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ATI's Glossary of Remote Sensing Terms
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A
- absolute temperature -Temperature
measured on the Kelvin scale, whose base is absolute zero, i.e. -273¡C;
0¡C is expressed as 273K.
- 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 12.50m) 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 m.
- 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 2.4m.
- 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.
- 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.
- D
- 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 0.7m
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 0.7mm 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 micrometres 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.
- 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 m. 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.
- 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.
- 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 m) 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 m) 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.
- 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.
- 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 -Nagle
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
(0.5mm) at which maximum amount of energy is reflected from the earth's
surface.
- reflected IR -Electromagnetic
energy of wavelengths from 0.7mm to about 3mm 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.
- 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.
- 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 m 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.
- 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.
- 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 VISSR -
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.
- 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.
- X
- X band -Radar
wavelength region from 2.4 to 3.8 cm.
- Y
- yaw -Rotation
of an aircraft about its vertical axis so that the longitudinal axis deviates
left or right from the flight line.
- Z
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