Calibration: The act or process of comparing certain specific measurements
in an instrument with a standard.
Calibration data: Measurements pertaining to the spectral and/or
geometric characteristics of a sensor and/or of a radiation source. Calibration
data are obtained through the use of a fixed energy source, such as a calibration
lamp, a temperature plate, or a geometric test pattern.
Camera: A lightproof chamber or box in which the image of an exterior
object is projected onto a sensitised plate or film, through an opening
usually equipped with a lens or lenses, shutter and variable aperture.
Camera, multi-band: A camera that exposes different areas of one
film, or more than one film, through one lens and a beam splitter, or two
or more lenses equipped with different filters, to provide two or more photographs
in different spectral bands.
Camera, panoramic: A camera with a very wide angle of view, up to
horizon to horizon, usually by means of a moving (sweeping) lens.
Cardinal points: (1) optics: In a thick lens or lens system, the
two principal points, the two nodal points, and two focal points. (2) The
principal directions, north, south, east, and west.
Cartesian co-ordinates: A co-ordinate system in which the locations
of points in space are expressed by reference to three planes, called co-ordinate
planes, no two of which are parallel.
Cartography: Map and chart construction.
Cathode Ray Tube (CRT): A vacuum tube capable of producing a black-and-
white or colour image by beaming electrons onto a sensitised screen.
CCD: Charge-Coupled Device(s).
Change-detection images: Images prepared by digitally comparing two
original images acquired at different times. The grey tones of each pixel
on a change- detection, or difference, image portray the amount of difference
between the original images.
Channel: Synonymous with band.
Charge-Coupled Devices (CCD): Electronic devices used in remote sensing
detectors which are based on formation, storage and transfer of charge packets
in an array of closely spaced MIS (Metal Insulator Semiconductors) capacitors.
Chroma: The saturation or purity of a colour.
Chromaticity: Objective measurement of the dominant wavelength and
purity of a colour. This corresponds to the hue and saturation of the colour
without regard to brightness.
CIE: Commission Internationale de l'Eclairage. International body
responsible for recommendations on photometry and colorimetry.
CIE colour co-ordinate system or diagram: A system adopted by the
Commission Internationale de l'Eclairage, which quantitatively describes
the entire gamut of perceivable colours in a colour space defined by three
tristimulus values derived from the additive primaries, red, green and blue.
Class: A term which can be used to mean either a set of pixels considered
to be similar or a rule to decide if a pixel belongs to the set. The definition
of similarity varies with different classification algorithms and parameters.
Generally both the rule and the pixels constitute a 'class'. The distinction
is particularly important in those situations where the class is defined
by training patches in the training region and then extended or 'extrapolated'
to other areas.
Class seed values: A set of image values, one per channel, which
represent a class. To be representative, the seed values should be the average
of all pixels allocated to a class within the training region.
Classification: The process of assigning individual pixels of a multi-spectral
image to categories, generally on the basis of spectral reflectance characteristics.
Classifying Stage: The process of developing a set of basic feature
classes which represent the variation in an image and allocating each pixel
in an image to one of these classes.
Cluster: Homogeneous group of patterns that are very similar to one
another as determined by the distance between patterns or by their density.
Clustering algorithm: A mathematical procedure for grouping samples
on the basis of some pre-defined similarity measure.
CNES: Centre National d'Etudes Spatiales, France.
Colour: The property of an object which is dependent on the wavelength
of the light it reflects or, in the case of a luminescent body, the wavelength
of light that it emits. If, in either case, this light is of a single wavelength,
the colour seen is a pure spectral colour; but if light of two or more wavelengths
is emitted, the colour will be mixed. White light is a balanced mixture
of all the visible spectral colours.
Colour composite: A colour picture produced by assigning the three
primary colours to three selected channels of an image. For example, with
Landsat MSS data, blue is ordinarily assigned to band 4 (0.5-0.6 µm),
green to band 5 (0.6-0.7 µm), and red to band 7 (0.8-1.1µm)
to form a picture closely approximating a colour infrared photograph.
Colour display: A computer monitor which translates image data values
to colour intensity and displays them as a picture.
Colour guns: There is one colour gun in a colour display system for
each of the three additive primary colours of blue, green and red.
Colour infrared film: Photographic film sensitive to energy in the
visible and near-infrared wavelengths, generally from 0.4- 0.9 µm;
usually used with a minus-blue (yellow) filter, which results in an effective
film sensitivity of 0.5-0.9 µm. Colour infrared film is especially
useful for detecting changes in the condition of the vegetative canopy which
are often manifested in the near infrared region of the spectrum.
Colour monitor: See Colour display.
Colour space: A three-dimensional co-ordinate system which defines
a model showing colours organised in space by attributes such as hue, lightness
and saturation. A physical model of a colour space is called a colour solid.
Colour temperature: Temperature of a blackbody that has the same
(normalised) chromaticity co-ordinates as the object being studied.
Compositing: Production of a single image by using data from multiple
images. This process is applied to AVHRR imagery for cloud removal.
Computer classification: An algorithm which groups spectrally similar
pixels into classes characterised by some decision rule which may subsequently
be applied to other pixels in an image.
Cones: Receptors in the retina which are sensitive to colour. It
is believed that individual cones are sensitive to either red, green or
blue components of light.
Conformality: Map projection in which angles are preserved such as
Mercator, Lambert's conformal and stereographic; also called orthomorphic.
Contrast: The difference in light intensity between the brightest
highlights and the deepest shadow on an image.
Contrast stretching: Improving the contrast of images by digital
processing. The original range of digital values is expanded to utilise
the full contrast range of the recording film or display device.
Control, ground: (1) Control obtained by ground surveys as distinguished
from control obtained by photogrammetric methods; may be for horizontal
or vertical control, or both. (2) Ground (in-situ) observations to aid in
interpretation of remotely sensed data.
Control point: A reference point precisely located on a photograph
and on the ground which is used to assemble photographs for map compilation.
(see Part FOUR, Image Rectification and Registration).
Co-ordinates: Linear or angular quantities that designate the position
of a point in a given reference or grid system.
Co-ordinates, geographical: A system of spherical co-ordinates for
describing the positions of points on the Earth. The declinations and polar
bearings in this system are the latitudes and longitudes, respectively.
Co-ordinate system: The grid-based system for referencing locations
in spatial data from a pre-defined origin. Map co-ordinates are usually
recorded as either degrees of latitude and longitude or UTM eastings and
northings.
COSSA: CSIRO Office of Space Science and Applications.
Covariance: The measure of how two variables change in relation to
each other (covariability). If larger values of one variable tend to be
associated with larger values of the other, the covariance will be positive.
If larger values of one variable are associated with smaller values of the
other, the covariance will be negative. When there is no particular association
between the two variables, the covariance value will approach zero.
Coverage: The ground area represented on aerial photographs, photomosaics,
maps or remotely sensed imagery.
Crab: (1) aerial photography: The condition caused by failure to
orient the camera so that the axis is perpendicular to the long dimension
of the film and is parallel to the track of the aeroplane. This is indicated
in vertical photography by the sides of the photographs not being parallel
to the principal-point base line. (2) air navigation: Any turning of an
aeroplane which causes its longitudinal axis to vary from the track of the
plane.
Crossplot: An XY plot of two image channels in which pixels are plotted
at the intersection of their values in these channels.
CSIRO Australia: Commonwealth Scientific and Industrial Research
Organisation; Australia's government research organisation.
Cubic convolution: A resampling technique which uses a high order
convolution process to determine image output values.
Cultural features: Details representing man-made elements of the
landscape.
Cursor: An aiming device, such as a lens with cross-hairs, or a digitiser
or an interactive computer display.