Accuracy
Measure of the closeness of the results of observations, computations or estimates, to the true values. It relates to the accuracy of the raw data, the exactness of the operation by which the result is obtained, and the way the data is encoded. The absolute geometric accuracy of a product corresponds to the uncertainty associated with the absolute position of any given point, or the error associated with that position (see RMS error).

Along-Track Stereo Pair
Pair of images acquired from viewing angles that vary along the flight path (as the aircraft moves forward).

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.

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.

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 be recorded in radar systems.

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.