camera (LFC)-An experiment first carried on the Space Shuttle in October 1984.
  laser-Light artificially stimulated electromagnetic radiation: a beam of coherent radiation with a single wavelength.
  latent image- Invisible image produced by the photochemical effect of light on silver halide grains in the emulsion of film. The latent image is not visible until after photographic development.
  layover-In radar images, the geometric displacement of the top of objects toward the near range relative to their base.
  L band-Radar wavelength region from 15 to 30 cm.
  lens-One or more pieces of glass or other transparent material shaped to form an image by refraction of light.
  LFC-Large-format camera.
  lidar-Light intensity detection and ranging, which uses lasers to stimulate fluorescence in various compounds and to measure distances to reflecting surfaces.
  light-Electromagnetic radiation ranging from 0.4 to 0.7µm in wavelength that is detectable by the human eye.
  light meter-Device for measuring the intensity of visible radiation and determining the appropriate exposure of photographic film in a camera.
  lineament-Linear topographic or tonal feature on the terrain and on images, maps, and photographs that may represent a zone of structural weakness.
  linear-Adjective that describes the straight line-like nature of features on the terrain or on images and photographs.
  lineation-The one-dimensional alignment of internal components of a rock that cannot be depicted as an individual feature on a map.
  line drop out-The loss of data from a scan line caused by malfunction of one of the detectors in a line scanner.
  line-pair-Pair of light and dark bars of equal widths. The number of such line-pairs aligned side by side that can be distinguished per unit distance expresses the resolving power of an imaging system.
  line scanner-An imaging device which uses a mirror to sweep the ground surface normal to the flight path of the platform. An image is built up as a strip comprising lines of data.
  look angle-The angle between the vertical plane containing a radar antenna and the direction of radar propagation. Complementary to the depression angle.
  look direction-Direction in which pulses of microwave energy are transmitted by a radar system. The look direction is normal to the azimuth direction. Also called range direction.
  look-up table (LUT)-A mathematical formula used to convert one distribution of data to another, most conveniently remembered as a conversion graph.
  low-sun-angle photograph-Aerial photograph acquired in the morning, evening, or winter when the sun is at a low elevation above the horizon.
  luminance-Quantitative measure of the intensity of light from a source.

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# M

  Mach band-An optical illusion of dark and light fringes within adjacent areas of contrasted tone. It is a psychophysiological phenomenon which aids human detection of boundaries or edges.
  median filter-A spatial filter, which substitutes the median value of DN from surrounding pixels for that recorded at an individual pixel. It is useful for removing random noise.
  Mercury-U.S. program of one-man, earth-orbiting spacecraft in 1962 and 1963.
  microwave-Region of the elctromagnetic spectrum in the wavelength range of 0.1 to 30 cm.
  mid-infrared (MIR)-The range of EM wavelengths from 8 to 14 µm dominated by emission of thermally generated radiation from materials; also known as thermal infrared.
  Mie scattering-The scattering of EM energy by particles in the atmosphere with comparable dimensions to the wavelength involved.
  minimum ground separation-Minimum distance on the ground between two targets at which they can be resolved on an image.
  minus-blue photographs-Black-and-white photographs acquired using a filter that removes blue wavelengths to produce higher spatial resolution.
  mixed pixel-A pixel whose DN represents the average energy reflected or emitted by several types of surface present within the area that it represents on the ground; sometimes called a mixel.
  modular optoelectric multispectral scanner (MOMS)-An along-track scanner carried on the Space Shuttle that recorded two bands of data.
  modulate-To vary the frequency, phase, or amplitude of electromagnetic waves.
  modulation transfer function (MTF)-A method of describing spatial resolution.
  MOMS-Modular optoelectric multispectral scanner.
  MOS-1-Marine Observation Satellite, launched by Japan in 1987.
  mosaic-Composite image or photograph made by piecing together individual images or photographs covering adjacent areas.
  MSS-Multispectral scanner system of Landsat that acquires images of four wavelength bands in the visible and reflected IR regions.
  multiband camera-System that simultaneously acquires photographs of the same scene at different wavelengths.
  multispectral classification-Identification of terrain categories by digital processing of data acquired by multispectral scanners.
  multispectral scanner-Scanner system that simultaneously acquires images of the same scene at different wavelengths.

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# N

  nadir-Point on the ground directly in line with the remote sensing system and the center of the earth.
  NASA-National Aeronautical and Space Administration.
  near infrared (NIR)-The shorter wavelength range of the infrared region of the EM spectrum, from 0.7 to 2.5 µm. It is often divided into very-near infrared (VNIR) covering the range accessible to photographic emulsions (0.7 to 1.0m), and the short-wavelength infrared (SWIR) covering the remainder of the NOR atmospheric window from 1.0 to 2.5m.
  near range-Refers to the portion of a radar image closest to the aircraft or satellite flight path.
  negative photograph-Photograph on film or paper in which the relationship between bright and dark tones is the reverse of that of the features on the terrain.
  NHAP-National High Altitude Photography program of the U.S. Geological Survey.
  NOAA-National Oceanic and Atmospheric Administration.
  noise-Random or repetitive events that obscure or interfere with the desired information.
  nondirectional filter-Mathematical filter that treats all orientations of linear features equally.
  non-selective scattering-The scattering of EM energy by particles in the atmosphere which are much larger than the wavelengths of the energy, and which causes all wavelengths to be scattered equally.
  non-spectral hue-A hue which is not present in the spectrum of colours produced by the analysis of white light by a prism of diffraction grating. Examples are brown, magenta, and pastel shades.
  nonsystematic distortion-Geometric irregularities on images that are not constant and cannot be predicted from the characteristics of the imaging system.
  normal color film-Film in which the colors are essentially true representations of the colors of the terrain.
  NSSDC-National Space Science Data Center.

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# O

  oblique photograph-Photograph acquired with the camera intentionally directed at some angle between horizontal and vertical orientations.
  OMS-Orbital maneuvering system.
  orbit-Path of a satellite around a body such as the earth, under the influence of gravity.
  orthophotograph-A vertical aerial photograph from which the distortions due to varying elevation, tilt, and surface topography have been removed, so that it represents every object as if viewed directly from above.
  orthophotoscope-An optical-electronic device which converts a normal vertical aerial photograph to an orthophotograph.
  overlap-Extent to which adjacent images or photographs cover the same terrain, expressed as a percentage.

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# P

  panchromatic film-Black and white film that is sensitive to all visible wavelengths.
  parallax-Displacement of the position of a target in an image caused by a shift in the observation system.
  parallax difference-The difference in the distance on overlapping vertical photographs between two points, which represent two locations on the ground with different elevations.
  parallel-polarized-Describes a radar pulse in which the polarization of the return is the same as that of the transmission. Parallel-polarized images may be HH (horizontal transmit, horizontal return) or VV (vertical transmit, vertical return).
  pass-In digital filters, refers to the spatial frequency of data transmitted by the filter. High-pass filters transmit high-frequency data; low-pass filters transmit low-frequency data.
  passive microwaves-Radiation in the 1 mm to 1 m range emitted naturally by all materials above absolute zero.
  passive remote sensing-Remote sensing of energy naturally reflected or radiated from the terrain.
  path-and-row index-System for locating Landsat MSS and TM images.
  pattern-Regular repetition of tonal variations on an image or photograph.
  periodic line dropout-Defect on Landsat MSS or TM images in which no data are recorded for every sixth or sixteenth scan line, causing a black line on the image.
  periodic line striping-Defect on Landsat MSS or TM images in which every sixth or sixteenth scan line is brighter or darker than the others. Caused by the sensitivity of one detector being higher or lower than the others.
  photodetector-Device for measuring energy in the visible-light band.
  photogeology-Mapping and interpretation of geologic features from aerial photographs.
  photograph-Representation of targets on film that results from the action of light on silver halide grains in the film's emulsion.
  photographic IR-Short-wavelength portion (0.7 to 0.9 µm) of the IR band that is detectable by IR color film or IR black-and-white film.
  photographic UV-Long-wavelength portion of the UV band (0.3 to 0.4 µm) that is transmitted through the atmosphere and is detectable by film.
  photomosaic-Mosaic composed of photographs.
  photon-Minimum discrete quantity of radiant energy.
  photopic vision-Vision under conditions of bright illumination.
  picture element-In a digitized image, the area on the ground represented by each digital number. Commonly contracted to pixel.
  pitch-Rotation of an aircraft about the horizontal axis normal to its longitudinal axis that causes a nose-up or nose-down attitude.
  pixel-Contraction of picture element.
  Planck's Law-An expression for the variation of emittance of a blackbody at a particular temperature as a function of wavelength.
  point spread function (PSF)- The image of a point source of radiation, such as a star, collected by an imaging device. A measure of the spatial fidelity of the device.
  polarization-The direction of orientation in which the electrical field vector of electromagnetic radiation vibrates.
  polar orbit-An orbit that passes close to the poles, thereby enabling a satellite to pass over most of the surface, except the immediate vicinity of the poles themselves.
  polarized radiation-Electromagnetic radiation in which the electrical field vector is contained in a single plane, instead of having random orientation relative to the propagation vector. Most commonly refers to radar images.
  positive photograph-Photographic image in which the tomes are directly proportional to the terrain brightness.
  previsual symptom-A vegetation anomaly that is recognizable on IR film before it is visible to the naked eye or on normal color photographs. It results when stressed vegetation loses its ability to reflect photographic IR energy and its recognizable on IR color film by a decrease in brightness of the red hues.
  primary colors-A set of three colors that in various combinations will produce the full range of colors in the visible spectrum. There are two sets of primary colors, additive and subtractive.
  principal component analysis-The analysis of covariance in a multiple data set so that the data can be projected as additive combinations on to new axes, which express different kinds of correlation among the data.
  principal-component (PC) image-Digitally processed image produced by a transformation that recognizes maximum variance in multispectral images.
  principal point-Optical center of an aerial photograph.
  printout-Display of computer data in alphanumeric format.
  probability density function (PDF)-A function indicating the relative frequency with which any measurement may be expected to occur. In remote sensing it is represented by the histogram of DN in one band for a scene.
  pulse-Short burst of electromagnetic radiation transmitted by a radar antenna.
  pulse length-Duration of a burst of energy transmitted by a radar antenna, measured in microseconds.
  pushbroom scanner-An alternate term for an along-track scanner
  pushbroom system-An imaging device consisting of a fixed linear array of many sensors which is swept across an area by the motion of the platform, thereby building up an image. It relies on sensors whose response and reading is nearly instantaneous, so that the image swathe can be segmented into pixels representing small dimensions on the ground.

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# Q

  quantum-The elementary quantity of EM energy that is transmitted by a particular wavelength. According to the quantum theory, EM radiation is emitted, transmitted, and absorbed as numbers of quanta, the energy of each quantum being a simple function of the frequency of the radiation.

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# R

  radar-Acronym for radio detection and ranging. Radar is an active form of remote sensing that operates in the microwave and radio wavelength regions.
  radar altimeter-A non-imaging device that records the time of radar returns from vertically beneath a platform to estimate the distance to and hence the elevation of the surface; carried by Seasat and the EAS-ERS-1 platforms.
  radar cross section-A measure of the intensity of backscattered radar energy from a point target. Expressed as the area of a hypothetica surface which scatters radar equally in all directions and which would return the same energy to the antenna.
  radar scattering coefficient-A measure of the back-scattered energy from a target with a large area. Expressed as the average radar cross section per unit area in decibels (dB). It is the fundamental measure of the radar properties of a surface.
  radar scatterometer-A non-imaging device that records radar energy backscattered from terrain as a function of depression angle.
  radar shadow-Dark signature on a radar image representing no signal return. A shadow extends in the far-range direction form an object that intercepts the radar beam.
  radial relief displacement-The tendency of vertical objects to appear to learn radially away from the center of a vertical aerial photograph. Caused by the conical field of view of the camera lens.
  radian-Angle subtended by an arc of a circle equal in length to the radius of the circle 1 rad = 57.3¡.
  radiant energy peak-Wavelength at which the maximum electromagnetic energy is radiated at a particular temperature.
  radiant flux-Rate of flow of electromagnetic radiation measured in watts per square centimeter.
  radiant temperature-Concentration of the radiant flux from a material. Radiant temperature is the kinetic temperature multiplied by the emissivity to the one-fourth power.
  radiation-Propagation of energy in the form of electromagnetic waves.
  radiometer-Device for quantitatively measuring radiant energy, especially thermal radiation.
  random line dropout-In scanner images, the loss of data from individual scan lines in a nonsystematic fashion.
  range-In radar usage this is the distance in the direction of radar propagation, usually to the side of the platform in an imaging radar system. The slant range is the direct distance from the antenna to the object, whereas the distance from the ground track of the platform to the object is termed the ground range.
  range direction-See look direction.
  range resolution-In radar images, the spatial resolution in the range direction, which is determined by the pulse length of the transmitted microwave energy.
  raster-The scanned and illuminated area of a video display, produced by a modulated beam of electrons sweeping the phosphorescent screen line by line from top to bottom at a regular rate of repetition.
  raster format-A means of representing spatial data in the from of a grid of DN, each line of which can be used to modulate the lines of a video raster.
  raster pattern-Pattern of horizontal lines swept by an electron beam across the face of a CRT that constitute the image display.
  ratio image-An image prepared by processing digital multi-spectral data as follows: for each pixel, the value for one band is divided by that of another. The resulting digital values are displayed as an image.
  Rayleigh criterion-In radar, the relationship between surface roughness, depression angle, and wavelength that determines whether a surface will respond in a rough or smooth fashion to the radar pulse.
  Rayleigh scattering-Selective scattering of light in the atmosphere by particle that are small compared with the wavelength of light.
  RBV-Return-beam vidicon.
  real-aperture radar-Radar system in which azimuth resolution is determined by the transmitted beam width, which is in turn determined by the physical length of the antenna and by the wavelength.
  real time-Refers to images or data made available for inspection simultaneously with their acquisition.
  recognizability-Ability to identify an object on an image.
  rectilinear-Refers to images with no geometric distortion in which the scales in the horizontal and vertical directions are identical.
  redundancy-Information on an image which is either not required for interpretation or cannot be seen. Redundancy may be spatial or spectral. The term also refers to multispectral data where the degree of correlation between bands is so high that one band contains virtually the same information as all the bands.
  reflectance-Ratio of the radiant energy reflected by a body to the energy incident on it. Spectral reflectance is the reflectance measured within a specific wavelength interval.
  reflected energy peak-Wavelength (0.5 µm) at which maximum amount of energy is reflected from the earth's surface.
  reflected IR-Electromagnetic energy of wavelengths from 0.7 µm to about 3 µm that consists primarily of reflected solar radiation.
  reflectivity-Ability of a surface to reflect incident energy.
  refraction-Bending of electromagnetic rays as they pass from one medium into another when each medium has a different index of refraction.
  registration-Process of superposing two or more images or photographs so that equivalent geographic points coincide.
  relief-Vertical irregularities of a surface.
  relief displacement-Geometric distortion on vertical aerial photographs. The tops of objects appear in the photograph to be radially displaced from their bases outward from the photograph's centerpoint.
  remote sensing-collection and interpretation of information about an object without being in physical contact with the object.
  resampling-The calculation of new DN for pixels created during geometric correction of a digital scene, based on the values in the local area around the uncorrected pixels.
  reseau marks-Pattern of small crosses added to photographs.
  resolution-Ability to separate closely spaced objects on an image or photograph. Resolution is commonly expressed as the most closely spaced line-pairs per unit distance that can be distinguished. Also called spatial resolution.
  resolution target-Series of regularly spaced alternating light and dark bars used to evaluate the resolution of images or photographs.
  resolving power-A measure of the ability of individual components. and of remote sensing systems, to separate closely spaced targets.
  reststrahlen band-In the IR region, refers to absorption of energy as a function of silica content.
  return-In radar, a pulse of microwave energy reflected by the terrain and received at the radar antenna. The strength of a return is referred to as return intensity.
  return-beam vidicon (RBV)- A system in which images are formed on the photosensitive surface o a vacuum tube; the image is scanned with an electron beam and transmitted or recorded. Landsat 3 used a pair of RBV's to acquire images.
  ringing-Fringe-like artefacts produced at edges by some forms of spatial-frequency filtering.
  rods-The receptors in the retina that are sensitive to brightness variations.
  roll-Rotation of an aircraft that causes a wing-up or wing-down attitude.
  roll compensation system-Component of an airborne scanner system that measures and records the roll of the aircraft. This information is used to correct the imagery for distortion due to roll.
  rough criterion-In radar, the relationship between surface roughness, depression angle, and wavelength that determines whether a surface will scatter the incident radar pulse in a rough or intermediate fashion.
  roughness-In radar, the average vertical relief of a small-scale irregularities of the terrain surface. Also called surface roughness.

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# S

  SAMII-Stratospheric Aerosol Measurement experiment, carried by Nimbus-7.
  SAMS-Stratospheric and Mesospheric Sounder, carried by Nimbus-7.
  satellite-An object in orbit around a celestial body.
  saturation-In the IHS system, represents the purity of color. Saturation is also the condition where energy flux exceeds the sensitivity range of a detector.
  SBUV-Solar Back-scatter Ultraviolet Instrument, carried by NOAA satellites.
  scale-Ratio of distance on an image to the equivalent distance on the ground.
  scan line-Narrow strip on the ground that is swept by IFOV of a detector in a scanning system.
  scanner-An imaging system in which the IFOV of one or more detectors is swept across the terrain.
  scanner distortion-Geometric distortion that is characteristic of cross-track scanner images.
  scan skew-Distortion of scanner images caused by forward motion of the aircraft or satellite during the time required to complete a scan.
  scattering-Multiple reflections of electromagnetic waves by particles or surfaces.
  scattering coefficient curves-Display of scatterometer data in which relative backscatter is shown as a function of incidence angle.
  scatterometer-Nonimaging radar device that quantitatively records backscatter of terrain as a function of incidence angle.
  scene-Area on the ground that is covered by an image or photograph.
  scotopic vision-Vision under conditions of low illumination, when only the rods are sensitive to light. Visual acuity under these conditions is highest in the blue part of the spectrum.
  Seasat-NASA unmanned satellite that acquired L-band radar images in 1978.
  sensitivity-Degree to which a detector responds to electromagnetic energy incident on it.
  sensor-Device that receives electromagnetic radiation and converts it into a signal that can be recorded and displayed as either numerical data or an image.
  Shuttle imaging radar (SIR)-L-band radar system deployed on the Space Shuttle.
  sidelap-Extent of lateral overlap between images acquired on adjacent flight lines.
  side-looking airborne radar (SLAR)-An airborne side scanning system for acquiring radar images.
  side-scanning sonar-Active system for acquiring images of the seafloor using pulsed sound waves.
  side-scanning system-A system that acquires images of a strip of terrain parallel with the flight or orbit path but offset to one side.
  signal-Information recorded by a remote sensing system.
  signal to noise radio (S/N)-The ratio of the level of the signal carrying real information to that carrying spurious information as a result of defects in the system.
  silver halide-Silver salts that are especially sensitive to visible light and convert to metallic silver when developed.
  SIR-Shuttle Imaging Radar, synthetic-aperture radar experiments carried aboard the NASA Space Shuttle in 1981 and 1984.
  Skylab-U.S.earth-orbiting workshop that housed three crews of three astronauts in 1973 and 1974.
  skylight-Component of light that is strongly scattered by the atmosphere and consists predominantly of shorter wavelengths.
  slant range-In radar, an imaginary line running between the antenna and the target.
  slant-range distance-Distance measured along the slant range.
  slant-range distortion-Geometric distortion of a slant-range image.
  slant-range image-In radar, an image in which objects are located at positions corresponding to their slant-range distances from the aircraft path. On slant-range images, the scale in the range direction is compressed in the near-range region
  SLAR-Side-looking airborne radar.
  SMIRR-Shuttle Multispectral Infrared Radiometer, a non-imaging spectroradiometer carried by the NASA Space Shuttle covering ten narrow wavebands in the 0.5-2.4 m range.
  SMMR-Scanning Multichannel Microwave Radiometer, carried by Nimbus-7.
  smooth criterion-In radar, the relationship between surface roughness, depression angle, and wavelength that determines whether a surface will scatter the incident radar pulse in a smooth or intermediate fashion.
  software-Programs that control computer operations.
  sonar-Acronym for sound navigation ranging. Sonar is an active form of remote sensing that employs sonic energy to image the seafloor.
  Space Shuttle-U.S. manned satellite program in the 1980s, officially called the Space Transportation System (STS).
  Space Station-A planned series of three polar-orbiting, sun-synchronous satellites to be launched by NASA, the European Space Agency, and the Japanese Space Agency in the 1990s. They will carry a large range of remote-sensing devices.
  spatial-frequency filtering-The analysis of the spatial variations in DN of an image and the separation or suppression of selected frequency ranges.
  specific heat-The ratio of the heat capacity of unit mass of a material to the heat capacity of unit mass of water.
  spectral hue-A hue which is present in the spectral range of white light analysed by a prism or diffraction grating.
  spectral reflectance-Reflectance of electromagnetic energy at specified wavelength intervals.
  spectral sensitivity-Response, or sensitivity, of a film or detector to radiation in different spectral regions.
  spectral vegetation index-An index of relative amount and vigor of vegetation. The index is calculated from two spectral bands of AVHRR imagery.
  spectrometer-Device for measuring intensity of radiation absorbed or reflected by a materiel as a function of wavelength.
  spectroradiometer-A device which measures the energy reflected or radiated by materials in narrow EM wavebands.
  spectrum-Continuous sequence of electromagnetic energy arranged according to wavelength or frequency.
  specular-Refers to a surface that is smooth with respect to the wavelength of incident energy.
  SPOT-Systeme Probatoire d'Observation del la Terre. Unmanned French remote sensing satellite orbiting in the late 1980s.
  Stefan-Boltzmann constant- 5.68 x 10 -12 W . cm-2 .K-4.
  Stefan-Boltzmann law-States that radiant flux of a blackbody is equal to the temperature to the fourth power times the Stefan-Boltzmann constant.
  Stereo base-Distance between a pair of correlative points on a stereo pair that are oriented for stereo viewing.
  stereo model-Three-dimensional visual impression produced by viewing a pair of overlapping images through a stereoscope.
  stereo pair-Two overlapping images or photographs that may be viewed stereoscopically.
  stereopsis-The ability for objects to be perceived in three dimensions as a result of the parallax differences produced by the eye base.
  stereoscope-Binocular optical device for viewing overlapping images or diagrams. The left eye sees only the left image, and the right eye sees only the right image.
  SSU-Stratosphere Sounding Unit, carried by NOAA-series satellites.
  subscene-A portion of an image that is used for detailed analysis.
  subtractive primary colors-Yellow, magenta, and cyan. When used as filters for white light, these colors remove blue, green and red light, respectively.
  sunglint-Bright reflectance of sunlight caused by ripples on water
  sun-synchronous-Earth satellite orbit in which the orbit plane is nearly polar and the altitude is such that the satellite passes over all places on earth having the same latitude twice daily at the same local sun time.
  sun-synchronous orbit-a polar orbit where the satellite always crosses the Equator at the same local solar time.
  supervised classification-Digital-information extraction technique in which the operator provides training-site information that the computer uses to assign pixels to categories.
  surface phenomenon-Interaction between electromagnetic radiation and the surface of a material.
  surface roughness-See roughness.
  synthetic-aperture radar (SAR)-Radar system in which high azimuth resolution is achieved by storing and processing data on the Doppler shift of multiple return pulses in such a way as to give the effect of a much longer antenna.
  synthetic stereo images-Stereo images constructed through digital processing of a single image. Topographic data are used to calculate parallax.
  system-Combination of components that constitute an imaging device.
  systematic distortion-Geometric irregularities on images that are caused by known and predictable characteristics.

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# T

  target-Object on the terrain of specific interest in a remote sensing investigation.
  TDRS-Tracking and Data Relay Satellite
  telemeter-To transmit data by radio or microwave links.
  terrain-Surface of the earth.
  texture-Frequency of change and arrangement of tones on an image.
  Thematic Mapper (TM)- A cross-track scanner deployed on Landsat that records seven bands of data from the visible through the thermal IR regions.
  thermal capacity (c )- See heat capacity.
  thermal conductivity (K)- Measure of the rate at which heat will pass through a material, expressed in calories per centimeter per second per degree Centigrade.
  thermal crossover-On a plot of radiant temperature versus time, the point at which temperature curves for two different materials intersect.
  thermal diffusivity (k)- Governs the rate at which temperature changes within a substance, expressed in centimeters squared per second.
  thermal inertia (P)-Measure of the response of a material to temperature changes, expressed in calories per square centimeter per square root of second.
  thermal IR-IR region from 3 to 14 µm that is employed in remote sensing. This spectral region spans the radiant power peak of the earth.
  thermal IR image- Image acquired by a scanner that records radiation within the thermal IR band.
  thermal IR multispectral scanner (TIMS)- Airborne scanner that acquires multispectral images within the 8-to-14mm band of the thermal IR region.
  thermal model-Mathematical expression that relates thermal and other physical properties of a material to its temperature. Models may be used to predict temperature for given properties and conditions.
  thermography-Medical applications of thermal IR images. Images of the body, called thermograms, have been used to detect tumors and monitor blood circulation.
  THIR-Temperature-Humidity Infrared Radiometer, carried by Nimbus-7.
  tie-point-A point on the ground which is common to two images. Several are used in the coregistration of images.
  TIMS-Thermal IR multispectral scanner.
  TM-Thematic mapper.
  tone-Each distinguishable shade of gray from white to black on an image.
  topographic inversion-An optical illusion that may occur on images with extensive shades. Ridges appear to be valleys, and valleys appear to be ridges. The illusion is corrected by orienting the image so that the shadows trend from the top margin of the image to the bottom.
  topographic reversal-A geomorphic phenomenon in which topographic lows coincide with structural highs and vice versa. Valleys are eroded on crests of anticlines to cause topographic lows, and synclines form ridge, or topographic highs.
  TOVS-TIROS Operational Vertical Sounder.
  Tracking and Data Relay Satellite (TDRS)-Geostationary satellite used to communicate between ground receiving stations and satellite such as Landsat.
  training area-A sample of the Earth's surface with known properties; the statistics of the imaged data within the area are used to determine decision boundaries in classification.
  trade-off-As a result of changing one factor in a remote sensing system, there are compensating changes elsewhere in the system; such a compensating change is known as a trade-off.
  training site-Area of terrain with known properties or characteristics that is used in supervised classification.
  transmissivity-Property of a material that determines the amount of energy that can pass through the material.
  transparency-Image on a transparent photographic material, normally a positive image.
  transpiration-Expulsion of water vapor and oxygen by vegetation.
  travel time-In radar, the time interval between the generation of a pulse of microwave energy and its return from the terrain.
  tristimulus colour theory-A theory of colour relating all hues to the combined effects of three additive primary colours corresponding to the sensitivities of the three types of cone on the retina.

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# U

  unsupervised classification-Digital information extraction technique in which the computer assigns pixels to categories with no instructions from the operator.
  UV-Ultraviolet region of the electromagnetic spectrum ranging in wavelengths from 0.01 to 0.4m.

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# V

  variance-A measure of the dispersion of the actual values of a variable about its mean. It is the mean of the squares of all the deviations from the mean value of a range of data.
  VAS- Atmospheric Sounder, carried by GEOS satellites
  vector format-The expression of points, lines, and areas on a map by digitized Cartesian coordinates, directions, and values.
  vegetation anomaly-Deviation from the normal distribution or properties of vegetation. Vegetation anomalies may be caused by faults, trace elements in soil, or other factors.
  vertical exaggeration-In a stereo model, the extent to which the vertical scale appears larger than the horizontal scale.
  vidicon-An imaging device based on a sheet of transparent material whose electrical conductivity increases with the intensity of EM radiation falling on it. The variation in conductivity across the plate is measured by a sweeping electron beam and converted into a video signal. Now largely replaced by cameras employing arrays of charge-coupled devices (CCDs).
  vignetting-A gradual change in overall tone of an image from the centre outwards, caused by the imaging device gathering less radiation from the periphery of its field of view than from the centre. Most usually associated with the radially increasing angel between a lens and the Earth's surface, and the corresponding decrease in the light-gathering capacity of the lens.
  visible radiation-Energy at wavelengths from 0.4 to 0.7mm that is detectable by the human eye.
  visual dissonance-The disturbing effect of seeing a familiar object in an unfamiliar setting or in an unexpected colour.
  VISSR- Visible Infrared Spin-Scan Radiometer carried by the GOES satellites.
  volume scattering-In radar, interaction between electromagnetic radiation and the interior of a material.

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# W

  watt (W)-Unit of electrical power equal to rate of work done by one ampere under a potential of one volt.
  wavelength-Distance between successive wave crests or other equivalent points in a harmonic wave.
  Wien's displacement law-Describes the shift of the radiant power peak to shorter wavelengths as temperature increases.

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# X

  X band-Radar wavelength region from 2.4 to 3.8 cm.

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# Y

  yaw-Rotation of an aircraft about its vertical axis so that the longitudinal axis deviates left or right from the flight line.

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# Z

Q uestions, C omments or P roblems?

Please send e-mail to:

jeffw@ldeo.columbia.edu

Primary Contact: Nicholas M. Short, Sr. email: nmshort@ptd.net

Glossary Author: Jeff Weissel (jeffw@ldeo.columbia.edu)