Data acquisition system: The collection of devices and media that
measures and records physical variables prior to input to a data processing
system.
Data base management system (DBMS): Computer software which enables
a data base to be organised to allow efficient sorting, updating, extracting
or retrieving of information and the generation of reports or desired output.
Data compression: Any technique that condenses the available data
so as to make data storage or transmission more efficient with minimal loss
of information.
Data processing: Application of procedures whereby data are changed
from one form into another.
Data reduction: Transformation of data values into useful, ordered,
or simplified information.
Data set: A specific collection of related data elements used for
a particular task. This may include data from many sources and in many formats.
Data tablet: A flat tablet that will output the digital position
of a pointer at any position on its surface.
Datum: A reference element, such as a line or plane, in relation
to which the position of other elements are determined; also called reference
plane or datum plane.
Debug: To isolate and remove malfunctions from a device, or mistakes
from a computer routine or program.
Decision rule: The criterion used to establish discriminant functions
for classification; eg, nearest neighbour rule, minimum-distance-to-means
rule, maximum-likelihood rule; also called classification rule.
DEM: Digital Elevation Model.
Density slicing: The process of converting the continuous grey tone
of an image into a series of density intervals, or slices, each corresponding
to a specific digital range.
Descending mode or orbit: When the active or imaging scan of a remote
sensing satellite occurs during north to south travel.
Destriping: Process of overcoming periodic striping patterns in imagery
due to miscalibration between sets of sensors in the scanning instrument.
Detector (radiation): A device providing an electrical output that
is a useful measure of incident radiation. It is broadly divisible into
two groups: thermal (sensitive to temperature changes), and photodetectors
(sensitive to changes in photon flux incident on the detector), or it may
also include antennas and film. Typical thermal detectors are thermocouples,
thermopiles, and thermistors; the latter is termed a bolometer.
Diapositive: A positive image on a transparent medium such as glass
or film.
Dielectric constant: Electrical property of matter that influences
radar returns. This is also referred to as complex dielectric constant.
Difference image: Two images of the same area are registered together
and subtracted from each other so that an image is produced which shows
any differences between the two images. The pair may be recorded at different
dates so that the difference image indicates changes over time, or may have
been processed using different parameters or algorithms.
Diffuse light: Light that does not reach the subject from a single
direction such as sunlight that has been scattered by the atmosphere or
clouds; also referred to as skylight.
Diffuse reflection: The type of reflection obtained from a relatively
rough (in terms of the wavelength of the radiation) surface, in which the
reflected rays are scattered in all directions.
Diffuse reflector: Any surface that reflects incident rays in many
directions, either because of irregularities in the surface or because the
material is optically non-homogeneous. Contrasts with specular reflector.
Digital computer: A computer that operates
on the principal of counting as opposed to measuring. Contrasts with analogue computer.
Digital data: Data displayed, recorded or stored in numeric format.
Digital Elevation Model (DEM): Gridded representation of elevation.
Digital image: An image having numeric values representing grey tones
in which each numeric value represents a different grey tone.
Digital number (DN): The value of a variable recorded for each pixel
in an image; in the range of 0-255 for 8-bit data.
Digital Terrain Model (DTM): Gridded representation of terrain height
above sea level.
Digitise: Using numeric values to represent data. Scan digitising
produces a gridded image; vector digitising records co-ordinate pairs along
selected lines.
Digitiser: Input device which can be used to trace line data and
capture x,y co-ordinates at desired intervals.
Directional filter: A spatial-frequency filter which enhances features
in an image in selected directions.
Directional reflectance: Reflectance where the incidence and collection
of radiant flux are measured for one direction only.
DISIMP: Device Independent
System for IMage Processing - a mini- computer and
workstation based image processing system developed by CSIRO Australia.
Disk: Magnetic storage medium on which information can be accessed
at random (as opposed to sequentially). Contrasts with magnetic tape.
Disk storage: A rotating plate with a magnetised surface, on which
data may be stored.
Displacement: Any shift in the position of an image on a photograph
which does not alter the perspective characteristics of the photograph (that
is, shift due to tilt of the photograph, scale change in the photograph,
and relief of the objects photographed).
Display: An output device that produces a visible representation
of a data set for quick visual access; usually the primary hardware component
is a cathode ray tube.
Distortion: (1) Any shift on the position of an image on a photograph
that alters the perspective characteristics of the photograph. Causes can
include lens aberration, differential shrinkage of film or paper, and motion
of the film or camera. (2) linear scale: Compression or expansion of the
scale of the imagery in the azimuth direction. This may be caused by an
incorrect film speed. (3) nonlinear scale: Changes in scale from one part
of the imagery to another.
Distribution function: The relative frequency with which different
values of a variable occur.
Distribution temperature: Temperature of a blackbody that best matches
the spectral distribution of the object being studied.
Dither matrix: A grouping of nibs (black or white dots) on a printer
or colour plotter to simulate greyscale intensity.
Dithering: Technique of using dither matrices to generate an expanded
colour range for continuous tone imagery.
Diurnal: Having a period of, occurring in, or related to a day.
Diurnal (thermal) wave: The daily temperature rise of the surface
soils under the heating of the sun which progresses downward as a heavily
dampened wave that dies out about 30 cm below the surface. Below this point
daily temperatures are relatively constant.
DMSP: Defense Meteorological Satellite Program.
DN: Digital number.
Doppler effect: A change in the observed frequency of electromagnetic
or other waves caused by the relative motion between the source and the
observer.
DTM: Digital Terrain Model.
Dynamic range: The ratio maximum measurable signal to minimum detectable
signal.