Barry Rice

- a scientist in love with the Earth and stars -



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Remote sensing glossary

Note: Bold-faced words in definitions (like this) are in turn also defined in the glossary.

Absorption feature  
A dip in a spectrum caused by the absorption of photons by molecules interacting with a beam of light. The feature can be as simple as a gaussian, or can be much more complicated. It may be due to a reflecting opaque object, or by transmission of the light beam through a partly transparent atmosphere.

Absorption spectrum  
A continuum spectrum that displays absorption features.

Active sensors  
A remote sensing device that operates by emitting a beam of energy, and then analyzing the beam after it strikes the study object and returns.

ArcInfo  
A software package that provides great ability to modify and edit your GIS data.

ArcView  
A GIS mapping software package suitable for desktop computers.

Atmospheric windows  
Spectral bandwidths at which the Earth's atmosphere causes minimal absorption. These allows for unhindered remote sensing via satellites.

AVHRR  
Advanced Very High Resolution Radiometer, a satellite used to map the Earth with very large pixels (approximately 1 km square).

AVIRIS  
Airborne Visible InfraRed Imaging Spectrometer, a popular sensor used to obtain hyperspectral data from low-altitude or high-altitude aircraft.

Band  
A name (such as red, infrared, blue, etc.) or index number (one, two, etc.) assigned to each of the guilds of data produced by a multispectral device. Compare with channels.

Bandwidth  
The wavelength range or width characterizing a band of a multispectral sensor. For example, the bandwidth of a visible light band (0.38 to 0.8 micron boundaries) would be 0.42 microns.

Black  
The signal sent to the human brain when human eyes are unable to detect optical photons.

Channels  
The individual data streams from a multispectral device, corresponding to the many individual slices of spectral data. Compare with band.

CCD Chip  
A digital device consisting of an array of Charge Coupled Devices. Located at the image plane, it can be thought of as the digital version of film. See also CMOS chip.

CMOS Chip  
A digital image-plane chip, similar to CCD chips. The acronym stands for Complementary Metal Oxide Semiconductor. Much rarer in use than CCD chips. The differences between CCD and CMOS chips are primarily in the way the data from the individual detectors are transmitted from the data chip.

Color  
A concept created by the human brain to help interpret spectral information received by the human eyes. Applied to photons in a band as well as a monochromatic beam.

Color Composite  
An image produced from three bands or channels of data, by assigning the intensity values of each band/channel to intensities of red, green, and blue in the output image. Compare false color and true color images.

Color Infrared Film  
A film emulsion that is sensitive to a broad bandwidth that includes both optical and infrared wavelengths.

Continuum spectrum  
A spectrum shaped like a gentle broad hill, rising to a peak that is at a wavelength determined by the temperature of the opaque, radiating body that creates it.

Data cube  
A term used to describe any set of data that has three dimensions, for example temperatures at various depths (z) in a specified area (x,y) of ocean. Often used to describe the intensity of radiation in hyperspectral data measured on the Earth, where the spatial coordinates are two dimensions (x,y) and the wavelength () for the channel is the third dimension. Compare with hyperdimensional data cube.

Detector  
The portion of the sensing equipment which detects the incoming energy stream being monitored by the sensor. Sometimes this term is incorrectly used interchangeably with sensor.

Electromagnetic radiation  
Merely a term referring to all the forms of radiation, from radio wavelengths to gamma radiation.

EROS  
The Greek god of Love. Also, a panchromatic and multispectral satellite platform (Earth Resources Observation Satellite).

False color  
An image created by representing data from three wavelength bands (such as ultraviolet, red, and infrared) with three visible colors (red, green, and blue) to produce an image that aids in the interpretation and understanding of the data in the three bands. Compare color composite and true color images.

Frequency  
For our purposes, the rate at which the electric and magnetic fields in a photon alternate directions. Photons with higher frequencies have greater energies. Frequency is indicated in equations by the Greek letter nu ().

Gamma radiation  
Extremely high energy radiation, wavelengths shorter than 0.001 nm.

GIS  
A larger concept than often believed, GIS (geographic information systems) is the hardware, software, data, personnel systems, etc., involved with geographical coordinate data.

GPS  
Global Positioning System, the broadcasting satellites, receiving hardware, and software used to locate coordinates on or near the surface of the Earth.

High-altitude sensor  
A sensor being operated in a modified U2 aircraft, i.e. at altitudes of 15-20 kilometers. Compare with low-altitude sensor.

Hyperdimensional data cube  
A data set for which there are more than three different axes of data. For example, observations of changes in temperatures at various oceanic depths over time, for different points in an area, would represent in an (x,y,z,T) hyperdimensional data cube, where (x,y,z) indicate the spatial coordinates for the aquatic location, the time is indicated by T. Compare with data cube.

Hyperspectral  
A data product in which the data are binned into approximately fifteen or more spectral bins. Compare with multispectral.

IKONOS  
A satellite that produces high resolution, multispectral imagery.

Image plane  
The (approximately) two-dimensional surface that indicates the location of the image produced by a lens. This is where you should put your detector in order to record a nice, focused image to take home and play with.

Image size  
The dimensions of an image in either pixel dimensions or in bytes of disk space.

Infrared radiation  
Radiation with wavelengths in the approximate range of 0.8 microns to 1 mm.

LiDAR  
An acronym meaning "Light Detection and Ranging". This is similar to rangefinding, in that an active sensor produces a laser beam to sense distances to objects. However, the data product is a two dimensional surface map of an area instead of a point distance.

Light  
In common usage this refers to electromagnetic radiation in optical wavelengths. However, in the discussion of remote sensing I tend to use this to indicate any kind of photon energy beam.

Low-altitude sensor  
A sensor being operated in a low-flying aircraft, i.e. altitudes of a few kilometers or less. Compare with high-altitude sensor.

Micron  
A distance measure equal to one thousandth of 1 mm.

Microwaves  
Radiation with wavelengths in the approximate range of 0.1 mm to 1 meter.

Monochromatic  
Said of a beam of photons when the bandwidth of the photons is so narrow that all the photons essentially have the same wavelength. This is one of the defining characteristics of a laser beam.

Multispectral  
A data product in which the data are binned into as few as two, or as many as approximately fifteen. There is no agreed-upon number of bands which separates multispectral work from hyperspectral work.

Nanometers  
A distance measure equal to one millionth of 1 mm.

Panchromatic  
Used for an image that is built from intensity data with a very broad bandwidth, such as most or all visible-wavelength photons.

Pan-sharpening  
A computer-enhancement algorithm for improving the resolution of an image. Usually, this involves combining the high spatial resolution of a panchromatic image with the color information from (lower resolution) multispectral data. The result is a color image that has the high resolution of the panchromatic image. Researchers using pan-sharpened images should be aware that they are dealing with data that are probably carrying a certain amount of image-manipulation artifacts---the data may look very good, but they are not as trustworthy as genuinely high-resolution data.

Particle-wave duality  
The apparent paradox that photons can exhibit wave-like behavior (e.g. make diffraction rings) while they can also exhibit particle-like behavior (such as the photo-electric effect).

Passive sensors  
A remote sensing device that operates by analyzing energy that is naturally emitted by, or reflected by, the study object.

Photons  
Discrete packets of electromagnetic energy. Ensembles of photons are called light.

Pixel  
The representation of data detected by a single detector during a certain finite collection period. Pixels may be arranged in a two dimensional array to indicate a spatial map of data (e.g. a photograph), in a one-dimensional string to display a spectrum, or in some other sort of data cube or hyperdimensional data cube.

Platform  
The observing structure, i.e. aircraft or satellite, that carries remote sensing apparatus.

Radio waves  
Radiation with wavelengths longer than about 1 meter.

Rangefinders  
Small handheld telescopes equipped with a laser that lets you measure the distance from the telescope to the object. Effective at ranges less than about 1 km. Compare with LiDAR.

Reflected infrared (IR)  
Infrared radiation at approximately 800-1100 nanometers. This is called reflected radiation because it is produced by the sun, and is observed after it is reflected off the surface of the ground. Compare with thermal infrared.

Selective availability  
A random error that was imposed on information from satellites relaying positional data; this error was rescinded in 2000 but may be reinstated by the USA military.

Sensor  
The entire device that produces remote sensing data, including the optics, housing, power modules, cooling system, etc. Sometimes this term is incorrectly used interchangeably with detector.

Spatial coverage  
The ground area spanned by an entire remote sensing image. Also used to describe the entire area mapped by an remote sensing project.

Spatial resolution  
Roughly speaking, the degree to which fine detail can be seen in an image. More precisely, resolution is the smallest distance between two objects that can be barely distinguished in the data. Note that if the resolution is limited by pixel size (which is the usual case in vegetation remote sensing), pixel size (in meters) is not the same as resolution. For example, suppose two bright, sub-pixel objects on a dark background are sampled in a square grid detector array. If one object occurs in the lower left corner of a pixel, and the second object occurs in the upper right corner of a pixel to the upper right of the first pixel, these two objects are not resolved (the data would still just show a two-pixel blob against a dark background). Any further apart, however, and they would be resolved. So if the pixel size is d×d meters, the scale of the finest object that can be resolved (r) in the image is actually given by r2=(2d)2+(2d)2, i.e. r~2.8d.

Spectral signature  
A characteristic shape to a spectrum which can be used as an unambiguous indicator of some chemical constituent (or vegetative component) of the area being studied.

Spectroscopy  
The branch of science that deals with deducing as much as possible about the workings of the Universe, by studying the light emitted or otherwise modified by the objects that inhabit it.

Spectrum  
A graphical presentation of the array of intensity values for data in a certain bandwidth.

Speed of light  
The speed at which photons travel, approximately 3×108 m/s. This speed is indicated in equations by the letter c.

Stephan-Boltzmann's law  
A law relating the surface area and temperature of a object to the power it produces via radiation: P=4r2T4.

Thermal infrared (IR)  
Infrared radiation at approximately 3-12 microns. Thermal radiation is produced by the Earth as it radiates heat energy into space. Compare with reflected infrared.

True color image  
An image in which the colors represented more or less accurately correspond to the colors of the optical photons detected during the gathering of the image data. Compare color composite and false color images.

Ultraviolet radiation  
Radiation with wavelengths in the approximate range of 0.38 microns to 1 nm.

Visible light  
Photons in the wavelength range of approximately 0.4-0.8 microns.

Wavelength  
The approximate scale size of a photon (a definition which to a certain degree is a fiction since photons do not have well defined sizes). Wavelength is indicated in equations by the Greek letter lambda ().

White light  
This is the artificial construct that the human brain has developed to convey to its owner that the human eyes have detected an ensemble of photons, with a wavelength and intensity distribution approximately equal to the sun's continuum spectrum.

Wien's law  
The law that notes at which wavelength an object will produce the maximum amount of energy in its continuum spectrum. This wavelength point is only dependent upon the emitting object's temperature.

X-rays  
Radiation with wavelengths in the approximate range of 1 nm to 0.001 nm.

 

4 February 2009