AAT
Arc Attribute Table. A table containing attributes for arc coverage features. In addition to user defined attributes the AAT contains the form and to nodes, the left and the right polygons, the length and internal sequence number and a feature identifier.

Abscissa
In a rectangular coordinate system, the distance of the x-coordinate along a horizontal axis from the vertical or y-axis. For example, a point with the coordinates (7,3) has an abscissa of 7.

Absolute coordinates
Coordinates that are referenced to the origin of a given coordinate system.

Abstraction
A way of viewing a real world object, usually a simplification. For example, a road may be represented as a centerline in one application and an area bounded by kerblines in another.

accessibility
The aggregate measure of how reachable loctions are from a given location. The ACCESSIBILITY command computes values for accessibility as a function of the distance between locations and an emperical derived distance decay parameters.

access rights
The privileges accorded a user for reading, writing, deleting updating and executing files on a disc rights are stated as 'no access', 'read only' and 'read/write'.

accuracy
The measurement for an X-Y digitizing tablet of how close the reported coordinates for any given point come to the point's actual location. Accuracy is stated in terms of the possible distance for error (for example, ±.01 or ±.025).

accuracy, absolute
The accuracy of a map in representing the geographic location of an object relative to its true location on the surface of the Earth Absolute accuracy is based on geographic co-ordinates.

accuracy, relative
The accuracy of a map in representing the geographic locations of an object relative to the location of other objects.

ACODE file
An INFO datafile storing arc attributes for coverage created from TIGER, DIME, IGDS and Etak files. ACODE stands for 'Arc CODE'. The ACODE file is related by cover-ID to Arc Attribute Table(AAT) of the coverage.

ACL
Acronym for access control list. A list of accounts or users used to designate restricted and unrestricted services and the authentication criteria required to access an object.

Across-track scanner
A remote-sensing tool with an oscillating mirror that moves back and forth across a satellite's direction of travel, creating scan line strips that are contiguous or that overlap slightly, thereby producing an image.

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

Active Sensor
A sensor that generates its own electromagnetic energy, usually within the microwave wavebands. RADAR is an example of such a system.

ActiveX Connector
A type of ArcIMS Application Server Connector that is a Component Object Model (COM) dynamic link library (DLL) which can be used in a COM application such as Microsoft Active Server Pages (ASP).

active area
The area on an X-Y digitizing tablet that is sensitive to the pointing device.

additive color
Colors perceived by the human eye that are created by mixing different colors of light, or visible radiation are produced in an additive manner. The colors of the images produced on a color display screen or by a color projection onto a white screen are examples of additive color. When all wavelengths, or colors, of visible light are mixed, white light is produced. That all visible colors are present in white light is easily demonstrated using a prism, which separates white light into the spectrum of colors of which it is composed. White light can also be produced by mixing complementary colors. Absence of radiation or relatively low radiation at all wavelengths of human visual sensitivity will yield black. A predominance of radiation in a particular range of wavelengths will yield a color whose intensity is proportional to the level of radiation. (See also: subtractive color)

Adjacency
The sharing of a common side or boundary by two or more polygons.

address matching
A mechanism for relating two files using address as the related item. Geographic co-ordinates and attributes can be transferred from one address to the other. For eg. a data file containing a students address can be matched to a street coverage taht contains addresses creating a point coverage of where the students live.

ADS
1. Arc Digitizing System: A simple digitizing and editing system used to add arcs and label points to a coverage.
2. The command at the Arc: promt that starts an ADS session.

ADT
Acronym for abstract data type. For OpenLS, a data type and structure for location information that is shared by two or more services. ADTs are application schemas that are encoded in XML for location services (XLS).

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

Aggregation
The process of collecting a set of similar, usually adjacent, polygons (with their associated attributes) to form a single, larger entity.

AIXM
Acronym for Aeronautical Information Exchange Format. An XML format used to describe aeronautical data transactions created and maintained by EUROCONTROL (European Organisation for the Safety of Air Navigation), in the process of being adopted worldwide.

AJAX
Acronym for Asynchronous JavaScript and XML. A programming technique for creating fast interactive Internet applications. Ajax adds a small application to part of the software user's browser for fast loading and display.

airphoto
A 9" x 9" photograph taken vertically downward from the air on 10" roll film. Airphoto images may be in the form of paper prints or transparent film. An airphoto includes significant horizontal displacement introduced by camera characteristics, tilt, nearness to the target scene, and variations in elevation of the target terrain.

airslide
A 35mm slide taken vertically downward from the air.

airvideography
The growing new field making measurements from digitized frames of vertical airvideo images for managing agricultural and natural resources, making tax assessments, and monitoring environmental degradation.

airvideo image
An image acquired vertically downward with a color, monochrome, or color-infrared video camera and recorder.

albedo
"The ratio of the light reflected by a planet or a satellite to that received by it" (Random House). This definition can be generalized to any object, such as a part of the earth's surface or atmosphere, a leaf, a soil element, and so on.

algorithm
A numerical scheme applied to reach a solution of a problem.

aliasing
(referring to a graphic display of lines) A line that is drawn digitally, cell by cell, with fixed, uniform color and intensity is said to be aliased. The cells are easily resolved by the observer's eye so the line is seen to have a jagged, stair-step appearance. (See also: anti-aliasing.)

Almanac

1. In GPS, a file transmitted from a satellite to a receiver that contains information about the orbits of all satellites included in the satellite network. Receivers refer to the almanac to determine which satellite to track.
2. An annual publication containing weather forecasts, information on astronomical events, and miscellaneous facts, arranged according to the calendar of a given year.

Along-track scanner
A remote-sensing tool with a line of many fixed sensors that record reflected radiation from the terrain along a satellite's direction of movement, creating scan-line strips that are contiguous or that overlap slightly, thereby producing an image.

Altitude

• The height or vertical elevation of a point above a reference surface. Altitude measurements are usually based on a given reference datum, such as mean sea level.
• The height above the horizon, measured in degrees, from which a light source illuminates a surface. Altitude is used when calculating a hillshade, or for controlling the position of a light source in a scene.

Altitude Matrix
A rectangular grid containing elevation data. An altitude matrix can be obtained from the stereoscopic study of overlapping aerial photographs on analytical stereoscopic plotters, or from interpolation of irregularly spaced elevation data points.

AM/FM
Automated Mapping and Facilities Management. A geographic information system designed for the optimal processing of information about utilities and infrastructures, such as power lines and water and telephone networks.

Ambiguity
In GIS, a state of uncertainty in data classification that exists when an object may appropriately be assigned two or more values for a given attribute. For example, coastal areas experiencing tidal fluctuations may be dry land at some times and under water at other times. Ambiguity may be caused by changeable conditions in reality, by incomplete or conflicting definitions of attributes, or by subjective differences in the evaluation of data. It may also be caused by disputes, as when two parties claim ownership of the same tract of land.

AML
ARC Macro Language. A high level algorithmic language for generating end-user application. Features include the ability to create on-screen menus, use and assign variables, control statement execution and get and use map or page unit co-ordinates. AML includes an extensive set of commands that can be used interactively or in AML programs (Macros) as well as commands that report on the status of ARC/INFO environment settings.

analog
Information stored and processed as signal intensity or other measurement of a continuous physical variable. Analog information processing translates and represents slight increments in data easily and conveys information by relative position without relying on the numeric value necessary to convey the same information digitally. For example, the second hand on an analog watch "sweeps" around the dial and you can tell time on an analog watch even if it has no numbers on the face. Another example is a thermometer that displays temperature using a needle or liquid can indicate fractions of a degree, as well as provide information about relative warmth by the position of the dial or height of the liquid. On the other hand, this continuous analog information is harder to copy, store, manipulate and reproduce dependably. Anyone who has ever listened to a copy of a copy of a copy of a cassette tape has first-hand knowledge of analog information degradation. For this reason, much analog information (video, audio, or field and laboratory measurements of temperature, pressure, voltage, radiation, and so on) is converted to its digital equivalent. (See also: digital, digitizer)

angiogram
"An X-ray of blood vessels or lymphatics following injection of a radiopaque substance" (Random House).

annotation
Descriptive test used to label coverage features. It is used for display and not analysis.

Annotation layer
A layer that references annotation. Information stored for annotation includes a text string, a position at which it can be displayed, and display characteristics.

Anisotropic
An adjective used to describe spatial data which shows unequal movement or properties in any direction. Often used to describe surfaces.

ANSI
American National Standard Isntitute is a national co-ordinator of voluntary standards activities, and an approcval organisation and clearing house in United States. ANSI works closely with Internation Organisations, particularly ISO for the development and approval of international standards. While ANSI standards apply to every fact of today's world, their efforts in the area of SQL and Spatial extensions to SQL are of particular interests to GIS community.

anti-aliasing
(referring to a graphic display of lines) Anti-aliasing removes or greatly reduces the jagged, stair-step appearance of a digital line. This stair-stepping is caused by plotting a uniform color and intensity line on a display device whose minimum resolution or cell size is easily resolved by the observer's eye. 
Anti-aliasing smoothes out the jagged edges of the line by filling in some of the intermediate and flanking cells in lower-intensity colors. (See also: aliasing)

API
Acronym for application programming interface. A set of interfaces, methods, protocols, and tools that application developers use to build or customize a software program. APIs make it easier to develop a program by providing building blocks of prewritten, tested, and documented code that are incorporated into the new program. APIs can be built for any programming language.

Applet
A small program that usually executes from within a Web browser. Applets are compatible with most platforms, and can also be used within applications or devices that support applets.

Application server
A computer program that receives user requests through a client application and returns results to the client.

Application schema
A set of conceptual schema for data required by one or more applications. An application schema contains selected parts of the base schemas presented in the ORM Information Viewpoint. Designers of application schemas may extend or restrict the types defined in the base schemas to define appropriate types for an application domain. Application schemas are information models for a specific information community.

Application Web service
A Web service that solves a particular problem; for example, a Web service that finds all of the hospitals within a certain distance of an address. An application Web service can be implemented using the native Web service framework of a Web server; for example, an ASP.NET Web service (WebMethod) or Java Web service (Axis).

Arbitrary symbol
A symbol that has no visual similarity to the feature it represents—for example, a circle used to represent a city, or a triangle used to represent a school.

arc
1. An ordered string of vertices (x,y co-ordinate pairs) that begins at one location and end at anther connecting the arc called nodes.
2. The coverage feature class user to represnt him features and polygon boundaries. One line features can contain many arecs. Arcs are topologically linked to nodes (arc-node topology) and to polygons (polygon-arc topology). The discriptive attributes of arc attributes table (AAT).

Architecture
An abstract technical description of a system or collection of systems. Modern software architectures employ interoperability interfaces to enable enterprises and whole industries to establish coherent, flexible, integrated information flows that can be implemented with heterogeneous but intercommunicating software systems. The OpenGIS Specification defines the interoperability interfaces that make it possible to include geographic information in these information flows. Conceptually based, architecture does not contain the level of detail needed for construction.

Archive
A collection of information or data that is stored on a permanent medium such as CDs, discs, or tapes. Information is archived to ensure its security or persistence.

ARC/INFO
A vector-based Geographic Information System (GIS) developed and marketed by ESRI, Inc. for use on workstations.

ARC/INFO extent file
The name of the data file used by ARC/INFO which contains arc, node, and polygon data elements.

arc-node topolgy
The topological data structure ARC/INFO used to represent connecting between arcs and nodes Arc-node topology support the definition of linear feature and polygon boundaries, and support analysis functions such as network tracing.

arcsecond
"The sixtieth part of a minute of angular measure often represented by the sign ", as in 30", which is read 30 seconds" (Random House). (See also: degree, minute.) Area correlation A DEM extraction method that methodically traverses a pair of stereo images, working from the initial set of correlation points supplied by the user, and building a correlation model from new points of correlation that it finds. 

Arc Tools
Arc Tools is a collection of ARC/INFO productivity tools implemented through an AML (ARC Macro language) graphic user Interface. Arctools provide a user friendly approach to commonly used ARC/INFO operations and functions.

Area of interest
The extent used to define a focus area for either a map or database production.

Artificial Intelligence
A term that applies to that branch of computer science which aims to imitate the thought processes of the human brain. There are a number of different approaches of achieving this, these include creating a computer with a similar (although greatly simpler structure), as the human brain through the use of neural networks, or alternatively, to simply imitate the thought processes through the use of software. The latter approach is perhaps the most common and has resulted in the development of expert or knowledge based systems.

arrow glyph
A graphic symbol on a cascade button that indicates the direction in which the button will open its associated cascade menu.

ASCII
American Standard Code for Information Interchange (pronounced "askee"). The 7-bit (128 characters) used as a computer's alphabet. The Latin alphabet character set encoded into digital values between 0 and 127 includes lowercase and uppercase letters, the numerals 0-9, English punctuation marks, special symbols (such as @#$%^&*) and non-displaying characters often used as printer control codes. The eighth bit, giving values from 128 to 255, is used in a nonstandard fashion and is not part of the standard ASCII code. PCs normally have the "extended" character set in their system font for digital values from 128 to 255. 
The term "ASCII file" is often used to mean a text-only file. Documents in most word processors are not text-only files, since they include header information and formatting characters. However most word processors have an export or print-to-file utility that will convert a document into a text-only ASCII format.

ASCII file
A text-only file. Documents in most word processors are not text-only files, since they include binary header information and formatting characters. However most word processors import so-called ASCII files and have an Export, Save/Text-only, or print-to-file utility that converts a document into an "ASCII" format. However, these files are not true ASCII files because they may include the characters from 128-255. The characters in this range are different between platforms such as PCs and Macintoshes and even from font to font with a platform, and are not part of the standard 7-bit ASCII code. (See also: ASCII.)

ASCS
Agricultural Stabilization and Conservation Service.

ASP
Acronym for Active Server Pages. A Microsoft server-side scripting technology that can be used to create and run dynamic, interactive Web applications, which are typically coded in JScript, JavaScript or VBScript. An ASP file contains not only the text and HTML tags that standard Web documents contain, but also commands written in a scripting language, which can be carried out on the server or the client.

ASP.NET
A Microsoft-created programming framework built on top of the common language runtime (CLR) that can be used on a Windows server to create Web applications in a variety of programming languages.

Aspatial Data
A term that is used to describe non-spatial data that is usually referenced to spatial data, for example, attributes.

aspect
The compass direction towards which a slope faces, measured in degrees from North in a clockwise direction.

aspect ratio
The ratio of horizontal scale to vertical scale for printing or display. For graphics and image processing, square cells/pixels are best (aspect 1:1). Some display devices have a non-square aspect, which causes images to appear stretched or distorted. Standard broadcast video has an aspect ratio of 4:3, which must be corrected in any framegrabbing or other digitization process.

ASRG
ARC standard Raster Graphic. Raster graphic data transformed to Equal ARC - Second Raster Chart/Map (ARC) Projection System (see SRG) ASRG (using RGB) is very similar to DMA (Defence Mapping Agency) ADRG (ARC Digitizer Raster Graphic). ADRG files can be imported to ARC/INFO with the ADRG GRID Command. The ASRG (using RGB) can be converted with the ADRG GRID command if it is in a true DMA ADRG format. The ASRG permits color codes, which the ADRG GRID command does not handle.

assignment statement
(in database query) A line in a query that changes the value of a style drawing variable. Example: size = 5. That statement changes the value of the size variable to 5. (The size variable controls the size at which node elements are drawn). Assignment statements may be used in vector display and several other processes that let you assign drawing styles by query.

Associated Data
Alphanumeric information associated with a specific spatially referenced object. Synonymous with attribute.

Atmospheric Absorption
The process where electromagnetic energy, as it passes through the earth's atmosphere, interacts with gas molecules and energy is converted into the internal energy of the molecule. The energy is usually considered to be 'lost', particularly for remote sensing applications.

Atmospheric Correction
A set of techniques used in digital image processing to compensate for the atmospheric effects present in remotely sensed imagery.

Atmospheric Window
The regions of the electromagnetic spectrum that are relatively free from the effects of atmospheric attenuation. Radiation in these wavebands will pass through the atmosphere with less modification than radiation at other wavelengths.

Atomic clock
A clock that keeps time by the radiation frequency associated with a particular atomic reaction. Atomic clocks are used in official timekeeping.

Attenuation
The dimming and blurring effects in remotely sensed images caused by the absorption and scattering of light or other radiation that passes through the earth's atmosphere.

attribute
1. A characteristic of a geographic feature described by numbers, characters, images and CAD drawings typically stored in tabular format and linked to the feature by a user asigned identifier (eg. attributes of a well might include depth and gallons per minute).
2. A column in a database table.

Authentication
The process of validating the identity of a user who logs on to a computer system, network, or Web site.

AutoCAD
The most popular commercial Computer Aided Design (CAD) software package. Written and distributed for the microcomputer by Autodesk, Inc.

autocorrelation
"The correlation of an ordered series of observations with the same series displaced by the same number of terms" (Random House).

Automated cartography
The process of making maps using computer systems that carry out many of the tasks associated with map production.

Automated feature extraction
The identification of geographic features and their outlines in remote-sensing imagery through postprocessing technology that enhances feature definition, often by increasing feature-to-background contrast or using pattern recognition software.

Automated text placement
An operation in which text is automatically placed on or next to features on a digital map by a software application according to rules set by the software user.

Automation

1. The automatic functioning of a machine, system, or process, without the need for human interaction.
2. In COM technology, a feature that allows an object that was designed for use in one application to be accessed in another application. For example, ArcObjects may be accessed in Visual Basic and in other languages, tools, and applications that support automation.

Autovectorization
The creation of vector data from raster data through automated tracing of pixels that are in close proximity and of the same or similar value.

AVHRR imagery
Advanced Very High Resolution Radiometer imagery produced by NOAA satellites.

AVIRIS imagery
Airborne Visible/InfraRed Imaging Spectrometer imagery. Multispectral images of approximately 240 coregistered spectral bands collected by NASA aircraft.

azimuth
The horizontal direction of a vector, measured clockwise in degrees of rotation from +ve y-axis for eg. degrees on a compass. The direction of one object from another usually expressed as an angle in degrees relative to true north. Azimuths are usually measured in clockwise direction, thus an azimuth of 90 degrees indicate that the second object is due east from the first.

azimuthal projections
also known as Planar Projections) A class of map projections that are constructed by placing a flat planar surface tangent to a single point on the globe, or by placing the globe to an intersecting (secant) plane. With azimuthal (or planar) projections, lines of equal distortion are concentric around the point of tangency or the center of the circle of intersection. Most azimuthal maps do not have standard parallels or standard meridians. Each map has only one standard point: the center. Thus, the azimuthal projections are suitable for minimizing distortion in a somewhat circular region, such as Antarctica, but not for an area with predominant length in one direction. Azimuthal projections include Orthographic, Stereographic, Gnomonic, Lambert Azimuthal Equal-Area, and Azimuthal Equidistant.

azimuth angle
Angular displacement from North.