GIS Definitions: T-Z

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

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A set of data elements that has a horizontal dimension (rows) and a vertical dimension (columns) in a relational database system. A table has a specified number of columns but can have any number of rows. A table is often called a relation. Rows stored in a table are structurally equivalent to records from flat files in that they must not contain repeating fields.
Text attribute table for an annotation subclass in a coverage. In addition to user-defined attributes, the TAT contains a sequence number and text feature identifier. See also feature attribute table.
The Transmission Control Protocol (TCP) is a communication protocol layered above the Protocol (IP). These are low-level communication protocols which allow computers to send and receive data.
1. A coverage containing common feature boundaries, such as land-water boundaries, for use as a starting place in automating other coverages. Templates save time and increase the precision of topological overlays.
2. A map template containing neatlines, North arrow, logos, and other cartographic map elements for a common map series.
3. An empty tabular data file containing only item definitions.
Terabyte, TB
A measure of data size. A terabyte of data is equivalent to 1,000 gigabytes of data or 1,000,000 megabytes of data. One petabyte equals 1,000 terabytes. Computer unit, 1012 bytes.
A device, usually a display monitor and a keyboard, used to communicate with the computer.
A text style defined by font, size, character spacing, color, and so on, used to label maps and coverage features in ArcInfo.
Thiessen polygons
Polygons whose boundaries define the area that is closest to each point relative to all other points. Thiessen polygons are generated from a set of points. They are mathematically defined by the perpendicular bisectors of the lines between all points. A tin structure is used to create Thiessen polygons.
A user-defined perspective on a coverage, grid, tin or image geographic data set specified, if applicable, by a coverage name and feature class or data set name, attributes of interest, a data classification scheme, and theme-specific symbology for drawing.
thematic data
See descriptive data.
Registration or geographic control points for a coverage representing known locations on the Earth's surface. Tics allow all coverage features to be recorded in a common coordinate system (e.g., Universal Transverse Mercator [UTM] meters or State Plane feet). Tics are used to register map sheets when they are mounted on a digitizer and to transform the coordinates of a coverage (e.g., from digitizer units [inches] to UTM meters).
TIC file
The coverage file used to store tic coordinates and tic IDs for a coverage.
tic match tolerance
The maximum distance allowed between an existing tic and a tic being digitized. If this distance is exceeded, the digitizing error is considered unacceptable and the map must be registered over again. The tic match tolerance is used to ensure a low RMS error during map registration on a digitizer.
Tagged interchange (image) file format. An industry-standard raster data format. TIFF supports black-and-white, gray-scale, pseudocolor, and true-color images, all of which can be stored in a compressed or uncompressed format. TIFF is commonly used in desktop publishing and serves as an interface to numerous scanners and graphic arts packages. (See CCITT.)
The Topologically Integrated Geographic Encoding and Referencing data format used by the U.S. Census Bureau to support census programs and surveys. It was used for the 1990 census. TIGER files contain street address ranges along lines and census tract/block boundaries. This descriptive data can be used to associate address information and census/demographic data with coverage features.
The spatial unit by which geographic data is organized, subdivided, and stored in a map library. Tiles subdivide the area covered by a map library and organize the library data by location (e.g., counties might be the tiles in a statewide database). A tile can be a regular, geometric shape (e.g., a map sheet), or an irregular shape, such as a county boundary. See also LIBRARIAN.
Triangulated irregular network. A surface representation derived from irregularly spaced sample points and breakline features. The tin data set includes topological relationships between points and their neighboring triangles. Each sample point has an x,y coordinate and a surface, or z-value. These points are connected by edges to form a set of non-overlapping triangles used to represent the surface. Tins are also called irregular triangular mesh or irregular triangular surface model.
The ArcInfo software product used for surface representation, modeling, and display.
TOL file
A coverage file that contains processing tolerances (fuzzy, tic match, and dangle length) and editing tolerances (weed, grain, edit distance, snap distance, and nodesnap distance). ArcInfo uses TOL file values as defaults in many automation, editing, and processing operations.
Of an arc's two endpoints, the one last digitized. See also from-node.
topographic map
1. A map containing contours indicating lines of equal surface elevation (relief), often referred to as topo maps.
2. Often used to refer to a map sheet published by the U.S. Geological Survey in the 7.5-minute quadrangle series or the 15-minute quadrangle series.
topological overlay
An analysis procedure for determining the spatial coincidence of geographic features. ArcInfo supports overlay among and between all feature classes. See also identity, intersect and union.
The spatial relationships between connecting or adjacent coverage features (e.g., arcs, nodes, polygons, and points). For example, the topology of an arc includes its from- and to-nodes, and its left and right polygons. Topological relationships are built from simple elements into complex elements: points (simplest elements), arcs (sets of connected points), areas (sets of connected arcs), and routes (sets of sections, which are arcs or portions of arcs). Redundant data (coordinates) are eliminated because an arc may represent a linear feature, part of the boundary of an area feature, or both. Topology is useful in GIS because many spatial modeling operations don't require coordinates, only topological information. For example, to find an optimal path between two points requires a list of the arcs that connect to each other and the cost to traverse each arc in each direction. Coordinates are only needed for drawing the path after it is calculated.
A tour is a minimum-impedance path that starts at an origin, visits a number of stops and returns to the origin visiting the stops only once. This is the solution to the traveling salesman problem. ArcInfo provides a heuristic solution to the traveling salesman problem by ordering the stops and then finding the least-cost path which visits them.
The process of determining which portions of a network connect.
trade-area map
A map showing the region from which a store derives a certain percentage of its revenues. Trade-area maps can be made from the information contained in an interaction matrix.
A logical unit of work performed on a database. A transaction can be terminated by either making permanent or rolling back all updates.
The process that converts coordinates from one coordinate system to another through translation, rotation, and scaling. ArcInfo supports these transformations: similarity, affine, piecewise linear, projective, NADCON datum adjustment using minimum-derived curvature transformation, and a polynomial transformation to warp grids and images.
traveling salesman problem
This is a classic tour problem in which a hypothetical salesman must find the most efficient sequence of destinations in his territory, stopping only once at each.
trusted login
A user is said to have a trusted login to another computer when that user is not prompted for a password when accessing the remote computer.
A row in a relational table; synonymous with record, observation.
A turn represents a transition from one network link to another at a network node. Turns represent relationships between network links rather than an abstraction of some real-world physical entity. The properties of a turn are stored in a turntable.
turn impedance
The impedance or cost of making a turn at a network node. A turn impedance for making a left turn can be different from that of a right turn or a U-turn at a node.
An INFO file containing the turn impedances between pairs of network links. There can be sixteen possible turn impedances between four network links meeting at a node (i.e., left, right, straight, and U-turn for each of the four network links).
triangulated irregular network
See tin.

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An arc that does not extend far enough to intersect another arc. See also dangling arc.
A topological overlay of two polygonal spatial data sets which preserves features that fall within the spatial extent of either input data set; that is, all features from both coverages are retained. See also intersect and identity.
universe polygon
The first record in a polygon attribute table. It represents the area beyond the outer boundary of the coverage. It's the only polygon that never has a label point, and so has a User-ID value of 0. Its area equals the negative sum of all the polygons in the coverage. Also referred to as the external polygon.
In tracing, upstream is the direction along the arcs that is against the direction of flow. Direction of flow is determined by a user-defined convention. See directed network.
Synonymous term for Cover-ID and feature-ID.
A digital elevation model produced by the Survey Branch of the United States Department of the Interior, consisting of a regular array of elevations referenced in the Universal Transverse Mercator (UTM) coordinate system. These data correspond to the standard 1:24,000-scale 7.5 x 7.5-minute quadrangles or 1:250,000 one-degree map sheets. Elevations are in meters or feet referenced to mean sea level.
U.S. National Map Accuracy Standards: Accuracy standards for published maps in English units defining measurements for horizontal and vertical accuracy. It is described in absolute terms; however, it is not described in statistical terms and some inconsistencies have been noted, thereby making it unusable for engineering mapping (large-scale mapping), and is not convenient to use in conjunction with mapping from space (remote sensing).

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value attribute table (VAT)
Value attribute table. A table containing attributes for a grid. In addition to user-defined attributes, the VAT contains the values assigned to cells in the grid and a count of the cells with those values.
A coordinate-based data structure commonly used to represent linear geographic features. Each linear feature is represented as an ordered list of vertices. Traditional vector data structures include double-digitized polygons and arc-node models.
One of a set of ordered x,y coordinates that constitutes a line.
virtual table
See view.
A logical table whose data are not physically stored. You define a view to access a subset of the columns stored in a row, access a set of columns stored in different rows, or avoid creating a redundant copy of data that is already stored.
Vector product format is a digital geographic vector-based format used by the U.S. Defense Mapping Agency for the distribution of its vector data sets. ArcInfo includes a bidirectional VPF translator.
Vector relational format. A relational model-based format very similar to the DMA VPF (Defense Mapping Agency Vector Product Format).

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Wide-Area Information Server. A client/server software system that provides sophisticated indexing of files based on all words in each file and provides scoring and "relevance feedback" of documents to its client. The client then selects which file or files to retrieve from the server from a list of file or document titles, ranked by a score assigned by the software. Searching can include Boolean constructs and access to structured information systems (like databases) through its compliance with ANSI standard Z39.50. WAIS is addressed at the operating system level.
Wide area network. Computer data communications technology that connects computers at remote sites. WANs are composed of special data communications hardware and software and usually operate across public or dedicated telephone networks.
watch file
A text file that records all nongraphic input and output during an ArcInfo session. Watch files can be edited and converted to AML programs.
weed tolerance
The minimum allowable distance between any two vertices along an arc. Weed tolerance is a parameter that can be set before adding arc features. When adding new arcs, if an input vertex is within the weed distance of the last vertex, it is disregarded. When weeding existing arcs, it is the tolerance used by the Douglas-Peucker algorithm. Nodes are always retained. The proximity of vertices on one arc to vertices on another arc within the tolerance does not cause them to be weeded; this is controlled by the proximal tolerance.
A directory containing geographic data sets for use with ArcInfo. A workspace contains an INFO directory for the feature attribute tables. ArcInfo workspaces have three primary uses: as a user work area, to store all the map sections for each tile in a map library, and as automation workspaces to store all the versions of a single coverage as it progresses through the coverage automation process.
World Wide Web
Developed by the European Laboratory for Particle Physics (CERN)
Consortium in Switzerland as a distributed hypermedia server. It allows one to prepare electronic documents that are composites of, or pointers to, many different files of potentially different types scattered across the world. It employs a hypertext markup language (html) to create the documents it serves and to follow "links" known as Universal Resource Locators (URLs) to fetch the document from elsewhere on the Internet. A WWW server does not provide search capabilities, rather it provides explicit linkage between files on the Internet using hypertext. This allows one to organize information in a particular way, but, unless the links exist, does not permit the discovery of other information that was not associated by the author. WWW can be accessed by Mosaic (see Mosaic).

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X Windows
A system developed at MIT that allows applications to be displayed in windows and shared among different workstations and terminals. Available on all UNIX operating systems supported by ArcInfo and ArcView. ArcInfo and ArcView are X Windows-based applications. ArcInfo or ArcView can be run on any X-compliant terminal or X Station on all supported UNIX platforms or on any PC or Macintosh through emulation software.
X Windows/Motif
ArcInfo and ArcView operate in the X Windows/Motif environment on all supported UNIX workstation platforms.
CCITT recommendation/ISC standard for wide area networks.
ISO standard for electronic mail.
X-Open Consortium
An association of American and European vendors formed in 1984 to promote open systems.
An X/Open software standard for UNIX Operating Systems. IBM provides a cshell variable named PSALLOC to make AIX XPG3-compliant, resulting in adding more swap space on disk, protects ArcInfo from an outside application bug-induced crash, and increases application performance.

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An ANSI protocol standard for WAN (wide area network) information query and exchange to share library referencing requests via distributed electronic access to information.
The value of a surface at a particular x,y location (e.g., elevation). Often referred to as spot values or spot elevations.
To enlarge and display greater detail of a portion of a geographic data set.

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