[an error occurred while processing this directive]
[an error occurred while processing this directive]
Remote Sensing 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, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z
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.
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.
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.
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.
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.
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.
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
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.
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 (IV)- 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.
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.
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.
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.
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.