GIS Definitions: A-D

Definitions of some commonly used GIS (Geographic Information System) terms.

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Arc attribute table. A table containing attributes for arc coverage features. In addition to user-defined attributes, the AAT contains the from and to nodes, the left and right polygons, the length, an internal sequence number and a feature identifier. See also feature attribute table.
ACCESS directory
The system directory that LIBRARIAN uses to store the files that manage access to the library. Each library has an ACCESS directory located in the library's DATABASE directory.
An aggregate measure of how reachable locations are from a given location. The ACCESSIBILITY command computes values for accessibility as a function of the distance between locations and an empirically derived distance decay parameter.
access rights
The privileges accorded a user for reading, writing, deleting, updating and executing files on a disk. Access rights are stated as 'no access', 'read only' and 'read/write'.
ACODE file
An INFO data file storing arc attributes for coverages created from TIGER, DIME, IGDS and Etak files. ACODE stands for 'Arc CODE'. The ACODE file is related by Cover-ID to the Arc Attribute Table (AAT) of the coverage.
address matching
A mechanism for relating two files using address as the relate item. Geographic coordinates and attributes can be transferred from one address to the other. For example, a data file containing student addresses can be matched to a street coverage that contains addresses creating a point coverage of where the students live.
1. Arc Digitizing System. A simple digitizing and editing system used to add arcs and label points to a coverage.
2. A command at the Arc: prompt that starts an ADS session.
The process of assigning arcs in a network to the closest center until the maximum impedance or resource capacity of the center is reached.
ARC Macro Language. A high-level algorithmic language for generating end-user applications. Features include the ability to create on-screen menus, use and assign variables, control statement execution, and get and use map or page unit coordinates. 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 ArcInfo environment settings.
Analysis is the process of identifying a question or issue to be addressed, modeling the issue, investigating model results, interpreting the results, and possibly making a recommendation. See model and spatial analysis.
1. Descriptive text used to label coverage features. It is used for display, not for analysis.
2. One of the feature classes in a coverage used to label other features. Information stored for annotation includes a text string, the location at which it is displayed, and a text symbol (color, font, size, etc.) for display. See also TAT.
American National Standards Institute is a national coordinator of voluntary standards activities, and an approval organization and clearinghouse for consensus standards in the United States. ANSI works closely with international organizations, particularly ISO, for the development and approval of international standards. While ANSI standards apply to every facet of today's world, their efforts in the area of SQL and spatial extensions to SQL are of particular interest to the GIS community.
Application program interface (API). An API is a set of system calls or routines for application programs to access services from operating systems or other programs. An API allows your program to work with other programs, possibly on other computers. API is fundamental to client/server computing. ArcView provides this service to ArcInfo users.
1. An ordered string of vertices (x,y coordinate pairs) that begin at one location and end at another. Connecting the arc's vertices creates a line. The vertices at each endpoint of an arc are called nodes.
2. A coverage feature class used to represent linear features and polygon boundaries. One line feature can contain many arcs. Arcs are topologically linked to nodes (arc-node topology) and to polygons (polygon-arc topology). The descriptive attributes of arcs are stored in the arc attribute table (AAT). See also node.
arc-node topology
The topological data structure ArcInfo uses to represent connectivity between arcs and nodes. Arc-node topology supports the definition of linear feature and polygon boundaries, and supports analysis functions such as network tracing. See also topology.
A preserved collection of historical information purged from an ArcStorm database.
ArcStorm (ArcStorageManager) is a data storage facility and transaction manager for ArcInfo data. ArcStorm manages a feature-oriented database that can be closely integrated with database systems supported by ArcInfo's DATABASE INTEGRATOR.
ArcStorm database
An ArcStorm database is a collection of libraries, layers, INFO tables and external DBMS tables. Data stored in an ArcStorm database benefit from the transaction management and data archiving capabilities of ArcStorm.
ArcTools is a collection of ArcInfo productivity tools implemented through an AML-based (ARC Macro Language) graphical user interface. ArcTools provides a user-friendly approach to commonly used ArcInfo operations and functions.
1. A homogeneous extent of the Earth bounded by one or more arc features (polygon) or represented as a set of polygons (region). Examples: states, counties, lakes, land-use areas, and census tracts.
2. The size of a geographic feature measured in unit squares. ArcInfo stores an area measure for each polygon and region.
American Standard Code for Information Interchange. A set of codes for representing alphanumeric information (e.g., a byte with a value of 77 represents a capital M). Text files, such as those created with the text editor of a computer system, are often referred to as ASCII files.
The compass direction toward which a slope faces, measured in degrees from North in a clockwise direction.
ARC Standard Raster Graphic. Raster graphic data transformed to the Equal ARC-second Raster Chart/Map (ARC) Projection System. See SRG. ASRG (using RGB) is very similar to DMA (Defense Mapping Agency) ADRG (ARC Digitized Raster Graphic). ADRG files can be imported into ArcInfo with the ADRGGRID command. The ASRG (using RGB) can be converted with the ADRGGRID command if it is in a true DMA ADRG format. The ASRG permits color codes, which the ADRGGRID command does not handle.
The property or properties of a location that create an incentive for trips to be made to that location. For example, the attractiveness of a retail store could be a function of the retail floor space, number of parking spaces, product pricing, or a combination of these factors.
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-assigned identifier (e.g., the attributes of a well might include depth and gallons per minute).
2. A column in a database table. See also item.
attribute table
An INFO or other tabular file containing rows and columns. In ArcInfo, attribute tables are associated with a class of geographic features, such as wells or roads. Each row represents a geographic feature. Each column represents one attribute of a feature, with the same column representing the same attribute in each row. See also feature attribute table.
The horizontal direction of a vector, measured clockwise in degrees of rotation from the positive y-axis, for example, degrees on a compass.

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A copy of a file, a set of files, or whole disk for safekeeping in case the original is lost or damaged.
One layer of a multispectral image representing data values for a specific range of the electromagnetic spectrum of reflected light or heat (e.g., ultraviolet, blue, green, red, near-infrared, infrared, thermal, radar, etc.). Also, other user-specified values derived by manipulation of original image bands. A standard color display of a multispectral image shows three bands, one each for red, green and blue. Satellite imagery such as LANDSAT TM and SPOT provide multispectral images of the Earth, some containing seven or more bands.
band separate
An image format that stores each band of data collected by multispectral satellite scanning instruments in a separate file.
A measure of the volume of data that can flow through a communications link. Image data tend to exist as large data sets; thus moving image data sets from one computer to another requires high bandwidth or performance will be slowed. Also known as throughput.
A map containing geographic features used for locational reference. Roads, for example, are commonly found on basemaps.
base table
A table that is physically stored in the database. Compare with view.
baud rate
A measure of the speed of data transmission between computer and other devices, measured in bits per second.
The smallest unit of information that a computer can store and process. A bit has two possible values, 0 or 1, which can be interpreted as YES/NO, TRUE/FALSE, or ON/OFF. See also byte.
Binary large object. The data type of a column in an RDBMS table which can store large image or textual data as attributes.
The file in a coverage or grid which contains the coverage extent.
Boolean expression
A type of expression that reduces to a true or false (logical) condition. A Boolean expression contains logical expressions (e.g., DEPTH > 100) and Boolean operators. A Boolean operator is a keyword that specifies how to combine simple logical expressions into complex expressions. Boolean operators negate a predicate (NOT), specify a combination of predicates (AND), or specify a list of alternative predicates (OR). For example, DEPTH > 100 AND DIAMETER > 20. See also logical selection.
border arcs
1. The arcs that create the outer edge boundary of a polygon coverage.
2. In LIBRARIAN, the tile boundary arcs that split a polygon coverage into tiles.
A linear feature that defines and controls the surface behavior of a tin in terms of smoothness and continuity. Breaklines are always maintained as linear features in a tin. Stereo-digitized features containing x,y,z values such as streams and shorelines containing an elevation attribute are often stored as breakline features.
A zone of a specified distance around coverage features. Both constant- and variable-width buffers can be generated for a set of coverage features based on each feature's attribute values. The resulting buffer zones form polygons-areas that are either inside or outside the specified buffer distance from each feature. Buffers are useful for proximity analysis (e.g., find all stream segments within 300 feet of a proposed logging area).
An error in a computer program or in a piece of electronics that causes it to malfunction.
A memory and data storage unit composed of contiguous bits, usually eight. For example, file sizes are measured in bytes or megabytes (one million bytes). Bytes contain values of 0 to 255 and most often represent integer numbers or ASCII characters (e.g., a byte with an ASCII value of 77 represents a capital M). A collection of bytes (often 4 or 8 bytes) represents real numbers and integers larger than 255.

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Computer-aided design. An automated system for the design, drafting, and display of graphically oriented information.
CAD drawing
The digital equivalent of a drawing, figure or schematic created using a CAD system. For example, a drawing file or DWG file in AutoCAD.
The process of choosing attribute values and computational parameters so that a model properly represents the real-world situation being analyzed. For example, in pathfinding and allocation, calibration generally refers to assigning or calculating appropriate values to be entered in impedance and demand items.
The maximum resource that can be assigned (allocated) to or be serviced by a center. For example, the capacity of a school is the number of students that can be enrolled there.
Cartesian coordinate system
A two-dimensional, planar coordinate system in which x measures horizontal distance and y measures vertical distance. Each point on the plane is defined by an x,y coordinate. Relative measures of distance, area, and direction are constant throughout the Cartesian coordinate plane.
Computer-Aided Software Engineering. CASE tools are defined programming rules for applying engineering principles, methods, techniques, and concepts. These tools assist in accomplishing a definable engineering task in the software design process by automating manual activities through structured prototyping. This technique reduces customized development time, supplying consistent code sets and supporting the entire software life cycle process.
Comité Consultatif Internationale de Télégraphique et Téléphonique (Consultative Committee on International Telephone and Telegraph). CCITT is a technical committee of the International Telecommunications Union, a United Nations organization in Geneva. It sets international communications recommendations concerning standardization of data interfaces, modems, and data networks. ArcInfo is fully compliant with CCITT Group IV, the Standard for raster data compression. ArcInfo supports the following TIFF compression schemes: CCITT Group 4 for black-and-white data (read only); CCITT Group 3, one-dimensional encoding for black-and-white data; and PackBits.
Compact Disk-Read Only Memory. CD-ROM is an optical media. A CD-ROM 5.25-inch disk can hold about 650 megabytes of information. The ISO 9660 standard defines the format of data held on CD-ROM.
See grid cell.
A discrete location that has a supply of a resource or commodity. In spatial interaction, a center is consider to have attractiveness.
Computer Graphics Metafile is a graphic image exchange standard, ANSI: x3.122-1986, ISO: 8632-1986, for graphic output file format. ArcInfo, ArcView Version 2, and PC ARC/INFO support CGM.
1. A letter (e.g., a, b, c, or d), digit (e.g., 1, 2, or 3), or special graphic symbol (e.g., *, |, or -) treated as a single unit of data.
2. A data type for an attribute designating that values for the attribute will be represented using characters. For example, the character data type would be appropriate for the attribute COUNTRY, if the values assigned are like United States, Brazil, Canada, Thailand, and so on.
Checkin is the act of returning ArcStorm data which was previously checked out for update purposes. When modified data is checked in, all locks on the data are released.
Checkout is the act of taking selected data out of an ArcStorm database into a local coverage for editing purposes. When data is checked out, it is locked to prevent updates from other users. ArcStorm data cannot be modified directly, it must first be checked out.
A software system is said to have a client/server architecture when there is a central process (server) which accepts requests from multiple user processes (clients). ArcStorm is one example of a client/server architecture within ArcInfo.
The spatial extraction of those features from one coverage that reside entirely within a boundary defined by features in another coverage (called the clip coverage)-clipping works much like a cookie cutter.
1. Abbreviation of the term COordinate GeOmetry. Land surveyors use COGO functions to enter survey data, to calculate precise locations and boundaries, to define curves, and so on.
2. The name of the ArcInfo coordinate geometry software product.
The vertical dimension of a table. A column has a name and a data type applied to all values in the column.
A specific instruction to a computer program, issued by the user to perform a desired action.
command line interface
A software product that allows the user to type in commands at a prompt. Contrast to forms interface.
To make permanent any changes made during a database transaction. Compare with roll back.
concurrency management
A database management process for maintaining consistency of the data while supporting simultaneous access by more than one user. A typical technique is to allow any number of users read access but to allow only one user to have write access. A second user wanting write access will have to wait until the first person completes their transaction.
conditional operator
A symbol or keyword specifying how to compare values. Conditional operators are used to query a database. Examples from SQL include:
= (equal to) BETWEEN
> (greater than) CONTAINING
A set of functions and procedures that aligns the arcs of one coverage with those of another and then transfers the attributes of one to the other. Alignment precedes the transfer of attributes and is most commonly performed by rubber-sheeting operations.
The topological identification of connected arcs by recording the from- and to-node for each arc. Arcs that share a common node are connected. See also arc-node topology.
Limits imposed on a model. For example, in an interaction model, specifying that the number of trips generated from an origin to all destinations cannot exceed the origin's production capacity.
The topological identification of adjacent polygons by recording the left and right polygons of each arc. See also polygon-arc topology.
continuous data
A surface for which each location has a specified or derivable value. Typically represented by a tin or lattice (e.g., surface elevation).
A line connecting points of equal surface value.
contour interval
The difference in surface values between contours.
A set of numbers that designate location in a given reference system, such as x,y in a planar coordinate system or an x,y,z in a three-dimensional coordinate system. Coordinates represent locations on the Earth's surface relative to other locations. See also vector and Cartesian coordinate system.
coordinate geometry
coordinate system
A reference system used to measure horizontal and vertical distances on a planimetric map. A coordinate system is usually defined by a map projection, a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions to locate x,y positions of point, line, and area features.
In ArcInfo, a system with units and characteristics defined by a map projection. A common coordinate system is used to spatially register geographic data for the same area.
Cover #
A unique sequence number automatically generated by ArcInfo for each coverage feature. This internal number is used to directly access features and to describe topological relationships between coverage features. It is often referred to as the record number.
An integer identifier, assigned by the user, to relate geographic features and corresponding attribute data. Cover-ID is an item found in feature attribute tables, with 'Cover' replaced by the coverage name (e.g., for a soils coverage, the Cover-ID would be SOILS-ID). Feature-ID and User-ID are synonymous terms to Cover-ID.
1. A digital version of a map forming the basic unit of vector data storage in ArcInfo. A coverage stores geographic features as primary features (such as arcs, nodes, polygons, and label points) and secondary features (such as tics, map extent, links, and annotation). Associated feature attribute tables describe and store attributes of the geographic features.
2. A set of thematically associated data considered as a unit. A coverage usually represents a single theme such as soils, streams, roads, or land use.
coverage extent
The coordinates defining the minimum bounding rectangle (i.e., xmin,ymin and xmax,ymax) of a coverage or grid. All coordinates for the coverage or grid fall within this boundary. In ARCPLOT and ARCEDIT, map extent is often set from the coverage extent. See also BND.
coverage units
The units (e.g., feet, meters, inches) of the coordinate system in which a coverage is stored.
cross-tile indexing
A method used to index features that cross tile boundaries. Features that cross tile boundaries are stored as one or more features in each tile instead of as a single feature.
The Content Standards for Spatial Metadata. A document produced by the Federal Geographic Data Committee (FGDC) that describes spatial metadata.
1. A graphic pointer used with a mouse to point to a location on a terminal screen.
2. An internal pointer to a record in a table which provides a mechanism for processing a selected set of records. The cursor is moved one by one through the set while operations such as display, query and update are performed.
1. In pathfinding, a cycle is a path or tour beginning and ending at the same node.
2. In tracing, a cycle is a set of arcs forming a closed polygon. Upstream and downstream directionality are undefinable in a cycle.

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Data Access Language. Apple's former standard to allow applications to communicate with relational databases. DAL is middleware on a network. It is a program installed on the database server to provide a common SQL access for all database servers on a network. Apple has licensed this technology to Independence Technologies, Inc.
dangle length
Minimum length allowed for dangling arcs during the CLEAN process. CLEAN removes dangling arcs that are shorter than the dangle length.
dangling arc
An arc having the same polygon on both its left and right sides and having at least one node that does not connect to any other arc. It often identifies where a polygon does not close properly (e.g., undershoot), where arcs don't connect properly, or where an arc was digitized past its intersection with another arc (i.e., overshoot). A dangling arc is not always an error. For example, dangling arcs can represent cul-de-sacs in street centerline maps. See also dangling node.
dangling node
The endpoint of a dangling arc not connected to another arc.
data access security
Measures taken to control system users' ability to view or modify data. These measures can include logical views of data and explicit access rights by group or individual users. See also access rights.
A logical collection of interrelated information, managed and stored as a unit, usually on some form of mass-storage system such as magnetic tape or disk. A GIS database includes data about the spatial location and shape of geographic features recorded as points, lines, areas, pixels, grid cells, or tins, as well as their attributes.
database design
The formal process of analyzing facts about the real world into a structured database model. Database design is characterized by the following phases: requirement analysis, logical design and physical design.
DATABASE directory
The same as the Library Reference workspace. It is the system directory that LIBRARIAN uses to manage information about a map library. Each map library has one database directory named DATABASE.
data conversion
The translation of data from one format to another. ArcInfo supports data conversion from many geographic data formats such as DLG, TIGER, DXF, and DEM.
data dictionary
A catalog of all data held in a database, or a list of items giving data names and structures. Also referred to as DD/D for data dictionary/directory. Commercial RDBMSs have online data dictionaries stored in special tables called system tables.
data integrity
Maintenance of data values according to data model and data type. For example, to maintain integrity, numeric columns will not accept alphabetic data. See referential integrity.
data model
1. The result of the conceptual design process. A generalized, user-defined view of the data related to applications.
2. A formal method of describing the behavior of the real-world entities. A fully developed data model specifies entity classes, relationships between entities, integrity rules and operations on the entities.
3. ArcInfo coverages and grids use a georelational data model, a hybrid data model that combines spatial data (in coverages or grids) and attribute data (in tables). Other data models used in ArcInfo include tins, images, and grid.
data set
A named collection of logically related data items arranged in a prescribed manner.
data type
The characteristic of columns and variables that defines what types of data values they can store. Examples include character, floating point and integer.
ArcInfo software's link to relational database management systems (RDBMS). DBI enables ArcInfo users to access existing commercial databases and take advantage of the power and capabilities of the RDBMS.
database lock
Locking is a mechanism by which database systems can prevent conflicting access to data when multiple users are making requests to the data. See also persistent lock.
database management system (DBMS)
A set of computer programs for organizing the information in a database. A DBMS supports the structuring of the database in a standard format and provides tools for data input, verification, storage, retrieval, query, and manipulation.
A set of parameters and control points used to accurately define the three-dimensional shape of the Earth (e.g., as a spheroid). The datum is the basis for a planar coordinate system. For example, the North American Datum for 1983 (NAD83) is the datum for map projections and coordinates within the United States and throughout North America.
See database management system.
DBMS table
See attribute table.
"Digital Chart of the World." The first 1:1,000,000-scale digital basemap of the world. The DCW contains topologically based vector data digitized from the U.S. Defense Mapping Agency's Operational Navigation Charts.
Dynamic Data Exchange. An IAC protocol developed by Microsoft for Windows-based applications. DDE allows one application to send messages to, and get information from, other applications in Windows. This is specific to Windows only (RPC, Remote Procedure Calls, is used in the UNIX environment). DDE is supported in ArcView Version 2 for the exchange of data with other business applications without having to convert the data or leave ArcView. (See IAC.)
Data definition language. SQL statements that can be used either interactively or within programming language source code to define databases and their components.
See digital elevation model.
1. In allocation, the potential for using a portion of the supply of a resource or commodity.
2. In spatial interaction, demand is the measure of the need for a particular type of service or goods that generates a trip to a destination. For example, the demand for a gallon of milk may generate a trip to a grocery store.
The process of restructuring a normalized data model to accommodate operational constraints or system limitations.
A process of adding vertices to an arc at specified distances, without altering the arc's shape. Compare with spline and grain tolerance.
descriptive data
Tabular data describing the characteristics of geographic features. Can include numbers, text, images, and CAD drawings about features. ArcInfo stores descriptive data in feature attribute tables and in related tables. Also referred to as attribute data.
In spatial interaction, the location of the end of a trip. For example, a shop or an office where a consumer or a worker is going. Destinations are represented as centers in a network coverage, as points in a point coverage, and as label points in a polygon coverage.
digital elevation model
1. A digital representation of a continuous variable over a two- dimensional surface by a regular array of z values referenced to a common datum. Digital elevation models are typically used to represent terrain relief. Also referred to as 'digital terrain model' (DTM).
2. An elevation database for elevation data by map sheet from the National Mapping Division of the U.S. Geological Survey (USGS).
3. The format of the USGS digital elevation data sets.
Digital Geospatial Metadata. DGM was approved in June 1994 by the Federal Geographic Data Committee (FGDC). DGM describes the specifications for the content, quality, condition, and other characteristics of metadata (data about data). The standard provides a common set of terminology and definitions for the documentation of geospatial data. DGM establishes the names of data elements and groups of data elements to be used for these purposes, definitions of these data elements and groups, and information about the values that are to be provided for the data elements.
The Digital Geographic Information Exchange Standard is produced under authority of NATO's Digital Geographic Information Working Group. DIGEST is a standard for digital geographic information which will enable interoperability and compatibility among national and multinational systems and users. DIGEST is composed of standards for two digital geographic formats: ARC Standard Raster Graphic (ASRG) and vector relational format (VRF). ASRG is very similar to ADRG and can be imported into ArcInfo as an ADRG file with the ADRGGRID command. The ArcInfo VPFIMPORT and VPFEXPORT convertors will process the VRF data.
digital terrain model
See digital elevation model.
1. To encode geographic features in digital form as x,y coordinates.
2. The process of using a digitizer to encode the locations of geographic features by converting their map positions to a series of x,y coordinates stored in computer files. Pushing a digitizer button records an x,y coordinate. A digitized line is created by recording a series of x,y coordinates.
1. A device that consists of a table and a cursor with crosshairs and keys used to digitize geographic features.
2. Title of the person who uses a digitizing device.
See digitize.
directed network
A network in which each arc has an associated direction of flow. Direction of flow can be determined by arc direction (e.g., each arc is digitized so that it is oriented downstream), a value in an item in the AAT, or through the use of a selection file.
A computer term identifying a location on a disk containing a set of data files and other directories (subdirectories). Operating systems use directories to organize data. The location of a directory is specified with a pathname.
discrete data
Geographic features containing boundaries: point, line or area boundaries.
A storage medium consisting of a spinning disk coated with a magnetic material for recording digital information.
An inexpensive, low-capacity storage medium, usually measuring 3.5 inches in diameter, often referred to as a floppy disk.
The process of removing boundaries between adjacent polygons that have the same values for a specified attribute.
distance-decay function
In spatial interaction, the mathematical representation of the effect of distance on the accessibility and number of interactions between locations. It can be either a power or an exponential function.
1. Digital Line Graph files from the U.S. Geological Survey (USGS), including data from the basemap categories such as transportation, hydrography, contours, and public land survey boundaries.
2. The digital format standards published by USGS for exchanging cartographic data files and in which the USGS delivers Digital Line Graph data sets.
Data manipulation language. SQL statements that can be used either interactively or within programming language source code to access and retrieve data stored in a database management system.
In a database, the set of allowed values for a table column, for example all positive integers.
double precision
Refers to a high level of coordinate accuracy based on the possible number of significant digits that can be stored for each coordinate. ArcInfo data sets can be stored in either single- or double-precision coordinates. Double-precision coverages store up to 15 significant digits per coordinate (typically, 13 to 14 significant digits), retaining the accuracy of much less than one meter at a global extent. See also single precision.
In tracing, downstream is the direction along the arcs that is the same as the direction of flow. Direction of flow is determined by a user-defined convention. See also directed network.
A perspective or panoramic rendering of two-dimensional features superimposed on a surface.
Digital terrain model. See digital elevation model.
Data Exchange Format. A format for storing vector data in ASCII or binary files. Used by AutoCAD and other CAD software for data interchange. DXF files are convertible to ArcInfo coverages.
dynamic segmentation
The process of computing the locations of events on linear features at run time based on event tables for which distance measures are available. Route-system features and event-handling commands provide the dynamic segmentation capability within ArcInfo.

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