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[edit] Glossary







absorption spectra

electromagnetic radiation wavelengths that are absorbed by specific materials of interest.

abstract symbol

an annotation symbol that has a geometric shape, such as a circle, square, or triangle. These symbols often represent amounts that vary from place to place, such as population density, yearly rainfall, and so forth

a priori

already or previously known.

accuracy assessment

comparison of a classification to geographical data that is assumed to be true. Usually, the assumed-true data are derived from ground truthing.

accuracy report

in classification accuracy assessment, a list of the percentages of accuracy, which is computed from the error matrix.


see attitude control system.

active sensors

solar imaging sensors that both emit and receive radiation.


see ARC Digitized Raster Graphic.


see ARC Digital Raster Imagery.

aerial stereopair

two photos taken at adjacent exposure stations.

Airborne Synthetic Aperture Radar

an experimental airborne radar sensor developed by Jet Propulsion Laboratories (JPL), Pasadena, California, under a contract with NASA. AIRSAR data have been available since 1983.

Airborne Visible/Infrared Imaging Spectrometer

(AVIRIS) a sensor developed by JPL (Pasadena, California) under a contract with NASA that produces multispectral data with 224 narrow bands. These bands are 10 nm wide and cover the spectral range of 0.4-2.4 nm. AVIRIS data have been available since 1987.


see Airborne Synthetic Aperture Radar.


a test of a training sample, usually used before the signature statistics are calculated. An alarm highlights an area on the display that is an approximation of the area that would be classified with a signature. The original data can then be compared to the highlighted area.


a Russian radar satellite that completed its mission in 1992.

Along-Track Scanning Radiometer

(ATSR) instrument aboard the European Space Agency’s ERS-1 and ERS-2 satellites, which detects changes in the amount of vegetation on the Earth’s surface.


Advanced Land Observing Satellite mission is a project operated by the Japan Aerospace Exploration Agency (JAXA), launched in 2006. ALOS carries three remote-sensing instruments: the Panchromatic Remote-sensing Instrument for Stereo Mapping (PRISM) for digital elevation mapping, the Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) for land coverage observation, and the Phased Array type L-band Synthetic Aperture Radar (PALSAR) for all-weather, day/night land observation.

American Standard Code for Information Interchange

(ASCII) a "basis of character sets. . .to convey some control codes, space, numbers, most basic punctuation, and unaccented letters a-z and A-Z" ( Free On-Line Dictionary of Computing, 1999a).

analog photogrammetry

optical or mechanical instruments used to reconstruct three-dimensional geometry from two overlapping photographs.

analytical photogrammetry

the computer replaces optical and mechanical components by substituting analog measurement and calculation with mathematical computation.

ancillary data

the data, other than remotely sensed data, that are used to aid in the classification process.


see Artificial Neural Networks.


the explanatory material accompanying an image or map. In ERDAS IMAGINE, annotation consists of text, lines, polygons, ellipses, rectangles, legends, scale bars, grid lines, tick marks, neatlines, and symbols that denote geographical features.

annotation layer

a set of annotation elements that is drawn in a Viewer or Map Composer window and stored in a file (.ovr extension).


see area of interest.


see line.

ARC system (Equal Arc-Second Raster Chart/Map)

a system that provides a rectangular coordinate and projection system at any scale for the Earth’s ellipsoid, based on the World Geodetic System 1984 (WGS 84).

ARC Digital Raster Imagery

NGA data that consist of SPOT panchromatic, SPOT multispectral, or Landsat TM satellite imagery transformed into the ARC system and accompanied by ASCII encoded support files. These data are available only to Department of Defense contractors.

ARC Digitized Raster Graphic

data from the NGA that consist of digital copies of NGA hard-copy graphics transformed into the ARC system and accompanied by ASCII encoded support files. These data are primarily used for military purposes by defense contractors.


vector data created with the ArcInfo UNGENERATE command.


a unit of measure that can be applied to data in the Lat/Lon coordinate system. Each pixel represents the distance covered by one second of latitude or longitude. For example, in 3 arc/second data, each pixel represents an area three seconds latitude by three seconds longitude.


a measurement of a surface.

area based matching

an image matching technique that determines the correspondence between two image areas according to the similarity of their gray level values.

area of interest

(AOI) a point, line, or polygon that is selected as a training sample or as the image area to be used in an operation. AOIs can be stored in separate .aoi files.

Artificial Neural Networks

(ANN) data classifiers that may process hyperspectral images with a large number of bands.


see American Standard Code for Information Interchange.


the orientation, or the direction that a surface faces, with respect to the directions of the compass: north, south, east, west.

aspect image

a thematic raster image that shows the prevailing direction that each pixel faces.

aspect map

a map that is color coded according to the prevailing direction of the slope at each pixel.


see Along-Track Scanning Radiometer.

attitude control system

(ACS) system used by SeaWiFS instrument to sustain orbit, conduct lunar and solar calibration procedures, and supply attitude information within one SeaWiFS pixel ( National Aeronautics and Space Administration, 1999).


the tabular information associated with a raster or vector layer.


the statistical mean; the sum of a set of values divided by the number of values in the set.


Advanced Very High Resolution Radiometer data. Small-scale imagery produced by an NOAA polar orbiting satellite. It has a spatial resolution of 1.1 × 1.1 km or 4 × 4 km.


see Airborne Visible/Infrared Imaging Spectrometer.


an angle measured clockwise from a meridian, going north to east.

azimuthal projection

a map projection that is created from projecting the surface of the Earth to the surface of a plane.



a set of data file values for a specific portion of the electromagnetic spectrum of reflected light or emitted heat (red, green, blue, near-infrared, infrared, thermal, and so forth), or some other user-defined information created by combining or enhancing the original bands, or creating new bands from other sources. Sometimes called channel.


see striping.

base map

a map portraying background reference information onto which other information is placed. Base maps usually show the location and extent of natural surface features and permanent human-made features.

Basic Image Interchange Format

(BIIF) the basis for the NITFS format.

batch file

a file that is created in the Batch mode of ERDAS IMAGINE. All steps are recorded for a later run. This file can be edited.

batch mode

a mode of operating ERDAS IMAGINE in which steps are recorded for later use.

bathymetric map

a map portraying the shape of a water body or reservoir using isobaths (depth contours).


a variation of the maximum likelihood classifier, based on the Bayes Law of probability. The Bayesian classifier allows the application of a priori weighting factors, representing the probabilities that pixels are assigned to each class.


see Basic Image Interchange Format.


band interleaved by line. A form of data storage in which each record in the file contains a scan line (row) of data for one band. All bands of data for a given line are stored consecutively within the file.

bilinear interpolation

a resampling method that uses the data file values of four pixels in a 2 × 2 window to calculate an output data file value by computing a weighted average of the input data file values with a bilinear function.

bin function

a mathematical function that establishes the relationship between data file values and rows in a descriptor table.


ordered sets of pixels. Pixels are sorted into a specified number of bins. The pixels are then given new values based upon the bins to which they are assigned.


band interleaved by pixel. A form of data storage in which the values for each band are ordered within a given pixel. The pixels are arranged sequentially on the tape.


a binary digit, meaning a number that can have two possible values 0 and 1, or off and on. A set of bits, however, can have many more values, depending upon the number of bits used. The number of values that can be expressed by a set of bits is 2 to the power of the number of bits used. For example, the number of values that can be expressed by 3 bits is 8 (23 = 8).

block of photographs

formed by the combined exposures of a flight. The block consists of a number of parallel strips with a sidelap of 20-30%.


a method of storing data on 9-track tapes so that there are more logical records in each physical record.

blocking factor

the number of logical records in each physical record. For instance, a record may contain 28,000 bytes, but only 4,000 columns due to a blocking factor of 7.

book map

a map laid out like the pages of a book. Each page fits on the paper used by the printer. There are neatlines and tick marks on all sides of every page.


logical, based upon, or reducible to a true or false condition.


on a map, a line that usually encloses the entire map, not just the image area as does a neatline.


a neighborhood analysis technique that is used to detect boundaries between thematic classes.


bits per inch. A measure of data storage density for magnetic tapes.


an elevation polyline in which each vertex has its own X, Y, Z value.

brightness value

the quantity of a primary color (red, green, blue) to be output to a pixel on the display device. Also called intensity value, function memory value, pixel value, display value, and screen value.


band sequential. A data storage format in which each band is contained in a separate file.

buffer zone

a specific area around a feature that is isolated for or from further analysis. For example, buffer zones are often generated around streams in site assessment studies so that further analyses exclude these areas that are often unsuitable for development.


the process of constructing the topology of a vector layer by processing points, lines, and polygons. See clean.


the unit of photogrammetric triangulation after each point measured in an image is connected with the perspective center by a straight light ray. There is one bundle of light rays for each image.

bundle attitude

defined by a spatial rotation matrix consisting of three angles (k, w, j).

bundle location

defined by the perspective center, expressed in units of the specified map projection.


8 bits of data.



see Compressed Aeronautical Chart.


see computer-aided design.

cadastral map

a map showing the boundaries of the subdivisions of land for purposes of describing and recording ownership or taxation.


see Compressed ADRG.

calibration certificate/report

in aerial photography, the manufacturer of the camera specifies the interior orientation in the form of a certificate or report.


a coordinate system in which data are organized on a grid and points on the grid are referenced by their X,Y coordinates.


the art and science of creating maps.

categorical data

see thematic data.


see computer compatible tape.


a read-only storage device read by a CD-ROM player.


1. a 1° × 1° area of coverage. DTED (Digital Terrain Elevation Data) are distributed in cells. 2. a pixel; grid cell.

cell size

the area that one pixel represents, measured in map units. For example, one cell in the image may represent an area 30’ × 30’ on the ground. Sometimes called pixel size.

center of the scene

the center pixel of the center scan line; the center of a satellite image.

central processing unit

(CPU) "the part of a computer which controls all the other parts. . .the CPU consists of the control unit, the arithmetic and logic unit (ALU) and memory (registers, cache, RAM and ROM) as well as various temporary buffers and other logic" ( Free On-Line Dictionary of Computing, 1999b).


a number, letter, or punctuation symbol. One character usually occupies one byte when stored on a computer.

check point

additional ground points used to independently verify the degree of accuracy of a triangulation.

check point analysis

the act of using check points to independently verify the degree of accuracy of a triangulation.

chi-square distribution

a nonsymmetrical data distribution: its curve is characterized by a tail that represents the highest and least frequent data values. In classification thresholding, the tail represents the pixels that are most likely to be classified incorrectly.

choropleth map

a map portraying properties of a surface using area symbols. Area symbols usually represent categorized classes of the mapped phenomenon.


see Controlled Image Base.

city-block distance

the physical or spectral distance that is measured as the sum of distances that are perpendicular to one another.


a set of pixels in a GIS file that represents areas that share some condition. Classes are usually formed through classification of a continuous raster layer.

class value

a data file value of a thematic file that identifies a pixel as belonging to a particular class.


the process of assigning the pixels of a continuous raster image to discrete categories.

classification accuracy table

for accuracy assessment, a list of known values of reference pixels, supported by some ground truth or other a priori knowledge of the true class, and a list of the classified values of the same pixels, from a classified file to be tested.

classification scheme

(or classification system) a set of target classes. The purpose of such a scheme is to provide a framework for organizing and categorizing the information that can be extracted from the data.


the process of constructing the topology of a vector layer by processing lines and polygons. See build.


on a computer on a network, a program that accesses a server utility that is on another machine on the network.


a contiguous group of pixels in one class. Also called raster region.


unsupervised training; the process of generating signatures based on the natural groupings of pixels in image data when they are plotted in spectral space.


the natural groupings of pixels when plotted in spectral space.


cyan, magenta, yellow. Primary colors of pigment used by printers, whereas display devices use RGB.


Centre National d’Etudes Spatiales. The corporation was founded in 1961. It provides support for ESA. CNES suggests and executes programs ( Centre National D’Etudes Spatiales, 1998).


one number in a matrix, or a constant in a polynomial expression.

coefficient of variation

a scene-derived parameter that is used as input to the Sigma and Local Statistics radar enhancement filters.


a nonlinear mathematical model that photogrammetric triangulation is based upon. Collinearity equations describe the relationship among image coordinates, ground coordinates, and orientation parameters.


the location where the data file values are stored in the colormap. The red, green, and blue values assigned to the colorcell control the brightness of the color guns for the displayed pixel.

color guns

on a display device, the red, green, and blue phosphors that are illuminated on the picture tube in varying brightnesses to create different colors. On a color printer, color guns are the devices that apply cyan, yellow, magenta, and sometimes black ink to paper.


an ordered set of colorcells, which is used to perform a function on a set of input values.

color printer

a printer that prints color or black and white imagery, as well as text. ERDAS IMAGINE supports several color printers.

color scheme

a set of lookup tables that assigns red, green, and blue brightness values to classes when a layer is displayed.

composite map

a map on which the combined information from different thematic maps is presented.

Compressed ADRG

(CADRG) a military data product based upon the general RPF specification.

Compressed Aeronautical Chart

(CAC) precursor to CADRG.

Compressed Raster Graphics

(CRG) precursor to CADRG.

compromise projection

a map projection that compromises among two or more of the map projection properties of conformality, equivalence, equidistance, and true direction.

computer-aided design

(CAD) computer application used for design and GPS survey.

computer compatible tape

(CCT) a magnetic tape used to transfer and store digital data.

confidence level

the percentage of pixels that are believed to be misclassified.


a map or map projection that has the property of conformality, or true shape.


the property of a map projection to represent true shape, wherein a projection preserves the shape of any small geographical area. This is accomplished by exact transformation of angles around points.

conic projection

a map projection that is created from projecting the surface of the Earth to the surface of a cone.

connectivity radius

the distance (in pixels) that pixels can be from one another to be considered contiguous. The connectivity radius is used in connectivity analysis.

contiguity analysis

a study of the ways in which pixels of a class are grouped together spatially. Groups of contiguous pixels in the same class, called raster regions, or clumps, can be identified by their sizes and manipulated.

contingency matrix

a matrix that contains the number and percentages of pixels that were classified as expected.


a term used to describe raster data layers that contain quantitative and related values. See continuous data.

continuous data

a type of raster data that are quantitative (measuring a characteristic) and have related, continuous values, such as remotely sensed images collected from satellites such as Landsat, SPOT, GeoEye, WorldView, and so forth.

contour map

a map in which a series of lines connects points of equal elevation.

contrast stretch

the process of reassigning a range of values to another range, usually according to a linear function. Contrast stretching is often used in displaying continuous raster layers, since the range of data file values is usually much narrower than the range of brightness values on the display device.

control point

a point with known coordinates in the ground coordinate system, expressed in the units of the specified map projection.

Controlled Image Base

(CIB) a military data product based upon the general RPF specification.

convolution filtering

the process of averaging small sets of pixels across an image. Used to change the spatial frequency characteristics of an image.

convolution kernel

a matrix of numbers that is used to average the value of each pixel with the values of surrounding pixels in a particular way. The numbers in the matrix serve to weight this average toward particular pixels.

coordinate system

a method of expressing location. In two-dimensional coordinate systems, locations are expressed by a column and row, also called x and y.

correlation threshold

a value used in rectification to determine whether to accept or discard GCPs. The threshold is an absolute value threshold ranging from 0.000 to 1.000.

correlation windows

windows that consist of a local neighborhood of pixels. One example is square neighborhoods (for example, 3 × 3, 5 × 5, 7 × 7 pixels).

corresponding GCPs

the GCPs that are located in the same geographic location as the selected GCPs, but are selected in different files.


group of four satellites equipped with radar sensors for Earth observation for civil and defense use. The mission was developed by the Italian Space Agency (Agenzia Spaziale Italiana) and Telespazio.


measures the tendencies of data file values for the same pixel, but in different bands, to vary with each other in relation to the means of their respective bands. These bands must be linear. Covariance is defined as the average product of the differences between the data file values in each band and the mean of each band.

covariance matrix

a square matrix that contains all of the variances and covariances within the bands in a data file.


see central processing unit.


on maps, the text that can include the data source and acquisition date, accuracy information, and other details that are required for or helpful to readers.


see Compressed Raster Graphics.

crisp filter

a filter used to sharpen the overall scene luminance without distorting the interband variance content of the image.

cross correlation

a calculation that computes the correlation coefficient of the gray values between the template window and the search window.

cubic convolution

a method of resampling that uses the data file values of sixteen pixels in a 4 × 4 window to calculate an output data file value with a cubic function.

current directory

also called default directory, it is the directory that you are in. It is the default path.

cylindrical projection

a map projection that is created from projecting the surface of the Earth to the surface of a cylinder.


dangling node

a line that does not close to form a polygon, or that extends past an intersection.


1. in the context of remote sensing, a computer file containing numbers that represent a remotely sensed image, and can be processed to display that image. 2. a collection of numbers, strings, or facts that requires some processing before it is meaningful.

database (one word)

a relational data structure used to store data and the relations between them organized in tables. Examples of popular databases include Oracle Server, SQL Server, Microsoft Access database file (*.mdb).

data base (two words)

in ERDAS IMAGINE, a set of continuous and thematic raster layers, vector layers, attribute information, and other kinds of data that represent one area of interest. A data base is usually part of a GIS.

data file

a computer file that contains numbers that represent an image.

data file value

each number in an image file. Also called file value, image file value, DN, brightness value, pixel.


see reference plane.


see Discrete Cosine Transformation.

decision rule

an equation or algorithm that is used to classify image data after signatures have been created. The decision rule is used to process the data file values based upon the signature statistics.

decorrelation stretch

a technique used to stretch the principal components of an image, not the original image.

default directory

see current directory.

Defense Mapping Agency

(DMA) former name for agency that supplies VPF, ARC digital raster, DRG, ADRG, and DTED files. See National Geospatial-Intelligence Agency.

degrees of freedom

when chi-square statistics are used in thresholding, the number of bands in the classified file.


see digital elevation model.


the process of adding vertices to selected lines at a user-specified tolerance.


1. the number of bits per inch on a magnetic tape. 9-track tapes are commonly stored at 1600 and 6250 bpi. 2. a neighborhood analysis technique that outputs the number of pixels that have the same value as the analyzed pixel in a user-specified window.

derivative map

a map created by altering, combining, or analyzing other maps.


see attribute.

desktop scanners

general purpose devices that lack the image detail and geometric accuracy of photogrammetric quality units, but are much less expensive.


the device in a sensor system that records electromagnetic radiation.

developable surface

a flat surface, or a surface that can be easily flattened by being cut and unrolled, such as the surface of a cone or a cylinder.


see Discrete Fourier Transform.


see Differential Correction.

Differential Correction

(DGPS) can be used to remove the majority of the effects of Selective Availability.

digital elevation model

(DEM) continuous raster layers in which data file values represent elevation. DEMs are available from the USGS at 1:24,000 and 1:250,000 scale, and can be produced with terrain analysis programs, IMAGINE InSAR, IMAGINE OrthoMAX™, and IMAGINE StereoSAR DEM.

Digital Number

(DN) variation in pixel intensity due to composition of what it represents. For example, the DN of water is different from that of land. DN is expressed in a value

typically from 0-255.

digital orthophoto

an aerial photo or satellite scene that has been transformed by the orthogonal projection, yielding a map that is free of most significant geometric distortions.

digital orthophoto quadrangle

(DOQ) a computer-generated image of an aerial photo ( United States Geological Survey, 1999b).

digital photogrammetry

photogrammetry as applied to digital images that are stored and processed on a computer. Digital images can be scanned from photographs or can be directly captured by digital cameras.

Digital Terrain Elevation Data

(DTED) data produced by NGA. DTED data comes in two types, both in Arc/second format: DTED 1

a 1° × 1° area of coverage, and DTED 2

a 1° × 1° or less area of coverage.

digital terrain model

(DTM) a discrete expression of topography in a data array, consisting of a group of planimetric coordinates (X,Y) and the elevations of the ground points and breaklines.

digitized raster graphic

(DRG) a digital replica of NGA hard-copy graphic products. See also ADRG.


any process that converts nondigital data into numeric data, usually to be stored on a computer. In ERDAS IMAGINE, digitizing refers to the creation of vector data from hard-copy materials or raster images that are traced using a digitizer keypad on a digitizing tablet, or a mouse on a display device.


see Dual Independent Map Encoding.


a term referring to the number of bands being classified. For example, a data file with three bands is said to be three-dimensional, since three-dimensional spectral space is plotted to analyze the data.


an area of a computer disk that is designated to hold a set of files. Usually, directories are arranged in a tree structure, in which directories can also contain many levels of subdirectories.

Discrete Cosine Transformation

(DCT) an element of a commonly used form of JPEG, which is a compression technique.

Discrete Fourier Transform

(DFT) method of removing striping and other noise from radar images. See also Fast Fourier Transform.


the degree of geometric distortion for a point that is not on the nadir line.

display device

the computer hardware consisting of a memory board and a monitor. It displays a visible image from a data file or from some user operation.

display driver

the ERDAS IMAGINE utility that interfaces between the computer running ERDAS IMAGINE software and the display device.

display memory

the subset of image memory that is actually viewed on the display screen.

display pixel

one grid location on a display device or printout.

display resolution

the number of pixels that can be viewed on the display device monitor, horizontally and vertically (that is, 512 × 512 or 1024 × 1024).


see Euclidean distance, spectral distance.

distance image file

a one-band, 16-bit file that can be created in the classification process, in which each data file value represents the result of the distance equation used in the program. Distance image files generally have a chi-square distribution.


the set of frequencies with which an event occurs, or the set of probabilities that a variable has a particular value.

distribution rectangles

(DR) the geographic data sets into which ADRG data are divided.


a display technique that is used in ERDAS IMAGINE to allow a smaller set of colors appear to be a larger set of colors.


a statistical measure of distance between two or more signatures. Divergence can be calculated for any combination of bands used in the classification; bands that diminish the results of the classification can be ruled out.


a neighborhood analysis technique that outputs the number of different values within a user-specified window.


see Defense Mapping Agency.


see Digital Number.


see digital orthophoto quadrangle.

dot patterns

the matrices of dots used to represent brightness values on hard-copy maps and images.

dots per inch

(DPI) when referring to the resolution of an output device, such as a printer, the number of dots that are printed per unit, for example, 300 dots per inch.


numeric values with fractional values within specific range. Double-precision floating-point number (double), approximately -2.2E308 to 1.8E308.

double precision

a measure of accuracy in which fifteen significant digits can be stored for a coordinate.


the skipping of pixels during the display or processing of the scanning process.


see dots per inch.


see distribution rectangles.


see Digital Terrain Elevation Data.


see digital terrain model.

Dual Independent Map Encoding

(DIME) a type of ETAK feature wherein a line is created along with a corresponding ACODE (arc attribute) record. The coordinates are stored in Lat/Lon decimal degrees. Each record represents a single linear feature.


Data Exchange Format. A format for storing vector data in ASCII files, used by AutoCAD software.

dynamic range

see radiometric resolution.


Earth Observation Satellite Company

(EOSAT) a private company that directed the Landsat satellites and distributed Landsat imagery during the 1980s. Landsat satellites are directed by NASA and Landsat imagery is distributed by the United States Geological Survey.

Earth Resources Observation Systems

(EROS) a division of the USGS National Mapping Division. EROS is involved with managing data and creating systems, as well as research ( USGS, 1999a).

Earth Resources Technology Satellites

(ERTS) in 1972, NASA’s first civilian program to acquire remotely sensed digital satellite data, later renamed to Landsat.


see Enhanced Compressed Wavelet.


see Enhanced Compressed Wavelet Protocol.


see EROS Data Center.

edge detector

a convolution kernel, which is usually a zero-sum kernel, that smooths out or zeros out areas of low spatial frequency and creates a sharp contrast where spatial frequency is high. High spatial frequency is at the edges between homogeneous groups of pixels.

edge enhancer

a high-frequency convolution kernel that brings out the edges between homogeneous groups of pixels. Unlike an edge detector, it only highlights edges, it does not necessarily eliminate other features.

EGM 96

Earth Gravitational Model 1996-an Earth geoid model developed by NGA.

EGM 2008

Earth Gravitational Model 2008-an Earth geoid model developed by NGA.


the length of a principal component that measures the variance of a principal component band. See also principal components.


the direction of a principal component represented as coefficients in an eigenvector matrix which is computed from the eigenvalues. See also principal components.


(EM) type of spectrum consisting of different regions such as thermal infrared and long-wave and short-wave reflective.

electromagnetic radiation

(EMR) the energy transmitted through space in the form of electric and magnetic waves.

electromagnetic spectrum

the range of electromagnetic radiation extending from cosmic waves to radio waves, characterized by frequency or wavelength.


an entity of vector data, such as a point, line, or polygon.

elevation data

see terrain data, DEM.


a two-dimensional figure that is formed in a two-dimensional scatterplot when both bands plotted have normal distributions. The ellipse is defined by the standard deviations of the input bands. Ellipse plots are often used to test signatures before classification.


see electromagnetic.


see ERDAS Macro Language.


see electromagnetic radiation.

end-of-file mark

(EOF) usually a half-inch strip of blank tape that signifies the end of a file that is stored on magnetic tape.

end-of-volume mark

(EOV) usually three EOFs marking the end of a tape.

Enhanced Compressed Wavelet

(ECW) Enhanced Compression Wavelet imagery format, created by ERDAS, significantly reduces the size of image files with minimal deterioration in quality. Wavelet compression involves a way of analyzing an uncompressed image in a recursive manner. This analysis results in a series of sequentially higher resolution images, each augmenting the information in the lower resolution images.

Enhanced Compressed Wavelet Protocol

(ECWP) Enhanced Compression Wavelet Protocol format is used to transmit images compressed with ECW over networks such as the Internet.

Enhanced Thematic Mapper Plus

(ETM+) the observing instrument on Landsat 7.


the process of making an image more interpretable for a particular application. Enhancement can make important features of raw, remotely sensed data more interpretable to the human eye.


an AutoCAD drawing element that can be placed in an AutoCAD drawing with a single command.

Environmental Systems Research Institute

(ESRI) company based in Redlands, California, which produces software such as ArcInfo and ArcView. ESRI has created many data formats, including GRID and GRID Stack.


(ENVIronmental SATellite) launched In 2002 by the European Space Agency. Envisat is an advanced polar-orbiting Earth observation satellite that provides measurements of the atmosphere, ocean, land, and ice. Envisat mission provides for continuity of the observations started with the ERS-1 and ERS-2 satellite missions, notably atmospheric chemistry, ocean studies and ice studies.


see end-of-file mark.


see end-of-volume mark.

ephemeris data

contained in the header of the data file of a SPOT scene, provides information about the recording of the data and the satellite orbit.

epipolar stereopair

a stereopair without y-parallax.

equal area

see equivalence.

equatorial aspect

a map projection that is centered around the equator or a point on the equator.


the property of a map projection to represent true distances from an identified point.


the property of a map projection to represent all areas in true proportion to one another.

ERDAS Macro Language

(EML) computer language that can be used to create custom dialogs in ERDAS IMAGINE, or to edit existing dialogs and functions for your specific application.


see Earth Resources Observation Systems.

EROS Data Center

(EDC) a division of USGS, located in Sioux Falls, SD, which is the primary receiving center for Landsat 7 data.

error matrix

in classification accuracy assessment, a square matrix showing the number of reference pixels that have the same values as the actual classified points.


the European Space Agency’s (ESA) first radar satellite. The mission ended in March 2000. One of its primary instruments was the Along-Track Scanning Radiometer (ATSR). The ATSR monitors changes in vegetation of the Earth’s surface.


the European Space Agency’s (ESA) radar satellite launched in 1995, carrying a number of instruments including SAR, ATSR, and the Global Ozone Monitoring Experiment.


see Earth Resources Technology Satellites.


see European Space Agency.


see Environmental Systems Research Institute.

ETAK MapBase

an ASCII digital street centerline map product available from ETAK, Inc. (Menlo Park, California).


see Enhanced Thematic Mapper Plus.

Euclidean distance

the distance, either in physical or abstract (for example, spectral) space, that is computed based on the equation of a straight line.

exposure station

during image acquisition, each point in the flight path at which the camera exposes the film.


the process of moving selected dangling lines up a specified distance so that they intersect existing lines.


the three letters after the period in a file name that usually identify the type of file.


1. the image area to be displayed in a Viewer. 2. the area of the Earth’s surface to be mapped.

exterior orientation

all images of a block of aerial photographs in the ground coordinate system are computed during photogrammetric triangulation using a limited number of points with known coordinates. The exterior orientation of an image consists of the exposure station and the camera attitude at this moment.

exterior orientation parameters

the perspective center’s ground coordinates in a specified map projection, and three rotation angles around the coordinate axes.

European Space Agency

(ESA) company with two satellites, ERS-1 and ERS-2, that collect radar data. For more information, visit the ESA website at [[1]].


selected bands of a complete set of NOAA AVHRR data.


false color

a color scheme in which features have expected colors. For instance, vegetation is green and water is blue. These are not necessarily the true colors of these features.

false easting

an offset between the y-origin of a map projection and the y-origin of a map. Typically used so that no y-coordinates are negative.

false northing

an offset between the x-origin of a map projection and the x-origin of a map. Typically used so that no x-coordinates are negative.

fast format

a type of BSQ format used by EOSAT to store Landsat TM data.

Fast Fourier Transform

(FFT) a type of Fourier Transform faster than the DFT. Designed to remove noise and periodic features from radar images. It converts a raster image from the spatial domain into a frequency domain image.

feature based matching

an image matching technique that determines the correspondence between two image features.

feature collection

the process of identifying, delineating, and labeling various types of natural and human-made phenomena from remotely-sensed images.

feature extraction

the process of studying and locating areas and objects on the ground and deriving useful information from images.

feature space

an abstract space that is defined by spectral units (such as an amount of electromagnetic radiation).

feature space area of interest

a user-selected area of interest (AOI) that is selected from a feature space image.

feature space image

a graph of the data file values of one band of data against the values of another band (often called a scatterplot).


see Fast Fourier Transform.

fiducial center

the center of an aerial photo.


four or eight reference markers fixed on the frame of an aerial metric camera and visible in each exposure. Fiducials are used to compute the transformation from data file to image coordinates.


in an attribute database, a category of information about each class or feature, such as Class name and Histogram.

field of view

(FOV) in perspective views, an angle that defines how far the view is generated to each side of the line of sight.

file coordinates

the location of a pixel within the file in x,y coordinates. The upper left file coordinate is usually 0,0.

file pixel

the data file value for one data unit in an image file.

file specification or filespec

the complete file name, including the drive and path, if necessary. If a drive or path is not specified, the file is assumed to be in the current drive and directory.


referring to polygons; a filled polygon is solid or has a pattern, but is not transparent. An unfilled polygon is simply a closed vector that outlines the area of the polygon.


the removal of spatial or spectral features for data enhancement. Convolution filtering is one method of spatial filtering. Some texts may use the terms filtering and spatial filtering synonymously.


the process of reversing the from-to direction of selected lines or links.

focal length

the orthogonal distance from the perspective center to the image plane.

focal operations

filters that use a moving window to calculate new values for each pixel in the image based on the values of the surrounding pixels.

focal plane

the plane of the film or scanner used in obtaining an aerial photo.

Fourier analysis

an image enhancement technique that was derived from signal processing.


see field of view.


the first vertex in a line.

full set

all bands of a NOAA AVHRR data set.

function memories

areas of the display device memory that store the lookup tables, which translate image memory values into brightness values.

function symbol

an annotation symbol that represents an activity. For example, on a map of a state park, a symbol of a tent would indicate the location of a camping area.

Fuyo 1 (JERS-1)

the Japanese radar satellite launched in February 1992.



see global area coverage.


see Geographic Base File.


see ground control point.

GCP matching

for image-to-image rectification, a GCP selected in one image is precisely matched to its counterpart in the other image using the spectral characteristics of the data and the transformation matrix.

GCP prediction

the process of picking a GCP in either coordinate system and automatically locating that point in the other coordinate system based on the current transformation parameters.


the process of weeding vertices from selected lines using a specified tolerance.

geocentric coordinate system

a coordinate system that has its origin at the center of the Earth ellipsoid. The ZG-axis equals the rotational axis of the Earth, and the XG-axis passes through the Greenwich meridian. The YG-axis is perpendicular to both the ZG-axis and the XG-axis, so as to create a three-dimensional coordinate system that follows the right-hand rule.

geocoded data

an image(s) that has been rectified to a particular map projection and cell size and has had radiometric corrections applied.

Geographic Base File

(GBF) along with DIME, sometimes provides the cartographic base for TIGER/Line files, which cover the US, Puerto Rico, Guam, the Virgin Islands, American Samoa, and the Trust Territories of the Pacific.

geographic information system

(GIS) a unique system designed for a particular application that stores, enhances, combines, and analyzes layers of geographic data to produce interpretable information. A GIS may include computer images, hard-copy maps, statistical data, and any other data needed for a study, as well as computer software and human knowledge. GISs are used for solving complex geographic planning and management problems.

geographical coordinates

a coordinate system for explaining the surface of the Earth. Geographical coordinates are defined by latitude and by longitude (Lat/Lon), with respect to an origin located at the intersection of the equator and the prime (Greenwich) meridian.

geometric correction

the correction of errors of skew, rotation, and perspective in raw, remotely sensed data.


the process of assigning map coordinates to image data and resampling the pixels of the image to conform to the map projection grid.


TIFF files that are geocoded.


(Gb) about one billion bytes.


see geographic information system.

GIS file

a single-band ERDAS Ver. 7.X data file in which pixels are divided into discrete categories.

global area coverage

(GAC) a type of NOAA AVHRR data with a spatial resolution of 4 × 4 km.

global operations

functions that calculate a single value for an entire area, rather than for each pixel like focal functions.

GLObal NAvigation Satellite System

(GLONASS) a satellite-based navigation system produced by the Russian Space Forces. It provides three-dimensional locations, velocity, and time measurements for both civilian and military applications. GLONASS started its mission in 1993 ( Magellan Corporation, 1999).

Global Ozone Monitoring Experiment

(GOME) instrument aboard ESA’s ERS-2 satellite, which studies atmospheric chemistry ( European Space Agency, 1995).

Global Positioning System

(GPS) system used for the collection of GCPs, which uses orbiting satellites to pinpoint precise locations on the Earth’s surface.


see GLObal NAvigation Satellite System.

.gmd file

the ERDAS IMAGINE graphical model file created with Model Maker (Spatial Modeler).


open, nonproprietary language used to created geo-spatial objects for the purpose of data sharing. GML also serves as a data transport for geospatial objects as well as exists as a means for describing geospatial Web services.


an azimuthal projection obtained from a perspective at the center of the Earth.


see Global Ozone Monitoring Experiment.


see Global Positioning System.

graphical modeling

a technique used to combine data layers in an unlimited number of ways using icons to represent input data, functions, and output data. For example, an output layer created from modeling can represent the desired combination of themes from many input layers.

graphical model

a model created with Model Maker (Spatial Modeler). Graphical models are put together like flow charts and are stored in .gmd files.

Graphical User Interface

(GUI) the dialogs and menus of ERDAS IMAGINE that enable you to execute commands to analyze your imagery.


the network of parallels of latitude and meridians of longitude applied to the global surface and projected onto maps.

gray scale

a color scheme with a gradation of gray tones ranging from black to white.

great circle

an arc of a circle for which the center is the center of the Earth. A great circle is the shortest possible surface route between two points on the Earth.


a compressed tiled raster data structure that is stored as a set of files in a directory, including files to keep the attributes of the GRID.

grid cell

a pixel.

grid lines

intersecting lines that indicate regular intervals of distance based on a coordinate system. Sometimes called a graticule.

Grid North

Direction pointing north on a map that runs parallel to a prime meridian. Grid North lines show direction on a 2-dimensional plane in contrast to True North lines showing direction on a 3-dimensional sphere.

GRID Stack

multiple GRIDs to be treated as a multilayer image.

ground control point

(GCP) specific pixel in image data for which the output map coordinates (or other output coordinates) are known. GCPs are used for computing a transformation matrix, for use in rectifying an image.

ground coordinate system

a three-dimensional coordinate system that uses a known map projection. Ground coordinates (X,Y,Z) are usually expressed in feet or meters.

ground truth

data that are taken from the actual area being studied.

ground truthing

the acquisition of knowledge about the study area from field work, analysis of aerial photography, personal experience, and so forth. Ground truth data are considered to be the most accurate (true) data available about the area of study.


see Graphical User Interface.



the process of using dots of varying size or arrangements (rather than varying intensity) to form varying degrees of a color.

hard-copy output

any output of digital computer (soft copy) data to paper.


see High Accuracy Reference Network.

header file

a file usually found in a companion file, or before the actual image data on DVDs, CDs, or tapes that contains information about the data, such as number of bands, upper left coordinates, map projection, and so forth.

header record

the first part of an image file that contains general information about the data in the file, such as the number of columns and rows, number of bands, database coordinates of the upper left corner, and the pixel depth. The contents of header records vary depending on the type of data.


see Hierarchal File Architecture System.

Hierarchal File Architecture System

(HFA) a format that allows different types of information about a file to be stored in a tree-structured fashion. The tree is made of nodes that contain information such as ephemeris data.

High Accuracy Reference Network

(HARN) HARN is based on the GRS 1980 spheroid, and can be used to perform State Plane calculations.

high-frequency kernel

a convolution kernel that increases the spatial frequency of an image. Also called high-pass kernel.

High Resolution Picture Transmission

(HRPT) the direct transmission of AVHRR data in real-time with the same resolution as LAC.

High Resolution Visible Infrared

(HR VIR) a pushbroom scanner on the SPOT 4 satellite, which captures information in the visible and near-infrared bands ( SPOT Image, 1999).

High Resolution Visible sensor

(HRV) a pushbroom scanner on a SPOT satellite that takes a sequence of line images while the satellite circles the Earth.


a graph of data distribution, or a chart of the number of pixels that have each possible data file value. For a single band of data, the horizontal axis of a histogram graph is the range of all possible data file values. The vertical axis is a measure of pixels that have each data value.

histogram equalization

the process of redistributing pixel values so that there are approximately the same number of pixels with each value within a range. The result is a nearly flat histogram.

histogram matching

the process of determining a lookup table that converts the histogram of one band of an image or one color gun to resemble another histogram.

horizontal control

the horizontal distribution of GCPs in aerial triangulation (x,y - planimetry).

host workstation

a CPU, keyboard, mouse, and a display.


see High Resolution Picture Transmission.


see High Resolution Visible sensor.


see High Resolution Visible Infrared.


a component of IHS (intensity, hue, saturation) that is representative of the color or dominant wavelength of the pixel. It varies from 0 to 360. Blue = 0 (and 360), magenta = 60, red = 120, yellow = 180, green = 240, and cyan = 300.

hyperspectral sensors

the imaging sensors that record multiple bands of data, such as the AVIRIS with 224 bands.



see Internal Average Relative Reflectance.


see Inverse Fast Fourier Transform.


see instantaneous field of view.


see Initial Graphics Exchange Standard files.


intensity, hue, saturation. An alternate color space from RGB (red, green, blue). This system is advantageous in that it presents colors more nearly as perceived by the human eye. See intensity, hue, and saturation.


a picture or representation of an object or scene on paper, or a display screen. Remotely sensed images are digital representations of the Earth.

image algebra

any type of algebraic function that is applied to the data file values in one or more bands.

image center

the center of the aerial photo or satellite scene.

image coordinate system

the location of each point in the image is expressed for purposes of photogrammetric triangulation.

image data

digital representations of the Earth that can be used in computer image processing and GIS analyses.

image file

a file containing raster image data. Image files in ERDAS IMAGINE have the extension .img. Image files from the ERDAS Ver. 7.X series software have the extension .LAN or .GIS.

image matching

the automatic acquisition of corresponding image points on the overlapping area of two images.

image memory

the portion of the display device memory that stores data file values, which may be transformed or processed by the software that accesses the display device.

image pair

see stereopair.

image processing

the manipulation of digital image data, including (but not limited to) enhancement, classification, and rectification operations.

image pyramid

a data structure consisting of the same image represented several times, at a decreasing spatial resolution each time. Each level of the pyramid contains the image at a particular resolution.

image scale

(SI) expresses the average ratio between a distance in the image and the same distance on the ground. It is computed as focal length divided by the flying height above the mean ground elevation.

image space coordinate system

identical to image coordinates, except that it adds a third axis (z) that is used to describe positions inside the camera. The units are usually in millimeters or microns.


see International Map Committee.

.img file

(also, image file) an ERDAS IMAGINE file that stores continuous or thematic raster layers.


see International Map of the World.


the angle between a vertical on the ground at the center of the scene and a light ray from the exposure station, which defines the degree of off-nadir viewing when the scene was recorded.


a function applied to thematic layers that adds the data file values of two or more layers together, creating a new output layer. Weighting factors can be applied to one or more layers to add more importance to those layers in the final sum.

index map

a reference map that outlines the mapped area, identifies all of the component maps for the area if several map sheets are required, and identifies all adjacent map sheets.

Indian Remote Sensing Satellite

(IRS) satellites operated by Space Imaging, including IRS-1A, IRS-1B, IRS-1C, and IRS-1D.


the process used to create output images by mathematically combining the DN values of different bands.


something that is independently meaningful, as opposed to data, which are not independently meaningful.


a process that ensures all values in a file or in computer memory are equal until additional information is added or processed to overwrite these values. Usually the initialization value is 0. If initialization is not performed on a data file, there could be random data values in the file.

inset map

a map that is an enlargement of some congested area of a smaller scale map, and that is usually placed on the same sheet with the smaller scale main map.

instantaneous field of view

(IFOV) a measure of the area viewed by a single detector on a scanning system in a given instant in time.


a component of IHS (intensity, hue, saturation), which is the overall brightness of the scene and varies from 0 (black) to 1 (white).


method of subtracting the phase of one SAR image from another to derive height information.

interior orientation

defines the geometry of the sensor that captured a particular image.

Internal Average Relative Reflectance

(IARR) a technique designed to compensate for atmospheric contamination of the spectra.

International Map Committee

(IMC) located in London, the committee responsible for creating the International Map of the World series.

International Map of the World

(IMW) a series of maps produced by the International Map Committee. Maps are in 1:1,000,000 scale.


the area or set that is common to two or more input areas or sets.

interval data

a type of data in which thematic class values have a natural sequence, and in which the distances between values are meaningful.

Inverse Fast Fourier Transform

(IFFT) used after the Fast Fourier Transform to transform a Fourier image back into the spatial domain. See also Fast Fourier Transform.


infrared portion of the electromagnetic spectrum. See also electromagnetic spectrum.


see Indian Remote Sensing Satellite.

isarithmic map

a map that uses isorithms (lines connecting points of the same value for any of the characteristics used in the representation of surfaces) to represent a statistical surface. Also called an isometric map.

ISODATA clustering

see Iterative Self-Organizing Data Analysis Technique.


A single line that connects with itself.

isopleth map

a map on which isopleths (lines representing quantities that cannot exist at a point, such as population density) are used to represent some selected quantity.


a term used to describe a process in which some operation is performed repeatedly.

Iterative Self-Organizing Data Analysis Technique

(ISODATA clustering) a method of clustering that uses spectral distance as in the sequential method, but iteratively classifies the pixels, redefines the criteria for each class, and classifies again, so that the spectral distance patterns in the data gradually emerge.


JERS-1 (Fuyo 1)

Japanese Earth Resource Satellite (JERS-1) obtained data from 1992 to 1998, and has been superseded by the ALOS mission.

Jet Propulsion Laboratories

(JPL) "the lead U.S. center for robotic exploration of the solar system." JPL is managed for NASA by the California Institute of Technology. For more information, see the JPL website at [[2]] ( National Aeronautics and Space Administration, 1999).


see JPEG File Interchange Format.


the process of interactively entering the side lot lines when the front and rear lines have already been established.

Joint Photographic Experts Group

(JPEG) 1. a group responsible for creating a set of compression techniques. 2. Compression techniques are also called JPEG.


see Joint Photographic Experts Group.

JPEG File Interchange Format

(JFIF) standard file format used to store JPEG-compressed imagery.


see Jet Propulsion Laboratories.


Kappa coefficient

a number that expresses the proportionate reduction in error generated by a classification process compared with the error of a completely random classification.


see convolution kernel.



in annotation, the text that conveys important information to the reader about map features.

label point

a point within a polygon that defines that polygon.


see local area coverage.

.LAN files

multiband ERDAS Ver. 7.X image files (the name originally derived from the Landsat satellite). LAN files usually contain raw or enhanced remotely sensed data.

land cover map

a map of the visible ground features of a scene, such as vegetation, bare land, pasture, urban areas, and so forth.


a series of Earth-orbiting satellites that gather MSS and TM imagery, operated by EOSAT.


a description used to represent a map or data file having a large ratio between the area on the map (such as inches or pixels), and the area that is represented (such as feet). In large-scale image data, each pixel represents a small area on the ground, such as SPOT data, with a spatial resolution of 10 or 20 meters.


Latitude/Longitude, a map coordinate system.


1. a band or channel of data. 2. a single band or set of three bands displayed using the red, green, and blue color channels of the ERDAS IMAGINE Viewer. A layer could be a remotely sensed image, an aerial photograph, an annotation layer, a vector layer, an area of interest layer, and so forth. 3. a component of a GIS data base that contains all of the data for one theme. A layer consists of a thematic image file, and may also include attributes.

least squares correlation

uses the least squares estimation to derive parameters that best fit a search window to a reference window.

least squares regression

the method used to calculate the transformation matrix from the GCPs. This method is discussed in statistics textbooks.


the reference that lists the colors, symbols, line patterns, shadings, and other annotation that is used on a map, and their meanings. The legend often includes the map’s title, scale, origin, and other information.


the manner in which place names and other labels are added to a map, including letter spacing, orientation, and position.

level 1A (SPOT)

an image that corresponds to raw sensor data to which only radiometric corrections have been applied.

level 1B (SPOT)

an image that has been corrected for the Earth’s rotation and to make all pixels 10 × 10 on the ground. Pixels are resampled from the level 1A sensor data by cubic polynomials.

level slice

the process of applying a color scheme by equally dividing the input values (image memory values) into a certain number of bins, and applying the same color to all pixels in each bin. Usually, a ROYGBIV (red, orange, yellow, green, blue, indigo, violet) color scheme is used.

LIDAR data

(LIDAR) Light Detection and Ranging uses 1.064 nanometer wavelength laser light pulses to measure distances by recording the time delay from the transmitted pulse and light reflected back from objects. LIDAR data are usually collected as 3D (x,y,z) points, or point clouds.


1. a vector data element consisting of a line (the set of pixels directly between two points), or an unclosed set of lines. 2. a row of pixels in a data file.

line dropout

a data error that occurs when a detector in a satellite either completely fails to function or becomes temporarily overloaded during a scan. The result is a line, or partial line of data with incorrect data file values creating a horizontal streak until the detector(s) recovers, if it recovers.


a description of a function that can be graphed as a straight line or a series of lines. Linear equations (transformations) can generally be expressed in the form of the equation of a line or plane. Also called 1st-order.

linear contrast stretch

an enhancement technique that outputs new values at regular intervals.

linear transformation

a 1st-order rectification. A linear transformation can change location in X and/or Y, scale in X and/or Y, skew in X and/or Y, and rotation.

line of sight

in perspective views, the point(s) and direction from which the viewer is looking into the image.

local area coverage

(LAC) a type of NOAA AVHRR data with a spatial resolution of 1.1 × 1.1 km.

logical record

a series of bytes that form a unit on a 9-track tape. For example, all the data for one line of an image may form a logical record. One or more logical records make up a physical record on a tape.

long wave infrared region

(LWIR) the thermal or far-infrared region of the electromagnetic spectrum.

lookup table

(LUT) an ordered set of numbers that is used to perform a function on a set of input values. To display or print an image, lookup tables translate data file values into brightness values.


"a term describing a data compression algorithm which actually reduces the amount of information in the data, rather than just the number of bits used to represent that information" ( Free On-Line Dictionary of Computing, 1999c).

low-frequency kernel

a convolution kernel that decreases spatial frequency. Also called low-pass kernel.


see lookup table.


see long wave infrared region.


Machine Independent Format

(MIF) a format designed to store data in a way that it can be read by a number of different machines.


the process of displaying one file pixel over a block of display pixels. For example, if the magnification factor is 3, then each file pixel takes up a block of 3 × 3 display pixels. Magnification differs from zooming in that the magnified image is loaded directly to image memory.


an element of an electromagnetic wave. Magnitude of a wave decreases exponentially as the distance from the transmitter increases.

Mahalanobis distance

a classification decision rule that is similar to the minimum distance decision rule, except that a covariance matrix is used in the equation.


a neighborhood analysis technique that outputs the most common value of the data file values in a user-specified window.


see Maximum A Posteriori.


a graphic representation of spatial relationships on the Earth or other planets.

map coordinates

a system of expressing locations on the Earth’s surface using a particular map projection, such as UTM, State Plane, or Polyconic.

map frame

an annotation element that indicates where an image is placed in a map composition.

map projection

a method of representing the three-dimensional spherical surface of a planet on a two-dimensional map surface. All map projections involve the transfer of latitude and longitude onto an easily flattened surface.


a set of numbers arranged in a rectangular array. If a matrix has i rows and j columns, it is said to be an i × j matrix.

matrix analysis

a method of combining two thematic layers in which the output layer contains a separate class for every combination of two input classes.

matrix object

in Model Maker (Spatial Modeler), a set of numbers in a two-dimensional array.


a neighborhood analysis technique that outputs the greatest value of the data file values in a user-specified window.

Maximum A Posteriori

(MAP) a filter (Gamma-MAP) that is designed to estimate the original DN value of a pixel, which it assumes is between the local average and the degraded DN.

maximum likelihood

a classification decision rule based on the probability that a pixel belongs to a particular class. The basic equation assumes that these probabilities are equal for all classes, and that the input bands have normal distributions.

.mdl file

an ERDAS IMAGINE script model created with the Spatial Modeler Language.


1. the statistical average; the sum of a set of values divided by the number of values in the set. 2. a neighborhood analysis technique that outputs the mean value of the data file values in a user-specified window.

mean vector

an ordered set of means for a set of variables (bands). For a data file, the mean vector is the set of means for all bands in the file.

measurement vector

the set of data file values for one pixel in all bands of a data file.


1. the central value in a set of data such that an equal number of values are greater than and less than the median. 2. a neighborhood analysis technique that outputs the median value of the data file values in a user-specified window.


(Mb) about one million bytes.

memory resident

a term referring to the occupation of a part of a computer’s RAM (random access memory), so that a program is available for use without being loaded into memory from disk.


the measurement of linear or areal distance.


a line of longitude, going north and south. See geographical coordinates.


often textual descriptive data about GI or other types of data. This information can include the following: description, location, accessibility, format, specific use, spatial and temporal location, collection location and agency, and purpose.


see Machine Independent Format.


a neighborhood analysis technique that outputs the least value of the data file values in a user-specified window.

minimum distance

a classification decision rule that calculates the spectral distance between the measurement vector for each candidate pixel and the mean vector for each signature. Also called spectral distance.


a neighborhood analysis technique that outputs the least common value of the data file values in a user-specified window.


the most commonly-occurring value in a set of data. In a histogram, the mode is the peak of the curve.


in a GIS, the set of expressions, or steps, that defines your criteria and creates an output layer.


the process of creating new layers from combining or operating upon existing layers. Modeling allows the creation of new classes from existing classes and the creation of a small set of images, perhaps even a single image, which at a glance contains many types of information about a scene.

modified projection

a map projection that is a modified version of another projection. For example, the Space Oblique Mercator projection is a modification of the Mercator projection.

monochrome image

an image produced from one band or layer, or contained in one color gun of the display device.

morphometric map

a map representing morphological features of the Earth’s surface.


the process of piecing together images side by side, to create a larger image.


see Multiresolution Seamless Image Database.


see multispectral scanner.

Multiresolution Seamless Image Database

(MrSID) a wavelet transform-based compression algorithm designed by LizardTech, Inc.

multispectral classification

the process of sorting pixels into a finite number of individual classes, or categories of data, based on data file values in multiple bands. See also classification.

multispectral imagery

satellite imagery with data recorded in two or more bands.

multispectral scanner

(MSS) Landsat satellite data acquired in four bands with a spatial resolution of 57 × 79 meters.


data from two or more different dates.



see North America Datum 1927.


see North America Datum 1983.


the area on the ground directly beneath a scanner’s detectors.

nadir line

the average of the left and right edge lines of a Landsat image.

nadir point

the center of the nadir line in vertically viewed imagery.


see National Aeronautics and Space Administration.

National Aeronautics and Space Administration

(NASA) an organization that studies outer space. For more information, visit the NASA website at [[3]].

National Geospatial-Intelligence Agency

(NGA) formerly NIMA. In 2003, NIMA became the National Geospatial-Intelligence Agency (NGA). NGA is a U.S. Department of Defense combat support agency and a member of the national Intelligence Community (IC), developing imagery and map-based geospatial intelligence (GEOINT) solutions. For more information, visit the NGA website at

National Imagery and Mapping Agency

(NIMA) formerly DMA. The agency was formed in October of 1996. NIMA supplied imagery and geospatial data ( National Imagery and Mapping Agency, 1998). In 2003, NIMA became the National Geospatial-Intelligence Agency (NGA). See National Geospatial-Intelligence Agency.

National Imagery Transmission Format Standard

(NITFS) a format designed to package imagery with complete annotation, text attachments, and imagery-associated metadata ( Jordan and Beck, 1999).

National Ocean Service

(NOS) the organization that created a zone numbering system for the State Plane coordinate system. A division of NOAA. For more information, visit the NOS website at [[4]].

National Oceanic and Atmospheric Administration

(NOAA) an organization that studies weather, water bodies, and encourages conservation. For more information, visit the NOAA website at [[5]].

Navigation System with Time and Ranging

(NAVSTAR) satellite launched in 1978 for collection of GPS data.


see Navigation System with Time and Ranging.


see Normalized Difference Vegetation Index.

nearest neighbor

a resampling method in which the output data file value is equal to the input pixel that has coordinates closest to the retransformed coordinates of the output pixel.


a rectangular border printed around a map. On scaled maps, neatlines usually have tick marks that indicate intervals of map coordinates or distance.

negative inclination

the sensors are tilted in increments of 0.6° to a maximum of 27° to the east.

neighborhood analysis

any image processing technique that takes surrounding pixels into consideration, such as convolution filtering and scanning.


see National Geospatial-Intelligence Agency.


see National Imagery and Mapping Agency.


CCTs that hold digital data.


see National Imagery Transmission Format Standard.


see National Oceanic and Atmospheric Administration.


the ending points of a line. See from-node and to-node.

nominal data

a type of data in which classes have no inherent order, and therefore are qualitative.


describing a function that cannot be expressed as the graph of a line or in the form of the equation of a line or plane. Nonlinear equations usually contain expressions with exponents. Second-order (2nd-order) or higher-order equations and transformations are nonlinear.

nonlinear transformation

a 2nd-order or higher rectification.

nonparametric signature

a signature for classification that is based on polygons or rectangles that are defined in the feature space image for the image file. There is no statistical basis for a nonparametric signature; it is simply an area in a feature space image.


the state of having a normal distribution.

normal distribution

a symmetrical data distribution that can be expressed in terms of the mean and standard deviation of the data. The normal distribution is the most widely encountered model for probability, and is characterized by the bell curve. Also called Gaussian distribution.


a process that makes an image appear as if it were a flat surface. This technique is used to reduce topographic effect.

Normalized Difference Vegetation Index

(NDVI) the formula for NDVI is IR-R/IR+R, where IR stands for the infrared portion of the electromagnetic spectrum, and R stands for the red portion of the electromagnetic spectrum. NDVI finds areas of vegetation in imagery.

North America Datum 1927

(NAD27) a datum created in 1927 that is based on the Clarke 1866 spheroid. Commonly used in conjunction with the State Plane coordinate system.

North America Datum 1983

(NAD83) a datum created in 1983 that is based on the GRS 1980 spheroid. Commonly used in conjunction with the State Plane coordinate system.


see National Ocean Service.

NPO Mashinostroenia

a company based in Russia that develops satellites, such as Almaz 1-B, for GIS application.

number maps

maps that output actual data file values or brightness values, allowing the analysis of the values of every pixel in a file or on the display screen.

numeric keypad

the set of numeric and mathematical operator keys (+, -, and so forth) that is usually on the right side of the keyboard.


in image registration, the original continuous function can be reconstructed from the sampled data, and phase function can be reconstructed to much higher resolution.



in models, an input to or output from a function. See matrix object, raster object, scalar object, table object.

oblique aspect

a map projection that is not oriented around a pole or the Equator.


in photogrammetric triangulation, a grouping of the image coordinates for a GCP.


any point that is not directly beneath a scanner’s detectors, but off to an angle. The SPOT scanner allows off-nadir viewing.


1:24,000 scale data, also called 7.5-minute DEM, available from USGS. It is usually referenced to the UTM coordinate system and has a spatial resolution of 30 × 30 meters.


1:250,000 scale DEM data available from USGS. Available only in arc/second format.


a measure of how opaque, or solid, a color is displayed in a raster layer.

Open Geospatial Consortium (OGC)

OGC is an international industry consortium of more than 230 companies, government agencies and universities participating in a consensus process to develop publicly available geoprocessing specifications.

operating system

(OS) the most basic means of communicating with the computer. It manages the storage of information in files and directories, input from devices such as the keyboard and mouse, and output to devices such as the monitor.


a circular, north-south and south-north path that a satellite travels above the Earth.


the complexity of a function, polynomial expression, or curve. In a polynomial expression, the order is simply the highest exponent used in the polynomial. See also linear, nonlinear.

ordinal data

a type of data that includes discrete lists of classes with an inherent order, such as classes of streams;first order, second order, third order, and so forth.

orientation angle

the angle between a perpendicular to the center scan line and the North direction in a satellite scene.


an azimuthal projection with an infinite perspective.


see orthorectification.


see digital orthophoto.


an image map product produced from orthoimages, or orthoimage mosaics, that is similar to a standard map in that it usually includes additional information, such as map coordinate grids, scale bars, north arrows, and other marginalia.


a form of rectification that corrects for terrain displacement and can be used if a DEM of the study area is available.


see operating system.

outline map

a map showing the limits of a specific set of mapping entities such as counties. Outline maps usually contain a very small number of details over the desired boundaries with their descriptive codes.


1. a function that creates a composite file containing either the minimum or the maximum class values of the input files. Overlay sometimes refers generically to a combination of layers. 2. the process of displaying a classified file over the original image to inspect the classification.

overlay file

an ERDAS IMAGINE annotation file (.ovr extension).

.ovr file

an ERDAS IMAGINE annotation file.



to store data in a way that conserves tape or disk space.

panchromatic imagery

single-band or monochrome satellite imagery.

paneled map

a map designed to be spliced together into a large paper map. Therefore, neatlines and tick marks appear on the outer edges of the large map.

pairwise mode

an operation mode in rectification that allows the registration of one image to an image in another Viewer, a map on a digitizing tablet, or coordinates entered at the keyboard.


displacement of a GCP appearing in a stereopair as a function of the position of the sensors at the time of image capture. You can adjust parallax in both the X and the Y direction so that the image point in both images appears in the same image space.


a line of latitude, going east and west.


1. a classification decision rule in which the data file values of the candidate pixel are compared to upper and lower limits. 2. the limits of a parallelepiped classification, especially when graphed as rectangles.


1. any variable that determines the outcome of a function or operation. 2. the mean and standard deviation of data, which are sufficient to describe a normal curve.

parametric signature

a signature that is based on statistical parameters (for example, mean and covariance matrix) of the pixels that are in the training sample or cluster.

passive sensors

solar imaging sensors that can only receive radiation waves and cannot transmit radiation.


the drive, directories, and subdirectories that specify the location of a file.

pattern recognition

the science and art of finding meaningful patterns in data, which can be extracted through classification.


see principal components.


see principal components analysis.

perspective center

1. a point in the image coordinate system defined by the x and y coordinates of the principal point and the focal length of the sensor. 2. after triangulation, a point in the ground coordinate system that defines the sensor’s position relative to the ground.

perspective projection

the projection of points by straight lines from a given perspective point to an intersection with the plane of projection.


an element of an electromagnetic wave.

phase flattening

In IMAGINE InSAR, removes the phase function that would result if the imaging area was flat from the actual phase function recorded in the interferogram.

phase unwrapping

In IMAGINE InSAR, the process of taking a wrapped phase function and reconstructing the continuous function from it.

photogrammetric quality scanners

special devices capable of high image quality and excellent positional accuracy. Use of this type of scanner results in geometric accuracies similar to traditional analog and analytical photogrammetric instruments.


the "art, science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring and interpreting photographic images and patterns of electromagnetic radiant imagery and other phenomena" ( American Society of Photogrammetry, 1980).

physical record

a consecutive series of bytes on a 9-track tape followed by a gap, or blank space, on the tape.

piecewise linear contrast stretch

a spectral enhancement technique used to enhance a specific portion of data by dividing the lookup table into three sections: low, middle, and high.


abbreviated from picture element; the smallest part of a picture (image).

pixel coordinate system

a coordinate system with its origin in the upper left corner of the image, the x-axis pointing to the right, the y-axis pointing downward, and the units in pixels.

pixel depth

the number of bits required to store all of the data file values in a file. For example, data with a pixel depth of 8, or 8-bit data, have 256 values (28 = 256), ranging from 0 to 255.

pixel size

the physical dimension of a single light-sensitive element (13 × 13 microns).

planar coordinates

coordinates that are defined by a column and row position on a grid (x,y).

planar projection

see azimuthal projection.

plane table photogrammetry

prior to the invention of the airplane, photographs taken on the ground were used to extract the geometric relationships between objects using the principles of Descriptive Geometry.

planimetric map

a map that correctly represents horizontal distances between objects.

plan symbol

an annotation symbol that is formed after the basic outline of the object it represents. For example, the symbol for a house might be a square, since most houses are rectangular.


1. an element consisting of a single (x,y) coordinate pair. Also called grid cell. 2. a vertex of an element. Also called a node.

point ID

in rectification, a name given to GCPs in separate files that represent the same geographic location.

point mode

a digitizing mode in which one vertex is generated each time a keypad button is pressed.

polar aspect

a map projection that is centered around a pole.


the direction of the electric field component with the understanding that the magnetic field is perpendicular to it.


a set of closed line segments defining an area.


a mathematical expression consisting of variables and coefficients. A coefficient is a constant that is multiplied by a variable in the expression.

positive inclination

the sensors are tilted in increments of 0.6° to a maximum of 27° to the west.

primary colors

colors from which all other available colors are derived. On a display monitor, the primary colors red, green, and blue are combined to produce all other colors. On a color printer, cyan, yellow, and magenta inks are combined.

principal components

(PC) the transects of a scatterplot of two or more bands of data that represent the widest variance and successively smaller amounts of variance that are not already represented. Principal components are orthogonal (perpendicular) to one another. In principal components analysis, the data are transformed so that the principal components become the axes of the scatterplot of the output data.

principal components analysis

(PCA) a method of data compression that allows redundant data to be compressed into fewer bands (Jensen, 1996; Faust, 1989).

principal component band

a band of data that is output by principal components analysis. Principal component bands are uncorrelated and nonredundant, since each principal component describes different variance within the original data.

principal components analysis

the process of calculating principal components and outputting principal component bands. It allows redundant data to be compacted into fewer bands (that is, the dimensionality of the data is reduced).

principal point (Xp, Yp)

the point in the image plane onto which the perspective center is projected, located directly beneath the interior orientation.


a device that prints text, full color imagery, and/or graphics. See color printer, text printer.


a row of data file values from a DEM or DTED file. The profiles of DEM and DTED run south to north (that is, the first pixel of the record is the southernmost pixel).

profile symbol

an annotation symbol that is formed like the profile of an object. Profile symbols generally represent vertical objects such as trees, windmills, oil wells, and so forth.

proximity analysis

a technique used to determine which pixels of a thematic layer are located at specified distances from pixels in a class or classes. A new layer is created that is classified by the distance of each pixel from specified classes of the input layer.

pseudo color

a method of displaying an image (usually a thematic layer) that allows the classes to have distinct colors. The class values of the single band file are translated through all three function memories that store a color scheme for the image.

pseudo node

a single line that connects with itself (an island), or where only two lines intersect.

pseudo projection

a map projection that has only some of the characteristics of another projection.


a scanner in which all scanning parts are fixed, and scanning is accomplished by the forward motion of the scanner, such as the SPOT scanner.

pyramid layers

image layers that are successively reduced by the power of 2 and resampled. Pyramid layers enable large images to display faster.



1. any of the hard-copy maps distributed by USGS such as the 7.5-minute quadrangle or the 15-minute quadrangle. 2. one quarter of a full Landsat TM scene. Commonly called a quad.

qualitative map

a map that shows the spatial distribution or location of a kind of nominal data. For example, a map showing corn fields in the US would be a qualitative map. It would not show how much corn is produced in each location, or production relative to other areas.

quantitative map

a map that displays the spatial aspects of numerical data. A map showing corn production (volume) in each area would be a quantitative map.


radar data

the remotely sensed data that are produced when a radar transmitter emits a beam of micro or millimeter waves, the waves reflect from the surfaces they strike, and the backscattered radiation is detected by the radar system’s receiving antenna, which is tuned to the frequency of the transmitted waves.


RADARSAT-1 satellite, developed by the Canadian Space Agency, carries SAR sensors. RADARSAT-2 carries SAR sensors as well, and was developed by the Canadian Space Agency and MacDonald, Dettwiler, and Associates, Ltd. (MDA).

radiative transfer equations

the mathematical models that attempt to quantify the total atmospheric effect of solar illumination.

radiometric correction

the correction of variations in data that are not caused by the object or scene being scanned, such as scanner malfunction and atmospheric interference.

radiometric enhancement

an enhancement technique that deals with the individual values of pixels in an image.

radiometric resolution

the dynamic range, or number of possible data file values, in each band. This is referred to by the number of bits into which the recorded energy is divided. See pixel depth.


see random-access memory.

random-access memory

(RAM) memory used for applications and data storage on a CPU ( Free On-Line Dictionary of Computing, 1999d).


a neighborhood analysis technique that outputs the number of values in a user-specified window that are less than the analyzed value.


see Real-Aperture Radar.

raster data

data that are organized in a grid of columns and rows. Raster data usually represent a planar graph or geographical area. Raster data in ERDAS IMAGINE are stored in image files.

raster object

in Model Maker (Spatial Modeler), a single raster layer or set of layers.

Raster Product Format

(RPF) Data from NIMA, used primarily for military purposes. Organized in 1536 × 1536 frames, with an internal tile size of 256 × 256 pixels.

raster region

a contiguous group of pixels in one GIS class. Also called clump.

ratio data

a data type in which thematic class values have the same properties as interval values, except that ratio values have a natural zero or starting point.


see relational database management system.


see Real Time Differential GPS.

Real-Aperture Radar

(RAR) a radar sensor that uses its side-looking, fixed antenna to transmit and receive the radar impulse. For a given position in space, the resolution of the resultant image is a function of the antenna size. The signal is processed independently of subsequent return signals.

Real Time Differential GPS

(RDGPS) takes the Differential Correction technique one step further by having the base station communicate the error vector via radio to the field unit in real time.


the assignment of new values to one or more classes.


1. the set of all attribute data for one class of feature. 2. the basic storage unit on a 9-track tape.


the process of making image data conform to a map projection system. In many cases, the image must also be oriented so that the north direction corresponds to the top of the image.

rectified coordinates

the coordinates of a pixel in a file that has been rectified, which are extrapolated from the GCPs. Ideally, the rectified coordinates for the GCPs are exactly equal to the reference coordinates. Because there is often some error tolerated in the rectification, this is not always the case.


the process of skipping file pixels when displaying an image so that a larger area can be represented on the display screen. For example, a reduction factor of 3 would cause only the pixel at every third row and column to be displayed, so that each displayed pixel represents a 3 × 3 block of file pixels.

reference coordinates

the coordinates of the map or reference image to which a source (input) image is being registered. GCPs consist of both input coordinates and reference coordinates for each point.

reference pixels

in classification accuracy assessment, pixels for which the correct GIS class is known from ground truth or other data. The reference pixels can be selected by you, or randomly selected.

reference plane

In a topocentric coordinate system, the tangential plane at the center of the image on the Earth or other planetary body ellipsoid, on which the three perpendicular coordinate axes are defined.

reference system

the map coordinate system to which an image is registered.

reference window

the source window on the first image of an image pair, which remains at a constant location. See also correlation windows and search windows.

reflection spectra

the electromagnetic radiation wavelengths that are reflected by specific materials of interest.


the process of making image data conform to another image. A map coordinate system is not necessarily involved.

regular block of photos

a rectangular block in which the number of photos in each strip is the same; this includes a single strip or a single stereopair.

relational database management system

(RDBMS) system that stores SDE database layers.

relation based matching

an image matching technique that uses the image features and the relation among the features to automatically recognize the corresponding image structures without any a priori information.

relief map

a map that appears to be or is three-dimensional.

remote sensing

the measurement or acquisition of data about an object or scene by a satellite or other instrument above or far from the object. Aerial photography, satellite imagery, and radar are all forms of remote sensing.

replicative symbol

an annotation symbol that is designed to look like its real-world counterpart. These symbols are often used to represent trees, railroads, houses, and so forth.

representative fraction

the ratio or fraction used to denote map scale.


the process of extrapolating data file values for the pixels in a new grid when data have been rectified or registered to another image.


the process of compressing data from one format to another. In ERDAS IMAGINE, this typically means compressing a 16-bit file to an 8-bit file.


the process of redigitizing a portion of a line.


in rectification, the distances between the source and retransformed coordinates in one direction. In ERDAS IMAGINE, they are shown for each GCP. The X residual is the distance between the source X coordinate and the retransformed X coordinate. The Y residual is the distance between the source Y coordinate and the retransformed Y coordinate.


a level of precision in data. For specific types of resolution see display resolution, radiometric resolution, spatial resolution, spectral resolution, and temporal resolution.

resolution merging

the process of sharpening a lower-resolution multiband image by merging it with a higher-resolution monochrome image.


in the rectification process, a coordinate in the reference (output) coordinate system that has transformed back into the input coordinate system. The amount of error in the transformation can be determined by computing the difference between the original coordinates and the retransformed coordinates. See RMS error.


red, green, blue. The primary additive colors that are used on most display hardware to display imagery.

RGB clustering

a clustering method for 24-bit data (three 8-bit bands) that plots pixels in three-dimensional spectral space, and divides that space into sections that are used to define clusters. The output color scheme of an RGB-clustered image resembles that of the input file.

rhumb line

a line of true direction that crosses meridians at a constant angle.

right-hand rule

a convention in three-dimensional coordinate systems (X,Y,Z) that determines the location of the positive Z axis. If you place your right hand fingers on the positive X axis and curl your fingers toward the positive Y axis, the direction your thumb is pointing is the positive Z axis direction.

RMS error

the distance between the input (source) location of a GCP and the retransformed location for the same GCP. RMS error is calculated with a distance equation.


(Root Mean Square Error) used to measure how well a specific calculated solution fits the original data. For each observation of a phenomena, a variation can be computed between the actual observation and a calculated value. (The method of obtaining a calculated value is application-specific.) Each variation is then squared. The sum of these squared values is divided by the number of observations and then the square root is taken. This is the RMSE value.


the process of moving across a display so that different areas of the image appear on the display screen.


the first part of a file name, which usually identifies the file’s specific contents.


a color scheme ranging through red, orange, yellow, green, blue, indigo, and violet at regular intervals.


see Raster Product Format.

rubber sheeting

the application of a nonlinear rectification (2nd-order or higher).



see training sample.


see Synthetic Aperture Radar.


a component of IHS that represents the purity of color and also varies linearly from 0 to 1.


see suitability/capability analysis.


1. the ratio of distance on a map as related to the true distance on the ground. 2. cell size. 3. the processing of values through a lookup table.

scale bar

a graphic annotation element that describes map scale. It shows the distance on paper that represents a geographical distance on the map.

scalar object

in Model Maker (Spatial Modeler), a single numeric value.

scaled map

a georeferenced map that is accurately arranged and referenced to represent distances and locations. A scaled map usually has a legend that includes a scale, such as 1 inch = 1000 feet. The scale is often expressed as a ratio like 1:12,000 where 1 inch on the map equals 12,000 inches on the ground.


the entire data acquisition system, such as the Landsat TM scanner or the SPOT panchromatic scanner.


1. the transfer of analog data, such as photographs, maps, or another viewable image, into a digital (raster) format. 2. a process similar to convolution filtering that uses a kernel for specialized neighborhood analyses, such as total, average, minimum, maximum, boundary, and majority.


a graph, usually in two dimensions, in which the data file values of one band are plotted against the data file values of another band.


the image captured by a satellite.

screen coordinates

the location of a pixel on the display screen, beginning with 0,0 in the upper left corner.

screen digitizing

the process of drawing vector graphics on the display screen with a mouse. A displayed image can be used as a reference.

script modeling

the technique of combining data layers in an unlimited number of ways. Script modeling offers all of the capabilities of graphical modeling with the ability to perform more complex functions, such as conditional looping.

script model

a model that is comprised of text only and is created with the SML. Script models are stored in .mdl files.


see Soil Conservation Service.


see standard deviation.


see Spatial Database Engine.


see spatial data transfer standard.

SDTS Raster Profile and Extensions

(SRPE) an SDTS profile that covers gridded raster data.

search radius

in surfacing routines, the distance around each pixel within which the software searches for terrain data points.

search windows

candidate windows on the second image of an image pair that are evaluated relative to the reference window.


a combination of an X-server and a host workstation.

Sea-viewing Wide Field-of-View Sensor

(SeaWiFS) a sensor located on many different satellites such as ORBVIEW’s OrbView-2, and NASA’s SeaStar.


see Sea-viewing Wide Field-of-View Sensor.


the intersection of two points or lines. In the case of conic or cylindrical map projections, a secant cone or cylinder intersects the surface of a globe at two circles.

Selective Availability

introduces a positional inaccuracy of up to 100 m to commercial GPS receivers.


a device that gathers energy, converts it to a digital value, and presents it in a form suitable for obtaining information about the environment.


a statistical measure of distance between two signatures.

separability listing

a report of signature divergence that lists the computed divergence for every class pair and one band combination. The listing contains every divergence value for the bands studied for every possible pair of signatures.

sequential clustering

a method of clustering that analyzes pixels of an image line by line and groups them by spectral distance. Clusters are determined based on relative spectral distance and the number of pixels per cluster.


on a computer in a network, a utility that makes some resource or service available to the other machines on the network (such as access to a tape drive).

Service Oriented Architecture

a way of connecting applications across a network via a common communications protocol. In theory, this lets developers treat applications as network services that can be chained together to create a complex business processes more quickly.

shaded relief image

a thematic raster image that shows variations in elevation based on a user-specified position of the sun. Areas that would be in sunlight are highlighted and areas that would be in shadow are shaded.

shaded relief map

a map of variations in elevation based on a user-specified position of the sun. Areas that would be in sunlight are highlighted and areas that would be in shadow are shaded.


an ESRI vector format that contains spatial data. Shapefiles have the .shp extension.

short wave infrared region

(SWIR) the near-infrared and middle-infrared regions of the electromagnetic spectrum.


see image scale.

Side-looking Airborne Radar

(SLAR) a radar sensor that uses an antenna that is fixed below an aircraft and pointed to the side to transmit and receive the radar signal.

signal based matching

see area based matching.

Signal-to-Noise ratio

(S/N) in hyperspectral image processing, a ratio used to evaluate the usefulness or validity of a particular band of data.


a set of statistics that defines a training sample or cluster. The signature is used in a classification process. Each signature corresponds to a GIS class that is created from the signatures with a classification decision rule.


a condition in satellite data, caused by the rotation of the Earth eastward, which causes the position of the satellite relative to the Earth to move westward. Therefore, each line of data represents terrain that is slightly west of the data in the previous line.


see Side-looking Airborne Radar.


the change in elevation over a certain distance. Slope can be reported as a percentage or in degrees.

slope image

a thematic raster image that shows changes in elevation over distance. Slope images are usually color coded to show the steepness of the terrain at each pixel.

slope map

a map that is color coded to show changes in elevation over distance.


for a map or data file, having a small ratio between the area of the imagery (such as inches or pixels) and the area that is represented (such as feet). In small-scale image data, each pixel represents a large area on the ground, such as NOAA AVHRR data, with a spatial resolution of 1.1 km.


see Spatial Modeler Language.


see Signal-to-Noise ratio.

soft-copy photogrammetry

see digital photogrammetry.

Soil Conservation Service

(SCS) an organization that produces soil maps (Fisher, 1991) with guidelines provided by the USDA.


see Space Oblique Mercator.

source coordinates

in the rectification process, the input coordinates.

Spaceborne Imaging Radar

(SIR-A, SIR-B, and SIR-C) radar sensors that flew on-board NASA space shuttles. SIR-A flew on-board the 1981 NASA Space Shuttle Columbia. SIR-B was flown on-board the shuttle Challenger in 1984. The SIR-C sensor flew twice in 1994, first on-board shuttle mission STS-59 and later on-board shuttle mission STS-68. SIR-C was the first polarimetric spaceborne SAR and first X-band sensor. The data from the Space Shuttle missions are valuable sources of radar data. ( National Aeronautics and Space Administration, 2006)

Space Oblique Mercator

(SOM) a projection available in ERDAS IMAGINE that is nearly conformal and has little scale distortion within the sensing range of an orbiting mapping satellite such as Landsat.

spatial data transfer standard

(SDTS) "a robust way of transferring Earth-referenced spatial data between dissimilar computer systems with the potential for no information loss" ( United States Geological Survey, 1999c).

Spatial Database Engine

(SDE) An ESRI vector format that manages a database theme. SDE allows you to access databases that may contain large amounts of information ( Environmental Systems Research Institute, 1996).

spatial enhancement

the process of modifying the values of pixels in an image relative to the pixels that surround them.

spatial frequency

the difference between the highest and lowest values of a contiguous set of pixels.

Spatial Modeler Language

(SML) a script language used internally by Model Maker (Spatial Modeler) to execute the operations specified in the graphical models you create. SML can also be used to write application-specific models.

spatial resolution

a measure of the smallest object that can be resolved by the sensor, or the area on the ground represented by each pixel.

speckle noise

the light and dark pixel noise that appears in radar data.

spectral distance

the distance in spectral space computed as Euclidean distance in n-dimensions, where n is the number of bands.

spectral enhancement

the process of modifying the pixels of an image based on the original values of each pixel, independent of the values of surrounding pixels.

spectral resolution

the specific wavelength intervals in the electromagnetic spectrum that a sensor can record.

spectral space

an abstract space that is defined by spectral units (such as an amount of electromagnetic radiation). The notion of spectral space is used to describe enhancement and classification techniques that compute the spectral distance between n-dimensional vectors, where n is the number of bands in the data.


the study of the absorption and reflection of electromagnetic radiation (EMR) waves.

spliced map

a map that is printed on separate pages, but intended to be joined together into one large map. Neatlines and tick marks appear only on the pages that make up the outer edges of the whole map.


the process of smoothing or generalizing all currently selected lines using a specified grain tolerance during vector editing.


the process of making two lines from one by adding a node.


a series of Earth-orbiting satellites operated by the Centre National d’Etudes Spatiales (CNES) of France.


see SDTS Raster Profile and Extensions.


see statistics file.

standard deviation

(SD) 1. the square root of the variance of a set of values that is used as a measurement of the spread of the values. 2. a neighborhood analysis technique that outputs the standard deviation of the data file values of a user-specified window.

standard meridian

see standard parallel.

standard parallel

the line of latitude where the surface of a globe conceptually intersects with the surface of the projection cylinder or cone.


in script models, properly formatted lines that perform a specific task in a model. Statements fall into the following categories: declaration, assignment, show, view, set, macro definition, and quit.

statistical clustering

a clustering method that tests 3 × 3 sets of pixels for homogeneity, and builds clusters only from the statistics of the homogeneous sets of pixels.

statistics file

(STA) an ERDAS IMAGINE Ver. 7.X trailer file for LAN data that contains statistics about the data.


1. the process of projecting onto a tangent plane from the opposite side of the Earth. 2. the process of acquiring images at angles on either side of the vertical.


a set of two remotely-sensed images that overlap, providing two views of the terrain in the overlap area.


achieved when two images of the same area are acquired on different days from different orbits, one taken east of the vertical, and the other taken west of the nadir.

stream mode

a digitizing mode in which vertices are generated continuously while the digitizer keypad is in proximity to the surface of the digitizing tablet.


a line of text. A string usually has a fixed length (number of characters).

strip of photographs

consists of images captured along a flight-line, normally with an overlap of 60% for stereo coverage. All photos in the strip are assumed to be taken at approximately the same flying height and with a constant distance between exposure stations. Camera tilt relative to the vertical is assumed to be minimal.


a data error that occurs if a detector on a scanning system goes out of adjustment, that is, it provides readings consistently greater than or less than the other detectors for the same band over the same ground cover. Also called banding.

structure based matching

see relation based matching.

Styled Layer Descriptor (SLD)

a styling language that the client and the server both understand, used to portray the output of the WMS, WFS and Web Coverage services.


the process of breaking out a portion of a large image file into one or more smaller files.

suitability/capability analysis

(SCA) a system designed to analyze many data layers to produce a plan map. Discussed in McHarg’s book Design with Nature ( Star and Estes, 1990).


a neighborhood analysis technique that outputs the total of the data file values in a user-specified window.

Sun raster data

imagery captured from a Sun monitor display.


a term used to describe Earth-orbiting satellites that rotate around the Earth at the same rate as the Earth rotates on its axis.

supervised training

any method of generating signatures for classification, in which the analyst is directly involved in the pattern recognition process. Usually, supervised training requires the analyst to select training samples from the data that represent patterns to be classified.


a one-band file in which the value of each pixel is a specific elevation value.

swath width

in a satellite system, the total width of the area on the ground covered by the scanner.


see short wave infrared region.


an annotation element that consists of other elements (sub-elements). See plan symbol, profile symbol, and function symbol.


a method of displaying vector data in which attribute information is used to determine how features are rendered. For example, points indicating cities and towns can appear differently based on the population field stored in the attribute database for each of those areas.

Synthetic Aperture Radar

(SAR) a radar sensor that uses its side-looking, fixed antenna to create a synthetic aperture. SAR sensors are mounted on satellites, aircraft, and the NASA Space Shuttle. The sensor transmits and receives as it is moving. The signals received over a time interval are combined to create the image.


table object

in Model Maker (Spatial Modeler), a series of numeric values or character strings.

tablet digitizing

the process of using a digitizing tablet to transfer nondigital data such as maps or photographs to vector format.

Tagged Imaged File Format

see TIFF data.


an intersection at one point or line. In the case of conic or cylindrical map projections, a tangent cone or cylinder intersects the surface of a globe in a circle.

Tasseled Cap transformation

an image enhancement technique that optimizes data viewing for vegetation studies.


see transverse electromagnetic wave.

temporal resolution

the frequency with which a sensor obtains imagery of a particular area.

terrain analysis

the processing and graphic simulation of elevation data.

terrain data

elevation data expressed as a series of x, y, and z values that are either regularly or irregularly spaced.


a German satellite system carrying a high-frequency X-band SAR instrument.

text printer

a device used to print characters onto paper, usually used for lists, documents, and reports. If a color printer is not necessary or is unavailable, images can be printed using a text printer. Also called a line printer.

thematic data

raster data that are qualitative and categorical. Thematic layers often contain classes of related information, such as land cover, soil type, slope, and so forth. In ERDAS IMAGINE, thematic data are stored in image files.

thematic layer

see thematic data.

thematic map

a map illustrating the class characterizations of a particular spatial variable such as soils, land cover, hydrology, and so forth.

Thematic Mapper

(TM) Landsat data acquired in seven bands with a spatial resolution of 30 × 30 meters.

thematic mapper simulator

(TMS) an instrument "designed to simulate spectral, spatial, and radiometric characteristics of the Thematic Mapper sensor on the Landsat-4 and 5 spacecraft" ( National Aeronautics and Space Administration, 1995b).


a particular type of information, such as soil type or land use, that is represented in a layer.

3D perspective view

a simulated three-dimensional view of terrain.


a limit, or cutoff point, usually a maximum allowable amount of error in an analysis. In classification, thresholding is the process of identifying a maximum distance between a pixel and the mean of the signature to which it was classified.

tick marks

small lines along the edge of the image area or neatline that indicate regular intervals of distance.

tie point

a point; its ground coordinates are not known, but can be recognized visually in the overlap or sidelap area between two images.

TIFF data

Tagged Image File Format data is a raster file format developed by Aldus, Corp. (Seattle, Washington), in 1986 for the easy transportation of data.


see Topologically Integrated Geographic Encoding and Referencing System.

tiled data

the storage format of ERDAS IMAGINE image files.


see triangulated irregular network.


see Thematic Mapper.


see thematic mapper simulator.


see Transformed Normalized Distribution Vegetative Index.


the last vertex in a line.

topocentric coordinate system

a coordinate system that has its origin at the center of the image on the Earth ellipsoid. The three perpendicular coordinate axes are defined on a tangential plane at this center point. The x-axis is oriented eastward, the y-axis northward, and the z-axis is vertical to the reference plane (up).


a term indicating elevation.

topographic data

a type of raster data in which pixel values represent elevation.

topographic effect

a distortion found in imagery from mountainous regions that results from the differences in illumination due to the angle of the sun and the angle of the terrain.

topographic map

a map depicting terrain relief.

Topologically Integrated Geographic Encoding and Referencing System

(TIGER) files are line network products of the US Census Bureau.


a term that defines the spatial relationships between features in a vector layer.

total RMS error

the total root mean square (RMS) error for an entire image. Total RMS error takes into account the RMS error of each GCP.

trailer file

1. an ERDAS IMAGINE Ver. 7.X file with a .TRL extension that accompanies a GIS file and contains information about the GIS classes. 2. a file following the image data on a 9-track tape.


the process of defining the criteria by which patterns in image data are recognized for the purpose of classification.

training field

the geographical area represented by the pixels in a training sample. Usually, it is previously identified with the use of ground truth data or aerial photography. Also called training site.

training sample

a set of pixels selected to represent a potential class. Also called sample.

transformation matrix

a set of coefficients that is computed from GCPs, and used in polynomial equations to convert coordinates from one system to another. The size of the matrix depends upon the order of the transformation.

Transformed Normalized Distribution Vegetative Index

(TNDVI) adds 0.5 to the NDVI equation, then takes the square root. Created by Deering et al in 1975 ( Jensen, 1996).


the interchanging of the rows and columns of a matrix, denoted with T.

transverse aspect

the orientation of a map in which the central line of the projection, which is normally the equator, is rotated 90 degrees so that it follows a meridian.

transverse electromagnetic wave

(TEM) a wave where both E (electric field) and H (magnetic field) are transverse to the direction of propagation.

triangulated irregular network

(TIN) a specific representation of DTMs in which elevation points can occur at irregular intervals.


establishes the geometry of the camera or sensor relative to objects on the Earth’s surface.

true color

a method of displaying an image (usually from a continuous raster layer) that retains the relationships between data file values and represents multiple bands with separate color guns. The image memory values from each displayed band are translated through the function memory of the corresponding color gun.

true direction

the property of a map projection to represent the direction between two points with a straight rhumb line, which crosses meridians at a constant angle.

True North

Direction pointing towards the geographic North Pole along a central meridian on the Earth's surface. Lines of longitude are True North lines.


see Topological Vector Profile.



the area or set that is the combination of two or more input areas or sets without repetition.

United Sates Department of Agriculture

(USDA) an organization regulating the agriculture of the US. For more information, visit the website

United States Geological Survey

(USGS) an organization dealing with biology, geology, mapping, and water. For more information, visit the website

Universal Polar Stereographic

(UPS) a mapping system used in conjunction with the Polar Stereographic projection that makes the scale factor at the pole 0.994.

Universal Transverse Mercator

(UTM) UTM is an international plane (rectangular) coordinate system developed by the US Army that extends around the world from 84°N to 80°S. The world is divided into 60 zones each covering six degrees longitude. Each zone extends three degrees eastward and three degrees westward from its central meridian. Zones are numbered consecutively west to east from the 180° meridian.

unscaled map

a hard-copy map that is not referenced to any particular scale in which one file pixel is equal to one printed pixel.


the process of joining two lines by removing a node.

unsupervised training

a computer-automated method of pattern recognition in which some parameters are specified by the user and are used to uncover statistical patterns that are inherent in the data.


see Universal Polar Stereographic.


see United States Department of Agriculture.


see United States Geological Survey.


see Universal Transverse Mercator.



1. a numeric value that is changeable, usually represented with a letter. 2. a thematic layer. 3. one band of a multiband image. 4. in models, objects that have been associated with a name using a declaration statement.

variable rate technology

(VRT) in precision agriculture, used with GPS data. VRT relies on the use of a VRT controller box connected to a GPS and the pumping mechanism for a tank full of fertilizers, pesticides, seeds, water, or similar.


the measure of central tendency.


1. a line element. 2. a one-dimensional matrix, having either one row (1 by j), or one column (i by 1). See also mean vector, measurement vector.

vector data

data that represent physical forms (elements) such as points, lines, and polygons. Only the vertices of vector data are stored, instead of every point that makes up the element. ERDAS IMAGINE vector data are based on the ArcInfo data model and are stored in directories, rather than individual files. See workspace.

vector layer

a set of vector features and their associated attributes.

Vector Quantization

(VQ) used to compress frames of RPF data.

velocity vector

the satellite’s velocity if measured as a vector through a point on the spheroid.

verbal statement

a statement that describes the distance on the map to the distance on the ground. A verbal statement describing a scale of 1:1,000,000 is approximately 1 inch to 16 miles. The units on the map and on the ground do not have to be the same in a verbal statement.


a point that defines an element, such as a point where a line changes direction.

vertical control

the vertical distribution of GCPs in aerial triangulation (z - elevation).


plural of vertex.

viewshed analysis

the calculation of all areas that can be seen from a particular viewing point or path.

viewshed map

a map showing only those areas visible (or invisible) from a specified point(s).


see visible/infrared imagery.

visible/infrared imagery

(VIS/IR) a type of multispectral data set that is based on the reflectance spectrum of the material of interest.


a medium for data storage, such as a magnetic disk or a tape.

volume set

the complete set of tapes that contains one image.


see vector product format.


see Vector Quantization.


see variable rate technology.



"a waveform that is bounded in both frequency and duration" ( Free On-Line Dictionary of Computing, 1999e).


the number of values in a set; particularly, in clustering algorithms, the weight of a cluster is the number of pixels that have been averaged into it.

weighting factor

a parameter that increases the importance of an input variable. For example, in GIS indexing, one input layer can be assigned a weighting factor that multiplies the class values in that layer by that factor, causing that layer to have more importance in the output file.

weighting function

in surfacing routines, a function applied to elevation values for determining new output values.


OGC specification for a Web Feature Server.


OGC specification for a Web Map Server.


see World Geodetic System.

Wide Field Sensor

(WiFS) sensor aboard IRS-1C with 188m spatial resolution.


see Wide Field Sensor.

working window

the image area to be used in a model. This can be set to either the union or intersection of the input layers.


a location that contains one or more vector layers. A workspace is made up of several directories.

World Geodetic System

(WGS) a spheroid. Earth ellipsoid with multiple versions including: WGS 66, 72, and 84. These have been updated with geoids EGM 96 and EGM 2008.

write ring

a protection device that allows data to be written to a 9-track tape when the ring is in place, but not when it is removed.


X residual

in RMS error reports, the distance between the source X coordinate and the retransformed X coordinate.

X RMS error

the root mean square (RMS) error in the X direction.


Y residual

in RMS error reports, the distance between the source Y coordinate and the retransformed Y coordinate.

Y RMS error

the root mean square (RMS) error in the Y direction.



see zone distribution rectangles.

zero-sum kernel

a convolution kernel in which the sum of all the coefficients is zero. Zero-sum kernels are usually edge detectors.

zone distribution rectangles

(ZDRs) the images into which each distribution DR are divided in ADRG data.


the process of expanding displayed pixels on an image so they can be more closely studied. Zooming is similar to magnification, except that it changes the display only temporarily, leaving image memory the same.