GIS Definitions: I-L

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

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Interapplication communication. The capability of one computer program to communicate with another program. With IAC, two (or more) programs can execute simultaneously, share data, and make requests of each other. ArcInfo Version 7 and ArcView Version 2 support IAC. IAC tools in AML (ARC Macro Language) support real-time GIS, a network GIS process server, interoperability, and open integration between ArcInfo and other applications. In a client/server environment the command references on the client side are IACCONNECT, IACDISCONNECT, and IACREQUEST; on the server side are IACOPEN, IACCLOSE, and &IACRETURN.
The topological overlay of a coverage (input) with a polygon coverage (identity). For each feature in the input coverage, the intersection with identity features is determined, creating new features of the same feature class as the input coverage. For example, a road (input coverage, arc feature class) passing through two counties (identity coverage) would be split into two arc features, each with the attributes of the road and the county it passes through. Compare with intersect and union.
identity link
A coverage link whose from-location is the same as its to-location. Used to control rubber sheeting and adjustment operations. Identity links act as nails to hold down the point location during adjustment. See also link.
Institute of Electrical and Electronics Engineers. IEEE Standard 1003.1-1990 (aka POSIX.1) defines C program interfaces for the operating system that enable source code portability beyond the ANSI C definition (see POSIX). ArcInfo and ArcView are fully compatible with network protocols that support IEEE 802.5, FDDI and X.25 transport standards.
Interactive Graphics Design Software. Intergraph IGDS file formats can be converted to and from ArcInfo coverages.
Initial Graphics Exchange Specification (IGES) is a common data format used for transfer of CAD data. IGES files can be converted to and from ArcInfo coverages.
A graphic representation or description of a scene, typically produced by an optical or electronic device. Common examples include remotely sensed data (e.g., satellite data), scanned data, and photographs. An image is stored as a raster data set of binary or integer values that represent the intensity of reflected light, heat, or other range of values on the electromagnetic spectrum.
image catalog
An organized set of spatially referenced, possibly overlapping, images that can be accessed as one logical image. An image catalog is a group of images on disk, each referenced by a record in an INFO data file. At a minimum, items in the data file include the image pathname and the bounding coordinates xmin,ymin and xmax,ymax.
image integrator
A collection of image management and display tools in ArcInfo that allows vector and raster data to be displayed concurrently. Image integrator commands georeference images to real-world coordinates, display images, and manage image catalogs.
The amount of resistance (or cost) required to traverse a line from its origin node to its destination node or to make a turn (i.e., move from one arc through a node onto another arc). Resistance may be a measure of travel distance, time, speed of travel times the length, and so on. Higher impedance indicates more resistance to movement, with 0 indicating no cost. Often, a negative impedance value indicates a barrier. Impedance is used in network routing and allocation. An optimum path in a network is the path of least resistance (or lowest impedance).
Special data structure used in a database to speed searching for records in tables or spatial features in geographic data sets.
ArcInfo supports both spatial and attribute indexes. See also item indexing, cross-tile indexing and spatial indexing.
index coverage
The polygon coverage that describes, and is used as a spatial index for, the tile structure of a layer. Each polygon in an index coverage corresponds to a tile.
A tabular DBMS used by ArcInfo to store and manipulate feature attribute tables and other related tables.
INFO database
The contents of a set of INFO data files, feature attribute tables, and related files stored in each ArcInfo workspace under a subdirectory named INFO. This subdirectory contains all feature attribute tables for the set of coverages contained in the workspace.
A relational database management system to which ArcInfo has access through the DATABASE INTEGRATOR.
A relational database management system to which ArcInfo has access through the DATABASE INTEGRATOR.
A number without a decimal (0, 1, 25, 173, 1032, etc.). Integer values can be less than, equal to, or greater than zero.
Interaction is a measure of the estimated number of trips that will be generated between origins and destinations for a particular activity. Interactions depend upon the properties of the origin to generate a trip, the property of the destination to attract a trip and the cost of traveling between them.
interaction matrix
A generated INFO file containing the number of interactions occurring between a set of origins and destinations. The interaction matrix can be analyzed to make trade-area maps.
inter-application communication (IAC)
A technology that enables software applications on remote or local machines to communicate with each other. IAC makes it possible to develop applications that seamlessly integrate the capabilities of a number of programs by providing a way for external applications to request services of ArcInfo and for an AML application to exploit the capabilities of other applications.
For data communication, a hardware and software link that connects two computer systems, or a computer and its peripherals.
internal number
See Cover #.
An international consortium of wide area networks that operate using a standard set of addresses allowing machine-to-machine connectivity on a global scale. The Internet is an outgrowth of a Defense Advanced Research Projects Agency (DARPA) research project in the early 1970s to provide connectivity between scientists running computer simulations in different locations. Additional regional, private, and public networks have joined the Internet over time. At this point there are over two million computers that now have direct access to the resources on the Internet. ESRI operates a discussion group on the Internet called ESRI-L. ESRI-L is open to the general public and is available to any Internet subscriber. It was established to give ArcInfo users a way to exchange technical questions and information.
The estimation of z values of a surface at an unsampled point based on the known z values of surrounding points.
The topological integration of two spatial data sets that preserves features that fall within the area common to both input data sets. See also identity and union.
Integrated Services Digital Network, provides WAN (see WAN) combined transmission of analog and digital services. ISDN is offered as a Basic Rate Service on either one or two channels for WAN services of either 64 kbps or 128 Kbps. ISDN services are available from a long-distance telephone company.
The International Organization for Standardization. A worldwide federation of national standards bodies (e.g., ANSI from the U.S.) that develops international standards. A Technical Committee (ISO/TC211) is developing international Geographic Information/Geomatics standards. Among many other computing standards, ISO maintains an SQL standard and is developing an extended version, SQL3, which will support queries on geographic data sets.
ISO 8211
The third of three parts of the SDTS (see SDTS) that specifies data transfer implementation (i.e., encoding method). ISO 8211 is a general-purpose, media-independent interchange standard whose variable length records may be written on any medium that is able to accept them, including communications lines.
ISO 9000
Established in 1987, ISO 9000 is an international set of five related standards for qualification of global quality assurance and quality control standards. Adherence is accomplished through an application process for ISO 9000 certification in company standards for inspecting production processes, updating records, maintaining equipment, training employees and handling customer relations. The governing international consortium is recognized worldwide.
ISO 9660
Volume and file structure of CD-ROM for Information Interchange. A standard for the organization of data on CD-ROM media established by the International Standards Organization.
A line on a surface connecting points of equal value.
A column of information in an attribute table, for example, a single attribute of a record in an INFO data file.
item indexing
A means of accelerating logical queries and tabular 'relates' by creating an index on an item in a database table.
Integrated Terrain Unit Mapping is an example of integrated data management. It is the process of adjusting terrain unit boundaries so that there is increased coincidence between the boundaries and occurrences of interdependent terrain variables such as hydrography, geology, physiography, soils and vegetation units.


See relational join.

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label point
See point.
Local area network. Computer data communications technology that connects computers at the same site. Computers and terminals on a LAN can freely share data and peripheral devices, such as printers and plotters. LANs are composed of cabling and special data communications hardware and software.
A series of satellites that produce images of the earth. The Landsat remote sensing satellite program was developed by NASA (National Aeronautics and Space Administration). Landsat data are provided in .BIL (band interleaved by line) or .BIP (band interleaved by pixel) formats. BIL and BIP are supported by ArcInfo and ArcView.
A spherical reference system used to measure locations on the Earth's surface. Latitude and longitude are angles measured from the Earth's center to locations on the Earth's surface. Latitude measures angles in a north-south direction. Longitude measures angles in the east-west direction.
A surface representation that uses a rectangular array of mesh points spaced at a constant sampling interval in the x and y directions relative to a common origin. A lattice is stored as a grid, but represents the value of the surface only at the mesh points rather than the value of the entire cell.
A thematic set of spatial data described and stored in an ArcStorm database or a LIBRARIAN map library. Layers organize a database or map library by subject matter (e.g., soils, roads, and wells). Conceptually, layers in a database or map library environment are exactly like coverages. See also ArcStorm database and map library.
layer index
See cross-tile indexing.
least-cost path
The path, among possibly many, between two points which has the lowest traversal cost, where cost is a function of time, distance, or other user-defined factors. See also impedance.
left-right topology
The topological data structure ArcInfo uses to represent contiguity between polygons. Left-right supports analysis functions such as adjacency. See also topology.
1. The reference area on a map that lists and explains the colors, symbols, line patterns, shadings, and annotation used on the map. The legend often includes the scale, origin, orientation, and other map information.
2. The symbol key used to interpret a map.
A set of software tools to manage and access large geographic data sets in a map library. LIBRARIAN commands create and define a map library, move data in and out of a library, query the data in a map library, and display the results of a query.
A collection of spatially related ArcStorm or LIBRARIAN layers. A library has a spatial extent which applies to all layers in the library.
1. A set of ordered coordinates that represents the shape of geographic features too narrow to be displayed as an area at the given scale (e.g., contours, street centerlines, or streams), or linear features with no area (e.g., state and county boundary lines).
2. A single arc in a coverage.
3. A line on a map (e.g., a neatline).
A spatial operation in which arcs in one coverage are overlaid with polygons of another coverage to determine which arcs, or portions of arcs, are contained within the polygons. Polygon attributes are associated with corresponding arcs in the resulting line coverage.
line symbol
A symbol for drawing coverage arcs.
linear event
See event.
linear feature
A geographic feature that can be represented by a line or set of lines. For example, rivers, roads within a pizza delivery area, and electric and telecommunication networks are all linear features. Linear features are represented in ArcInfo by arcs or by the route-system feature class.
A coverage feature class; links are two-point segments that represent from- and to-locations for the rubber sheeting adjustment process.
A string, a number, or a date which directly represents a constant value. 'XYZ123', '1234' and '6/10/57' are examples of a string literal, a numeric literal and a date literal, respectively.
local area network
See LAN.
log file
A coverage or workspace history file containing a list of all commands used to operate on a coverage or all commands used in the workspace.
logical connector
One of the reserved words AND, OR and XOR used to build complex logical expressions in a query.
logical expression
A combination of items, system items, system variables, literals and arithmetic logical operators from which a value of TRUE or FALSE is derived; for example:
$NUM1 LE 100
logical operator
Another term for Boolean operator. See Boolean expression.
logical selection
The process of selecting a subset of features from a coverage using logical expression that operates on the attributes of coverage features (e.g., AREA GT 16000). Only those features whose attributes meet the criteria are selected. Also known as feature selection by attribute.
long transaction
Long transactions support applications where changes to a database might span several days, weeks or months and may involve several sessions. Many planning and design activities, such as subdivision development, require long transactions.
See latitude-longitude.
lookup table
1. A special tabular data file containing additional attributes for features stored in an associated feature attribute table. The table can be an external attribute table or an INFO table that describes coverage features.
2. A special lookup table in which numeric item values are classified into categories. For example, well depth can be recorded explicitly in the feature attribute table, but displayed and used as a set of classes, such as 0 to 250 feet, 251 to 500 feet, and so on. An INFO lookup table contains at least two items: the relate item and an item named either SYMBOL or LABEL.

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