Remote Sensing Glossary

Latest revision as of 11:45, 16 January 2014

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Active remote sensing

A remote-sensing system, such as radar or LiDAR, that produces electromagnetic radiation and measures its reflection back from a surface.

Aerial photograph

A photograph of the earth's surface taken from a platform flying above the surface but not in orbit, usually an aircraft. Aerial photography is often used as a cartographic data source for base mapping, locating geographic features, and interpreting environmental conditions.

Affine transformation

A geometric transformation that scales, rotates, skews, and/or translates images or coordinates between any two Euclidean spaces. It is commonly used in GIS to transform maps between coordinate systems. In an affine transformation, parallel lines remain parallel, the midpoint of a line segment remains a midpoint, and all points on a straight line remain on a straight line.

Albedo

A measure of the reflectivity of an object or surface; the ratio of the amount of radiation reflected by a body to the amount of energy striking it.

Anaglyph

A stereo image made by superimposing two images of the same area. The images are displayed in complementary colors, usually red and blue or green. When viewed through filters of corresponding colors, the images appear as one three-dimensional image.

Atmospheric correction

A pre-processing step for satellite imagery to remove atmospheric effects in order to enhance accuracy of surface reflectance.

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LIDAR

The acronym for Light Detection and Ranging, formally known as LaDAR (Laser Detection and Ranging)

LIDAR scanning frequency

Number of pulses or beams emitted by a LIDAR sensor per second. Modern systems support frequencies of up to 167 kHz (167,000 pulses per second).

LIDAR scanning pattern

Spatial arrangement of pulse returns that would be expected from a flat surface and depends on the mechanism used to direct pulses across the flight line. Of the four scanning patterns supported by instruments used in acquiring laser data, the seesaw pattern and its stabilized equivalent are the most common. In these two patterns, the pulse is directed across the scanning swath by an oscillating mirror, and returns are continuously generated in both directions of the scan. In the parallel line pattern, a rotating polygonal mirror directs pulses along parallel lines across the swath, and data are generated in one direction of the scan only. The elliptical pattern is generated via a rotating mirror that revolves about an axis perpendicular to the rotation plane.

LIDAR beam divergence

The increase in beam diameter that occurs as the distance between the laser instrument and a plane that intersects the beam axis increases. Typical beam divergence settings range from 0.1 to 1.0 millirad. At 0.3 millirad, the diameter of the beam at a distance of 1000 m from the instrument is approximately 30 cm. Because the total amount of pulse energy remains constant regardless of the beam divergence, at a larger beam divergence, the pulse energy is spread over a larger area, leading to a lower signal-to-noise ratio.

LIDAR scanning angle

The angle the beam axis is directed away from the “focal” plane of the LIDAR instrument The maximum angle supported by most systems does not exceed 15 degrees. The angle is recorded as positive toward the starboard and negative toward the port side of the aircraft. The combination of scanning angle and aboveground flight height determines the scanning swath.

LIDAR footprint diameter

The diameter of a beam intercepted by a plane positioned perpendicularly to the beam axis at a distance from the instrument equal to the nominal flight height. It is thus a function of both beam divergence and the above-target flight height. The distribution of pulse energy is not uniform over the extent of the footprint. It decreases radially from the center and can be approximated by a two-dimensional Gaussian distribution.

LIDAR pulse length

The duration of the pulse, in nanoseconds (ns). Along with discretization settings, it determines the range resolution of the pulse in multiple return systems, or the minimum distance between consecutive returns from a pulse.

LIDAR pulse density

The maximum number of individual returns that can be extracted from a single beam. Certain systems can identify either the first or the first and last returns. Most modern systems can identify multiple returns (e.g., up to five) from a single beam.

LIDAR footprint spacing

The nominal distance between the centers of consecutive beams along and between the scanning lines, which, along with the beam divergence, determines the spatial resolution of LIDAR data. The footprint spacing is a function of scanning frequency, the aboveground flight height, and the velocity of the aircraft.

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Pixel

The smallest spatial unit in a digital image. A satellite image is made up of a matrix of pixels, each having its own vector of digital values for each band.

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Radiometric resolution

Sensitivity (maximum range of values or bandwidth) of the sensor in detecting different intensities of electromagnetic radiation in single spectral bands. Typically recorded in 6 bit, 7 bit, 8 bit, 10 bit or 16 bit per pixel. The contrast of a raster image is depending on radiometric resolution.

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Spatial resolution

The size of a pixel (corresponding ground area that is covered by a pixel) that is recorded in a raster image – typically pixels correspond to square areas. The recorded spectral reflectance is integrated over a defined ground area (GIFOV = ground-projected instantaneous field of view).

Spectral resolution

Number and wavelength bandwidth of the single spectral bands recorded by a sensor.

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Temporal resolution

Temporal frequency or revisiting time of the same location on the earth surface of the sensor.

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