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.

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.

Digital Surface Model (DSM)
Also known as Canopy Data . Digital representation of relief, plus significant above-ground features (e.g. buildings, vegetation), as a set of elevation values in correspondence with the cells of a map grid.

Digital Terrain Model (DTM)
Digital representation of relief (ground surface); usually a set of elevation values in correspondence
with the cells of a map grid.

Disparity
Difference between two images representing the same scene but acquired from two different viewing angles. (See also parallax). The disparity between homologous points is used to compute the elevation.

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.

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.