The terms shown in bold at their first appearance in the chapters are defined here, along with other technical terms that you may encounter in the Useful References.
absolute temperature Temperature measured on the Kelvin scale, whose base is absolute zero (-273oC). 0oC is expressed as 273K.
absorptance A measure of the ability of a material to absorb EM energy at a specific wavelength.
active remote sensing A system based on the illumination of a scene by artificial radiation and the collection of the reflected energy returned to the system. Examples are radar and systems using lasers.
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 colours The spectral colours red, green and blue, which Thomas Young (1773-1829) discovered to be capable of reproducing all other colours when mixed by projection through filters, and each of which cannot be produced by mixtures of the other two.
albedo The fraction of the total EM energy incident on a material which is reflected in all directions.
alluvial fan Fan- or cone-shaped deposits of water-transported material. They typically form at the base of topographic features where there is a marked break in slope. Consequently, alluvial fans tend to be coarse-grained, especially at their mouths. At their edges, however, they can be relatively fine-grained.
alluvium Unconsolidated sediments formed by long-range transportation of weathered material by flowing water.
anaglyph A stereoscopic pair of images superimposed and printed in different colours, usually red and cyan, producing a stereo effect when viewed with appropriate filters over each eye.
aquiclude A rock through which water will not flow at a measurable rate
aquifer A rock through which water flows at a rate that can be used for supplies.
confined aquifer Trapped between aquitards the lie above and below it, so that the aquifer is saturated throughout.
unconfined aquifer Entirely exposed at the surface
aquifuge A rock that has no interconnected spaces
aquitard A rock through which the flow is too low to be useful.
armoured surface A bare surface in arid or semi-arid areas where winnowing of sand and fines by sheet floods leaves larger fragments that may interlock to create an impervious surface, or ‘desert pavement’.
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.
artesian conditions Occur where wells penetrate a confined aquifer whose potentiometric surface is above the ground surface, so that the hydraulic gradient drives groundwater above the well head.
ASTER Advanced Spaceborne Thermal Emission and Reflection Radiometer carried by EOS-1AM (Terra), launched in December 1999. ASTER acquires images in 14 spectral bands in the reflected and thermally emitted regions, selected particularly for geological applications.
atmospheric window A range of EM wavelengths where radiation can pass through the Earth’s atmosphere with relatively little attenuation.
attribute Property assigned to a line, polygon or point in vector-format data. Several may be combined to give a description of a vector that is useful in a GIS.
azimuth In general this is the compass bearing of a line given in degrees clockwise from north. In radar terminology it refers to the direction at right angles to the radar propagation direction, which is parallel to the ground track in a sideways-looking system.
bajada Coalesced alluvial fans that may mantle the margins of large depositional basins.
band In remote sensing, a range of wavelengths from which data are gathered by a recording device.
binary A numerical system using the base 2. Examples are 0 = 0, 1 = 20 = 1, 10 = 21 = 2, 11 = 21 + 20 = 3.
bit An abbreviation of binary digit, which refers to an exponent of 2. A bit is represented by 0 or 1 for “on” or “off” in a digital computer.
blackbody A perfect radiator and absorber of EM energy, where all incident energy is absorbed and the energy radiated from the body at a particular temperature is at the maximum possible rate for each wavelength, as governed by the Stefan-Boltzmann Law. No natural material has these ideal properties, although water is a close approximation.
bond-stretching and bond-bending transitions Produce spectral features due to distortion of bonds in molecules, particularly the metal-hydroxyl bonds Al-OH and Mg-OH.
byte A group of 8 bits of digital data in binary form. A byte represents digital numbers up to 255, and is the standard adopted by most digital remote-sensing systems where the range of energies is coded from 0 to 255.
charge transfer transition A mechanism of spectral absorption of EMR, when an electron is transferred across a chemical bond.
charge-coupled device (CCD): A light-sensitive capacitor whose charge is proportional to the intensity of illumination. They are able to be charged and discharged very quickly, and are used in pushbroom devices, spectroradiometers and modern video cameras.
clastic dyke A cross-cutting body of sedimentary material formed when sediment fills a dilated fault or joint, either being washed down from the surface or injected from depth by overpressured water in an unconsolidated stratum.
colluvium A mantle of minimally transported, weathered material that has accumulated from above on a hill slope, also known as scree, talus or glacis.
context The surrounding environment of a particular feature on an image.
contrast The ratio between the energy emitted or reflected by an object and its immediate surroundings.
contrast stretching Expanding a measured range of DN in an image to a larger range, to improve the contrast of the image and its component parts.
corestone A relic, often rounded, mass of unaltered rock that remains in saprolite
corner reflector A cavity formed by three planar reflective surfaces intersecting at right angles, which returns radar directly back to its source.
crystal-field effect Dependence of spectral features on co-ordination of ions in a mineral molecule. Most commonly shown by minerals dominated by ferric iron (Fe3+)
cut off The digital number in the histogram of a digital image which is set to zero during contrast stretching. Usually this is a value below which atmospheric scattering makes a major contribution.
Darcy’s law The rate (Q in m3 s-1) at which groundwater flows through a cross sectional area (A m2) in a well at the lower point is related to the hydraulic gradient (h/l) by:
Q = KAh/l
dark-pixel correction Use of the darkest pixels in a scene, either total shadows or deep, clear water, to approximate the contribution by atmospheric scattering.
decorrelation stretching A way of making a three band multispectral image more colourful by stretching the data distribution along axes related to the natural elongation of the data distribution (the principal component directions). After stretching, the data are displayed on the original red, green and blue axes, thus retaining the original, straightforward colour relationships. Sometimes abbreviated to D-stretch.
depression angle In radar usage this is the angle between the horizontal plane passing through the antenna and the line connecting the antenna to the target. It is easily confused with the look angle.
desert pavement A bare surface in arid or semi-arid areas where winnowing of sand and fines by sheet floods leaves larger fragments that may interlock to create an impervious surface (see armoured surface).
desert varnish A dark often shiny coating on rocks found in arid regions. The coating is composed dominantly of fine-grained clay minerals trapped by bacterial or lichen films that have precipitated black manganese oxide and red iron oxide from pore water.
diffuse reflection Reflection of light (or other EMR) from a surface approximately evenly in all directions, as opposed to specular (mirror-like) reflection. A diffuse reflector has a surface that is rough on the scale of the wavelength of EMR concerned.
digital elevation model (DEM) A 3-D representation of a surface created by interpolation of topographic elevation contours to a grid or measured directly by interferometric radar or lidar surveys. There are two types: a digital surface model (DSM) represents the earth’s surface and includes all objects on it, including vegetation canopies; a digital terrain model (DTM) represents the bare ground surface, without objects upon it.
digital number (DN): The value of a variable recorded for each pixel in an image as a binary integer.
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.
eigenvalue Proportion of the variance in multispectral data that is redistributed to a principal component.
eigenvector Crudely, the contribution of an input band to a principal component.
electromagnetic (EM) radiation Energy transported by the propagation of disturbances in the electric and magnetic fields. Detection and measurement of natural or artificial EM radiation is the basis for remote sensing.
electronic transition A mechanism of spectral absorption of EMR, when an electron is moved from a lower orbit to a higher, as the result of absorption of a photon. Also a mechanism of generation of EMR, when an electron goes from a higher orbit to a lower, and emits a photon in the process.
eluvium Weathered bedrock that remains in place; also known as regolith or saprolite.
emissivity A measure of how well a surface emits, or radiates, EMR thermally; defined as the ratio between the thermal exitance from the surface and the thermal exitance from a blackbody (perfect emitter) at the same temperature. A blackbody therefore has an emissivity of 1 and natural materials range from 0 to 1.
emittance A term for the radiant flux of energy emitted by a body per square metre.
emitted region Part of the spectrum between 3 µm and 1 cm dominated by energy emitted by bodies above absolute zero.
equalization stretch Contrast stretch that forces the cumulative histogram of DN to a straight line, thereby spacing bins according to their frequencies. This achieves an equal population density of pixels along the DN axis. Also known as equipopulation or ramp stretch.
ESA European Space Agency, based in Paris. A consortium between several European states for the development of space science, including the launch of remote-sensing satellites.
etchplain Area of bare rock stripped of the products of long-term chemical weathering, including exhumed unconformities
exfoliation Formation of slabs of rock on joints that closely follow the exposed surface, mainly due to release of elastic strain during erosion and uplift.
exponential stretch Contrast stretch using a look-up table that is an exponential curve. Accentuates contrast in light regions.
false-colour image A colour image where parts of the non-visible EM spectrum are expressed as one or more of the red, green and blue components, so that the colours produced by the Earth’s surface do not correspond to normal visual experience.
fault rock Broken rock formed by shattering and shearing along a fault.
flatiron A term used to describe roughly triangular features often seen on images, derived from their similarity to the household appliance used for smoothing cloth. On aerial photographs they result from dipping rock layers that are well exposed. On radar images they may be an artefact caused by high relief and steep depression angle.
foreshortening A distortion in radar images causing the lengths of slopes facing the antenna to appear shorter on the image than on the ground. It is produced when radar wavefronts are steeper than the topographic slope.
formation water Trapped water from the environment in which sedimentary rocks were deposited
frequency Number of waves that pass a reference point in unit time, usually one second.
gabion Wire basket or cage (‘gabbione’ in Italian) filled with broken rock to create a low-cost barrier.
Gaussian stretch See normalized stretch.
geographic information system (GIS) A data handling and analysis system based on sets of data distributed spatially in two dimensions. The data sets may be map oriented, when they comprise qualitative attributes of an area recorded as lines, points and areas in vector format, or image oriented, when the data are quantitative attributes referring to cells in a rectangular grid in raster format.
georeferencing Correlation of positions on images with geographic co-ordinates, using ground-control points. Sometimes referred to as geocoding.
grid format The result of interpolation from values of a variable measured at irregularly distributed points, or along survey lines, to values referring to square cells in a rectangular array.
ground range Distance on the Earth’s surface from the position of an object to that directly below a radar antenna.
ground truth Observations made on the ground that can be used to verify the interpretation of an image. Ground truth includes field-based identification of surface types and their distribution (to help perform and verify classification). Comparable observations at sea (e.g. phytoplankton concentration, wave height etc.) are usually known as sea truth.
head The difference in gravitational potential energy between two points on a sloping water table is proportional to the difference in height of the water table at both
high-pass filter A spatial filter which selectively enhances contrast variations with high spatial frequencies in an image. It improves the sharpness of images and is a method of edge enhancement.
hill shading Creation of visual clues to topographic relief by simulating oblique illumination of a DEM by sunlight, most effectively from the upper left of a scane. May be used to selectively highlight linear features with different orientations.
histogram A means of expressing the frequency of occurrence of values in a data set within a series of equal ranges or bins, the height of each bin representing the frequency at which values in the data set fall within the chosen range. A cumulative histogram expresses the frequency of all values falling within a bin and lower in the range.
hue One of the three characteristics that define the colour of a pixel in an image in terms of intensity, hue and saturation. Hue is a measure of the relative amounts of red, green and blue contributing to the colour.
hydraulic conductivity The volume of water in m3 that will flow through a cross sectional area of 1 m2 in 1 s at a hydraulic gradient of 1, which gives units of m s-1.
hydraulic gradient The slope of the water table between two points on the water table – height difference/horizontal separation
Hyperion Imaging spectrometer with 7.5 km swath width to be carried on long-delayed NASA satellite (EO-1); part of NASA’s latest strategy of low-cost imaging systems.
hyperspectral data Often used for multi-channel, narrow band data captured by imaging spectrometers.
incidence angle The angle between the surface and an incident ray of EM radiation – most usually referring to radar.
infrared radiation EMR of wavelength between 0.7 mm and 100 mm, comprising reflected infrared 0.7–3.0 mm and thermal infrared 3.0–100 mm.
instantaneous field of view (IFOV) The solid angle through which a detector is sensitive to radiation. It varies with the intensity of the radiation, the time over which radiation is gathered, and forms one limit to the resolution of an imaging system.
intensity A measure of the energy reflected or emitted by a surface. Specifically, the overall brightness of a colour in the IHS colour system.
intensity-hue-saturation (IHS) colour system Based on a cone whose axis has equal values for red, green and blue (the grey axis). Intensity of a colour is measured by projection of its RGB co-ordinates on to the grey axis. A colour’s hue is the angular position (from 0 – 360°) of its RGB co-ordinates around the circumference of a circular section through the cone, known as a colour wheel. Saturation measures the distance of a colour’s RGB co-ordinates radially from the grey axis.
interferometric radar (InSAR) See radar interferometry.
irradiance The radiant flux density emitted by a body or falling on a surface.
joint Broken surface within a rock along which there no vertical or lateral displacement.
lag Coarse unconsolidated sediment from which clay and fine sand has been winnowed by wind or overland flow of water.
Lambertian reflector A perfectly diffuse reflector, which reflects incident EMR equally in all directions.
Landsat A US series of remote-sensing satellites in sun-synchronous, polar orbit that began in 1972.
laser Light artificially stimulated electromagnetic radiation. A beam of coherent radiation with a single wavelength.
layover A distortion in radar images when the angle of surface slope is greater than that of radar wavefronts. The base of a slope reflects radar after the top, and since radar images express the distance to the side in terms of time, the top appears closer to the platform than the base on an image, giving the impression of an overhanging slope.
lidar (light detection and ranging)
linear stretch Contrast stretch that redistributes minimum and maximum radiances in raw data linearly between 0 and 255 respectively, thereby stretching all intermediate DN over the full range of brightnesses.
logarithmic stretch Contrast stretch using a look-up table that is a logarithmic curve. Accentuates contrast in dark regions.
look angle The angle between the vertical plane containing a radar antenna and the direction of radar propagation. Complementary to the radar depression angle.
look direction The direction in which pulses of radar are transmitted.
look-up table (LUT): A mathematical formula used to convert one distribution of data to another, most conveniently remembered as a conversion graph.
median filter A spatial filter, which substitutes the median value of DN from surrounding pixels for that recorded at an individual pixel. It is useful for removing random noise.
microwaves EMR of wavelength between 100 mm and 1 m.
mid-infrared (MIR): The range of EM wavelengths from 8 to 14 mm dominated by emission of thermally generated radiation from materials. Also known as thermal infrared.
modulation transfer function (MTF) A measure of the sensitivity of an imaging system to spatial variations in contrast.
nadir The point on the ground vertically beneath the centre of a remote-sensing system.
near infrared (NIR) The shortest wavelength part of the near infrared (reflected infrared), with wavelengths between 0.7 mm and 1.4 mm.
noise Random or regular artefacts in data which degrade their information-bearing quality and are due to defects in the recording device.
non-spectral hue A hue which is not present in the spectrum of colours produced by the analysis of white light by a prism or diffraction grating. Examples are brown, magenta and pastel shades.
normalized stretch Contrast stretch in which a range of raw DN are transformed by forcing their histogram to the shape of a normal or Gaussian distribution that spans the full 0-255 range.
Optical Land Imager (OLI) Imaging system similar to the Thematic Mapper carried by Landsat-8.
overland flow Water that flows across the surface between discrete drainage channels: also known as sheetwash or sheet flood.
panchromatic Covering the range of wavelengths in the visible (and sometimes the VNIR) part of the spectrum
parallax The apparent change in position of an object relative to another when it is viewed from different positions. It forms the basis of stereoscopic vision.
passive remote sensing The capture of images representing the reflection or emission of EM radiation that has a natural source.
pericline Anticline with a curved hinge that plunges in opposed directions to create an elongated dome
permeability A qualitative expression for the degree of connectedness of spaces in a rock and the ease of flow through it.
photon A quantum of EM energy.
phreatophyte vegetation Plants that depend almost entirely on groundwater, often through deep root systems. Known to cause large drawdown in water tables of arid and semi-arid regions.
pixel A single sample of data in a digital image, having both a spatial attribute – its position in the image and a usually rectangular dimension on the ground – and a spectral attribute – the intensity of the response in a particular waveband for that position. A contraction of picture element.
Planck’s Law An expression for the variation of emittance of a blackbody at a particular temperature as a function of wavelength.
polar orbit An orbit that passes close to the poles, thereby enabling a satellite to pass over most of the surface, except the immediate vicinity of the poles themselves.
porosity The proportion of the rock’s volume that consists of open spaces or pores, expressed as a percentage between 0 and 100.
primary porosity Due to spaces between the grains that make up a rock.
secondary porosity Due to spaces created in a rock after it formed, e.g. by solution or fracturing
potentiometric surface Imaginary surface to which groundwater would rise in boreholes that penetrate a confined aquifer
power The rate of transfer of energy. The SI unit is the watt 1 W = 1 Js-1.
principal component analysis (PCA) The analysis of covariance in a multiple data set so that the data can be projected as additive combinations onto new axes, which express different kinds of correlation among the data.
pushbroom system An imaging device consisting of a fixed, linear array of many sensors, usually CCDs which is swept across an area by the motion of the platform, thereby building up an image. It relies on sensors whose response and reading is nearly instantaneous, so that the image swath can be segmented into pixels representing small dimensions on the ground.
quantum (pl. quanta) The elementary quantity of EM energy that is transmitted by a particular wavelength. EM radiation is emitted, transmitted and absorbed as numbers of quanta, the energy of each quantum being a simple function of the frequency of the radiation.
radar The acronym for radio detection and ranging, which uses pulses of artificial EM radiation in the 1mm to 1m range to locate objects which reflect the radiation. The position of the object is a function of the time that a pulse takes to reach it and return to the antenna.
radar interferometry Use of two antennas to record backscattered radar energy, so that the antenna separation is analogous to the distance between stereoscopic aerial photographs. Radiation travels different distances from each point on the ground to each antenna, so that when the signals are combined they interfere. Using the geometry of the radar set-up and the known wavelength of the radiation the interference record is converted to topographic elevation of each point. This produces a digital elevation model of the overflown surface.
radial relief displacement The tendency of vertical objects to appear to lean radially away from the centre of a vertical aerial photograph. Caused by the conical field of view of the camera lens.
radiance The radiant flux density falling on a surface measured per solid angle. A useful concept because we rarely measure the radiant flux density leaving a surface in all directions; instead we collect it with a detector responding to EMR arriving at the detector from a finite solid angle.
ramp stretch See equalization stretch.
range In radar usage this is the distance in the direction of radar propagation, usually to the side of the platform in an imaging radar system. The slant range is the direct distance from the antenna to the object, whereas the distance from the ground track of the platform to the object is termed the ground range.
raster The scanned and illuminated area of a video display, produced by a modulated beam of electrons sweeping the phosphorescent screen line by line from top to bottom at a regular rate of repetition.
raster format A means of representing spatial data in the form of a grid of values, each line of which can be used to modulate the lines of a video raster.
Rayleigh criterion A way of quantifying surface roughness with respect to wavelength of EMR, to determine whether the surface will act as a specular (smooth) reflector or as a diffuse (rough) reflector. A surface can be considered rough if the root mean square height of surface irregularities is greater than one eighth of the wavelength of EMR divided by the cosine of the incidence angle.
receptor Light-sensitive cell embedded in the eye’s retina. See rods and cones.
red edge The sharp increase in spectral reflectance of healthy leafy vegetation, that occurs with increasing wavelength between about 700 nm and 750 nm wavelength, i.e. in the red/very near infrared part of the spectrum.
reductive dissolution Some metals, such as iron and manganese, have several valence states that depend on ambient chemical conditions. Oxidising conditions result in increased valency when ions such as Fe3+ form highly insoluble oxides and hydroxides. Gain of electrons under reducing conditions reduces valency, such as from Fe3+ to water-soluble Fe2+.This results in the breakdown of minerals stable under oxidising conditions and release to solution of iron and any ions of trace elements that were contained by the previously stable mineral. Not all multivalent metals behave in this way: oxidised uranium ions are far more soluble than those stable under reducing conditions.
reflectance The ratio of the EM energy reflected by a surface to that which falls upon it. It is often qualified as spectral reflectance because reflectance of a material may vary at different wavelengths.
reflected infrared That part of the infrared spectrum that, in terrestrial remote sensing, is mostly made up of reflected solar EMR, as opposed to thermal infrared, which has longer wavelengths. The reflected infrared is defined as wavelengths from 0.7 mm to 3.0 mm, and is synonymous with the near infrared.
reflected region That part of the spectrum between 0.4 to 2.5µm where most energy is reflected from surfaces.
relative band depth An arithmetic combination of bands that define absorption features of particular minerals; generally the band in which absorption occurs and those on the features short- and long-wave shoulders.
resolution A poorly defined term relating to the fidelity of an image to the spatial attributes of a scene. Resolution is colloquially equated with pixel size of an imaging system. Properly, it involves the IFOV and MTF of the imaging system, and depends on the contrast within the image, as well as on other factors. It is properly expressed as line pairs per millimetre of the most closely spaced lines that can be distinguished, and therefore depends on human vision, scale and viewing distance.
reststrahlen band Range of wavelengths, mainly in the thermal infrared, that cannot be propagated by a mineral due to its molecular structure, giving rise to distinct spectral features.
RGB colour system Based on the additive primary colours red, green and blue, this is the system used by all video monitors in which various data are assigned to red, green and blue colour guns.
sand dam A dam constructed in areas of ephemeral stream flow to trap sediment behind it, thereby creating an shallow artificial aquifer.
saprolite Chemically weathered bedrock that forms a veneer on unaltered rock.
saturation In colour theory it means the degree of mixture between a pure hue and neutral grey. Sometimes used to refer to the maximum brightness that can be assigned to a pixel on a display device, and corresponds to a DN of 255.
scale In cartography this refers to the degree of reduction from reality that is represented on a map, usually expressed as a ratio (e.g. 1:250 000). A large-scale map (e.g. 1:10 000) represents ground dimensions by larger cartographic dimensions than does a small-scale map (e.g. 1: 100 000). This is the opposite of the common usage wherein a large-scale feature has larger dimensions than a smaller-scale feature.
scattering An atmospheric effect where EM radiation, usually of short visible wavelength, is propagated in all directions by the effects of gas molecules and aerosols.
scene The area on the ground recorded by an image.
shear zone A zone where rock displacement has taken place as a result of ductile (plastic) deformation that may result in foliation or thinning of pre-existing layering.
shear zone Tectonic feature across which a fault-like displacement has taken place, but through ductile deformation rather than brittle fracture. Often associated with a strong planar fabric.
sheetwash Surface water flow outside of channels – see overland flow.
short-wave infrared (SWIR) That part of the reflected infrared with wavelengths between 1.4 mm and 3.0 mm. These wavelengths are too long to affect infrared photographic film, hence the distinction from VNIR.
Shuttle Radar Topography Mission (SRTM) Interferometric radar mission to map surface elevation of the continents between 60º N and 58º S, with a resolution up to 30 m.
signal to noise ratio (S/N): The ratio of the level of the signal carrying real information to that carrying spurious information as a result of defects in the system.
signature In remote sensing this refers to the spectral properties of a material or homogeneous area, most usually expressed as the range of DN in a number of spectral bands.
slant range Direct distance from a radar antenna to a point at the Earth’s surface. Because radar uses energy transmitted obliquely downwards, slant range is different from actual distance on the ground, or ground range, and varies through the scene in the look direction.
spate irrigation A means of delivering water to agricultural areas by diverting floods from their natural watercourses to a system of channels that irrigate fields and may also allow surface water that would otherwise escape the area to infiltrate and recharge groundwater.
spatial-frequency filtering The analysis of the spatial variations in DN of an image and the separation or suppression of selected frequency ranges.
spectral hue A hue which is present in the spectral range of white light analysed by a prism or diffraction grating.
specular reflection Reflection of EMR from a smooth surface, which behaves like a mirror. A specular reflector has a surface that is smooth on the scale of the wavelength of EMR concerned. If the surface is rough on the scale of the wavelength of the EMR, it will behave as a diffuse reflector.
SPOT Satellite Probatoire pour l’Observation de la Terre, a French remote sensing satellite carrying imaging systems for three wavebands in the visible and VNIR and panchromatic images. Each system comprises two devices which are pointable so that off-nadir images are possible, thereby allowing stereoptic viewing.
Stefan-Boltzmann Law A radiation law describing how the energy radiated by a blackbody is proportional to the fourth power of its absolute temperature.
stream order Measure of the relative size of streams. In the widely used Strahler classification Order 1 streams are the smallest and have no tributaries, an Order 2 stream is the product of confluence of two Order 1 tributaries, confluence of two Order 2 streams produces Order 3 and so on, up to the Amazon that is Order 12.
subtractive primary colours The colours cyan, magenta and yellow, the subtraction of which from white light in different proportions allows all colours to be created.
Sun angle When considering the illumination of a surface by EMR from the Sun, the angle between a normal to the surface and the direction of the Sun.
Sun-synchronous orbit A polar orbit where the satellite always crosses the Equator at the same local solar time.
synthetic-aperture radar (SAR): A radar imaging system in which high resolution in the azimuth direction is achieved by using the Doppler shift of backscattered waves to identify waves from ahead of and behind the platform, thereby simulating a very long antenna.
texture The frequency of change and arrangement of tones on an image, often used to describe the aggregate appearance of different parts of the surface, but sometimes used for the spacing of drainage elements.
Thematic Mapper TM: An imaging device carried by Landsats-4 and -5, which recorded scenes in seven wavebands, six in the visible, NIR and SWIR with a resolution of 30m, and one in the MIR with a resolution of 120m. The Advanced Thematic Mapper + (ETM+) on Landsat-7 included a 15 m resolution panchromatic band and improved resolution for its thermal sensor.
thermal capacity The ability of a material to store heat.
thermal conductivity A measure of the rate at which heat passes through a material.
thermal emission Emission of EMR from a material due to thermal vibration, as a result of its temperature.
thermal infrared TIR That part of the spectrum with wavelengths between 3.0 mm and 100 mm. These are the wavelengths at which thermal emission is greatest for surfaces at normal environmental temperatures. However, hot areas (fires, parts of volcanoes and so on) emit thermally at wavelengths shorter than the thermal infrared.
threshold Constriction to subsurface flow that creates conditions for slowing it and ponding groundwater, commonly where drainages in open terrain meet higher topography to flow through narrow defiles.
tone Each distinguishable shade from black to white on a monochrome image, sometimes called greytone.
transmittance The ratio of the EM energy passing through a material to that falling on its exposed surface.
transpiration The production and emission of water vapour and oxygen by plants.
ultraviolet That part of the spectrum with wavelengths between about 0.5 nm and 400 nm. Not useful for most aspects of terrestrial remote sensing, because of scattering in the atmosphere and absorption by ozone.
variance A measure of the dispersion of the actual values of a variable about its mean. It is the mean of the squares of all the deviations from the mean value of a range of data.
vector format The expression of points, lines and areas on a map by digitised Cartesian co-ordinates, directions and values.
vegetation index A technique, usually involving ratioing, whereby channels from a multispectral image are combined to show up variations in the amount of vegetation. A simple example is to divide very-near infrared DN by red DN.
vertical exaggeration The extent to which the vertical scale exceeds the horizontal scale in stereoptic viewing of two overlapping images with parallax differences. It is directly proportional to the base height ratio.
vibrational transitions A mechanism of spectral absorption of EMR, when the vibration of a chemical bond in a molecule of crystalline lattice changes from one state to another. Vibrations may occur either as stretching or bending of a bond.
visible and near infrared VNIR An arbitrary but conventional division of the shortest wavelength imaged in remote sensing, spanning 0.4 to 1.4 μm
visible radiation EMR in that part of the electromagnetic spectrum to which human eyes respond. It lies between the ultraviolet and the infrared, with wavelengths from 400 nm to 700 nm.
wavelength The mean distance between maxima or minima of a periodic pattern. In the case of EM radiation, it is the reciprocal of the frequency multiplied by the velocity of light.
whiskbroom Imaging device that sweeps radiant energy from lines on the ground across a CCD array made up of several hundred devices, each designed to record radiance in a narrow spectral band. Used in imaging spectrometers.
Wien’s Displacement Law A radiation law that relates how the peak of energy emitted by a material shifts to shorter wavelengths as absolute temperature increases.