GIS Definitions: T-Z
Definitions of some commonly used GIS (Geographic Information System) terms.
T U V
W X Y
Z Full glossary:
- 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
- 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
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
- 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
- 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
- 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
- topological overlay
- An analysis procedure for determining the spatial coincidence of
geographic features. ArcInfo supports overlay among and between all
classes. See also identity,
- 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
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
- 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
- triangulated irregular network
- See tin.
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- An arc that does not extend
far enough to intersect another arc. See also dangling
- 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
- 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
- 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
- Synonymous term for Cover-ID
- USGS DEM
- 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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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
- 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
- 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
- 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
recommendation/ISC standard for wide area networks.
- ISO standard for electronic
- 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
- 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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Glossary pages: A-D