Absorptance (a): The ratio of the radiant flux absorbed by an object
to that incident upon it.
Absorption: The process by which radiant energy is absorbed and converted
into other forms of energy.
Absorption band: A range of wavelengths (or frequencies) in the electromagnetic
spectrum within which radiant energy is absorbed by a substance.
Absorption spectrum: The array of absorption lines and absorption
bands that results from the passage of radiant energy from a continuous
source through a selectively absorbing medium cooler than the source.
Absorptivity: The capacity of a material to absorb incident radiant
energy. A special case of absorptance, it is a fundamental property of material
that has a specular (optically smooth) surface and is sufficiently thick
to be opaque. It may be further qualified as spectral absorptivity
Absorptivity-emissivity ratio: The ratio of absorptivity of solar radiation
to infrared emissivity of a material.
Accuracy: The success in estimating the true value. The closeness
of an estimate of a characteristic to the true value of the characteristic
of the population.
Achromatic colour: A colour which occurs along the grey scale from
white to black; devoid of hue.
ACRES: Australian Centre for Remote Sensing; formerly ALS:Australian
Landsat Station.
Active microwave: Active remote sensing system operating in microwave
region of EM spectrum, usually refers to radar.
Active system: A remote sensing system that transmits its own electromagnetic
emanations at an object(s) and then records the energy reflected or refracted
back to the sensor.
Additive colour process: A method for creating essentially all colours
by adding together the light of the three additive colour primaries (blue,
green and red) in various proportions through the use of three separate
projectors. In this type of process, each primary filter absorbs the other
two primary colours and transmits only about one-third of the luminous energy
of the source. It also precludes the possibility of mixing colours with
a single light source because the addition of a secondary primary colour
results in total absorption of the light transmitted by the first colour.
Additive colour viewer: A projector used for viewing positive transparencies
obtained through multi-band photography; each image is super-imposed by
use of a different coloured light.
Additive primary colours: The spectral colours red, green and blue,
which are capable of reproducing all other colours when mixed by projection
through filters, and each of which cannot be produced by mixtures of the
other two.
ADEOS: ADvanced Earth Observation Satellite; under development by
NASDA, Japan.
Advanced Very High Resolution Radiometer (AVHRR): A multi-spectral
imaging system carried by the NOAA series of meteorological satellites.
Albedo: (1) The ratio of the amount of electromagnetic radiation
reflected by a body to the amount incident upon it, often expressed as a
percentage; eg, the albedo of the Earth is 34%. (2) The reflectivity of
a body as compared to that of a perfectly diffusing surface at the same
distance from the sun, and normal to the incident radiation.
Algorithm: (1) A fixed step-by-step procedure to accomplish a given
result; usually a simplified procedure for solving a complex problem; also
a full statement of a finite number of steps. (2) A computer-oriented procedure
for resolving a problem.
Alphanumeric: A character set composed of letters, integers, punctuation
marks, and special symbols.
Altitude: Height above a datum; the datum is usually mean sea level.
See elevation.
AMG: Australian Map Grid. The Australian implementation of UTM.
AMSR: Advanced Microwave Scanning Radiometer (proposed for EOS by
Japan).
AMSU: Advanced Microwave Sounding Unit; planned for NOAA satellites
in 1990s to replace current MSU and SSU.
Anaglyph: A stereogram in which the two views are printed super-imposed
in complementary colours; a three-dimensional image is rendered when the
stereogram is viewed through spectacles having filters out of the same two
colours, usually red and green.
Analogue: A form of data display in which values are shown in graphic
form, such as curves. Also a form of computing in which values are represented
by directly measurable quantities, such as voltages or resistances. Analog
computing methods contrast with digital methods in which values are treated
numerically.
Analogue computer: A computing machine
that works on the principle of measuring rather than counting. Contrasts
with digital computer.
Analogue-to-digital (A/D) conversion: The process of sampling continuous
analogue signals to convert them into a stream of digital values. Multi-spectral
scanner data, if collected in analogue form, undergoes this conversion prior
to digital analysis.
Analogue image: An image where continuous variation in the object
or surface being sensed is represented by continuous variation in image
tone.
Ancillary data: In remote sensing, secondary data pertaining to the
area or classes of interest, such as topographical, demographic, or climatological
data. Ancillary data may be digitised and used in the analysis process in
conjunction with the primary remote sensing data.
Angle of incidence (q): The angle between the direction of incoming
radiation and the normal to the intercepting surface.
Angle of reflection: The angle that radiation reflected from a surface
makes with the perpendicular (normal) to the surface.
Antenna: The device that radiates electromagnetic radiation from
a transmitter and receives electromagnetic radiation from antennae or other
sources.
Anti-aliasing: A filtering technique to give the appearance of smooth
lines and edges in a raster image. The technique involves use of intermediate
intensities between neighbouring pixels to soften the 'stairstep effect'
of sloped lines.
Aperture: The opening in a lens diaphragm through which light passes.
Aperture, effective: The unobstructed useful area of the orifice
through which the ray of light passes to the film to produce the image.
This is smaller than the actual area of the hole due to the effects of diffraction
or bending of the rays near the edge.
Aperture, relative: The ratio of equivalent focal length to the diameter
of the entrance 'pupil' of a photographic lens. Expressed 'f:4.5', etc.
Also called 'f:number', 'stop', 'aperture stop', 'speed'.
Apogee: The orbital point where a satellite is furthest from the
body around which it is orbiting. Contrasts with Perigee.
Application: Use of a process or methodology to achieve a particular
result. Examples of remote sensing applications include using remotely sensed
data to map water depth or monitor changes in land cover.
Arc/Info: Commercially available software package for generating
and processing geographical data in vector and tabular formats.
Artefact: A feature on an image which is introduced by the optics
of the scanning system or by image processing techniques.
Ascending mode or orbit: When the active or imaging scan of a remote
sensing satellite occurs during south to north travel.
ASCII: American Standard Code for Information Interchange. A standard
computing code for representing printable and control characters as binary
numbers.
Aspect ratio: The ratio of width by length for an image pixel as
imaged, displayed or plotted.
Atmospheric attenuation: The reduction of radiation intensity due
to absorption and/or scattering of energy by the atmosphere. This effect
is usually wavelength dependent and may affect both solar radiation travelling
to the Earth and reflected/emitted radiation travelling to the sensor from
the Earth's surface.
Atmospheric effects: See atmospheric attenuation.
Atmospheric path radiance: Radiation which has been scattered by
the atmosphere and reaches a remote sensing device without contacting the
Earth's surface.
Atmospheric windows: Those wavelength ranges in which radiation can
pass through the atmosphere with relatively little attenuation; in the optical
portion of the electromagnetic spectrum, these ranges are approximately
0.3-2.5, 3.0-4.0, 4.2-5.0, and 7.0-15.0 µm.
Attenuation: In physics, any process in which the flux density (or
power, amplitude, intensity, illuminance) of a 'parallel beam' of energy
decreases with increasing distance from the energy source.
Attitude: The angular orientation of a remote sensing system with
respect to a geographical reference system.
AUSLIG: Australian Surveying and Land Information Group; formed in
1987.
AUSSAT: Australian system of geostationary satellites used for domestic
telecommunications.
Autocorrelation: The correlation between data in one part of an image
or at one time and that at another. The spatial autocorrelation in an image
can be measured by Fourier Transforms and Variograms.
AVHRR: Advanced Very High Resolution Radiometer. A sensor on board
the NOAA series.
Azimuth: The geographical orientation of a line given as an angle
measured clockwise from north.