GIS Definitions: E-H
Definitions of some commonly used GIS (Geographic Information System) terms.
Full glossary: A-D
- edge matching
- An editing procedure to
ensure that all features that cross adjacent map sheets have the same
edge locations. Links
are used when matching features in adjacent coverages.
- To correct errors within,
or modify, a computer file, a geographic data set, or a tabular file
containing attribute data.
- embedded SQL
- SQL statements that are
embedded in a host language program.
- A collection of objects
(persons, places, things) described by the same attributes. Entities
are identified during the conceptual design phase of database and
- entity relationship
- A graphical representation
of the entities and the relationships between them. Entity relationship
diagrams are a useful medium to achieve a common understanding of
data among users and application developers.
- A set of parameters defining
various display, editing, and data manipulation conditions that remain
active during a session until explicitly changed by the user. For
example, the drawing environment in ARCEDIT might be 'arcs on, labels
off, annotation.streets on'.
- The Earth Observation
Satellite. An effort to study the earth as a system while tracking
long-term changes on a global scale. EOS, a mission of the National
Aeronautics and Space Administration (NASA), will produce petabytes
(1,000 terabytes) of satellite image data and also large-scale data
sets (terabytes [1,000 gigabytes] a day) to be manipulated and analyzed.
- equation item
- An arithmetic expression
used in place of an item
name in an ArcInfo command. For example, to list feature areas, a
user could specify LIST AREA; to list areas in acres instead of square
feet, a user could specify LIST AREA / 43560.
- A network protocol defining
a specific implementation of the Physical and Data Link Layers in
the OSI model (IEEE 802.3). Ethernet is a local area network, using
a bus topology, that provides reliable high-speed communications (maximum
of 10 million bits per second) in a limited geographic area (e.g.,
office complex, university complex).
- The parallel of reference
0 north or south.
- A geographic feature occurring
on or along a linear feature. There are three event types: linear,
continuous, and point. For example, a left lane closure on route I-10
from the 1.5 to 2.1 mileposts is a linear event. A continuous event
is a linear event where the start position of a segment is the same
as the end position of its preceding event, such as for speed limits.
A point event occurs at a point along a route, for example, an accident
at milepost 6.3 on route I-10. In ArcInfo, an event is defined in
terms of a route and measures along the route. See also route-system.
- event source
- This is a name assigned
by the user to reference a DBMS table containing event data for use
with the dynamic segmentation commands. This is similar to the relate
name. See also relate.
- Extended character sets
support languages which require 8-bit characters or double-byte characters,
such as Chinese and French. Compare with POSIX
- external file
- INFO stores data in files
within a database. However, database information can be stored in
files outside of the database. These files are referred to as external
files. For example, feature attribute tables are stored as external
INFO data files maintained in the coverage directory.
- external polygon
- See universe
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- A table used to store
attribute information for a specific coverage feature class. ArcInfo
maintains the first several items of these tables. Feature attribute
tables supported for coverages include:
<COVER>.PAT for polygons or points
<COVER>.AAT for arcs
<COVER>.NAT for nodes
<COVER>.RAT for routes
<COVER>.SEC for sections
<COVER>.PAT for regions
<COVER>.TAT for annotation (text)
where <COVER> is the coverage name.
- Fiber Distributed Data
Interface is a media access (transmission) control-level protocol
with token-ring architecture, a communication bandwidth of 100 Mbps
and supported on a fiber network medium. To provide required ArcInfo
communications, the workstation network communications software must
include TCP/IP and NFS
communication protocols, which the UNIX operating system provides.
Packaging of TCP/IP communications for FDDI transmission is supported
by a network interface card at the sending and receiving station,
and this packaging is transparent to ArcInfo applications and data.
- 1. A classification describing
the format of geographic features and supporting data in a coverage.
Coverage feature classes for representing geographic features include
point, arc, node, route-system, route, section, polygon and region.
One or more coverage features are used to model geographic features;
for example, arcs and nodes can be used to model linear features such
as street centerlines. The tic, annotation, link, and boundary feature
classes provide supporting data for coverage data management and viewing.
2. The conceptual representation of a geographic feature. When referring
to geographic features, feature classes include point, line, area,
- Synonymous term for Cover-ID
- feature selection by
- See logical
- Federal Geodetic Control
Committee: a standards committee concerned with accuracy levels in
geodetic control. Within the United States, coordinate control is
based on the National Geodetic Survey's published control points and
is a basis for collecting data. Data collected using these basic coordinate
points can be read by ArcInfo.
- The United States Federal
Geographic Data Committee. Composed of representatives of several
federal agencies and GIS vendors, the FGDC has the lead role in defining
spatial metadata standards, which it describes in the Content Standards
for Spatial Metadata (see CSSM,
- In a database, another
term for column.
- field data collector
- An electronic device that
collects and stores observation information from survey instruments.
Two types of devices are available: one records x,y,z coordinates
using a satellite-based global positioning system (GPS), and the other
device records distance and bearing ArcInfo GENERATE is often used
to convert GPS coordinates while ArcInfo COGO has a FIELDDATA conversion
- A set of related information
that a computer can access by a unique name (e.g., a text file, a
data file, a DLG file). Files are the logical units managed on disk
by the computer's operating system. Files may be stored on tapes or
- file transfer
- The process of copying
data from one computer to another or one DBMS to another.
- The Federal Information
Processing Standards. FIPS deals with a wide range of computer system
components including the components of most GISs: hardware, storage
media, data files, codes, interfaces, data transmission, networking,
data management, documentation, programming languages, software engineering,
performance, security, and so forth. FIPS 173 is the precursor to
the SDTS (Spatial Data Transfer Standard), which includes standardized
definitions for a variety of digital mapping terms, addressing federal
requirements for accuracy. FIPS provides a U. S. government standard
state and country identification code; standards approved for use
by U.S. government agencies. FIPS 152-2 includes POSIX.1 Compliance.
- A logical set of related
patterns representing text characters or point symbols. Courier, Helvetica,
and Times are three types of font.
- One or more table attributes
that can uniquely identify a record in another table. A foreign key
is the primary
key of another table. Foreign key-primary key relationships define
a relational join. See also relate.
- The pattern into which
data are systematically arranged for use on a computer. A file format
is the specific design of how information is organized in the file.
For example, ArcInfo has specific, proprietary formats used to store
coverages. DLG, DEM, and TIGER are geographic data sets with different
- A graphic user interface
characterized by user-controlled movement of a cursor from one data
field to another. Contrast to command
- Of an arc's two endpoints,
the one first digitized.
- functional surface
- A surface representation
which stores a single z value (as opposed to multiple z values) for
any given x,y location. TIN represents data as functional surfaces.
Functional surfaces are also referred to as 2.5-dimensional surfaces.
- fuzzy tolerance
- The fuzzy tolerance is
an extremely small distance used to resolve inexact intersection locations
due to limited arithmetic precision of computers. It defines the resolution
of a coverage resulting from the CLEAN operation or a topological
overlay operation such as UNION, INTERSECT, or CLIP.
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- A work of geographic reference
that supplies place name and location information. When a place name
is known, a gazetteer can provide the coordinates of the place. Most
atlases contain gazetteers. Well-known digital gazetteers are the
USGS Geographic Names Information System (GNIS) and the gazetteer
in the Digital Chart of the World (DCW). In ArcInfo the gazetteer
spatial index is done as a grid of alpha and numeric references which
is converted into a polygon coverage. Places (points or polygons)
are then overlaid with this grid, then sorted alphabetically. This
produces a list of place names sorted both alphabetically and by reference
- For the 1980 census, the
U.S. Census Bureau produced Geographic Base Files (GBF) and Dual Independent
Map Encoding (DIME) files, containing census geographic statistical
codes and coordinates of line segments for most metropolitan areas.
DIME files provide a schematic map of a city's streets, address ranges,
and geostatistical codes relating to the Census Bureau's tabular statistical
data. DIME was replaced by TIGER for the 1990 Census.
- In general, reducing the
number of points used to represent a line. In ArcInfo, the process
of removing vertices from arcs according to a specified tolerance.
- The process of identifying
the coordinates of a location given its address. For example, an address
can be matched against a TIGER street network to determine the location
of a home. Also referred to as address geocoding.
- geographic data
- The locations and descriptions
of geographic features. The composite of spatial data and descriptive
- geographic database
- A collection of spatial
data and related descriptive data organized for efficient storage
and retrieval by many users.
- geographic data set
- One of seven geographic
data types supported by ArcInfo. Geographic data sets in ArcInfo include
coverages, grids, DBMS tables, tins, images, lattices, and CAD drawings.
- A user-defined geographic
phenomenon that can be modeled or represented using geographic data
sets in ArcInfo. Examples of geographic features include streets,
sewer lines, manhole covers, accidents, lot lines, and parcels.
- geographic information
- See GIS.
- Geometry deals with the
measures and properties of points, lines and surfaces. In ArcInfo,
geometry is used to represent the spatial component of geographic
- To establish the relationship
between page coordinates on a planar map and known real-world coordinates.
- georelational model
- A geographic data model
that represents geographic features as an interrelated set of spatial
and descriptive data. The georelational model is the fundamental data
model used in ArcInfo.
- Geographic Information
Retrieval and Analysis data files from the U.S. Geological Survey.
GIRAS files contain land use/land cover information for areas in the
United States, including attributes for land use, land cover, political
units, hydrologic units, census and county subdivisions, federal landownership
and state landownership. These data sets are available to the public
in both map and digital form.
- Geographic information
system. An organized collection of computer hardware, software, geographic
data, and personnel designed to efficiently capture, store, update,
manipulate, analyze, and display all forms of geographically referenced
- Government Open System
Interconnection Protocols are U.S. government procurement specifications
for OSI protocols (see OSI).
The government has mandated that all federal agencies standardize
on the OSI model and implement OSI-based systems for GOSIP. Most vendors
(Sun, IBM, HP, DEC, etc.) have either complied or are working toward
- A system of satellites
and receiving devices used to compute positions on the Earth. GPS
is used in navigation, and its precision supports cadastral surveying.
- See global
- A parameter controlling
the number of vertices and the distance between them on arcs representing
curves. The smaller the grain tolerance, the closer vertices can be.
tolerance, grain tolerance can affect the shape of curves.
user interface (GUI)
- A graphical method of
controlling how a user interacts with a computer to perform various
tasks. Instead of issuing commands at a prompt, the user performs
desired tasks by using a mouse to choose from 'a dashboard' of options
presented on the display screen. These are in the form of pictorial
buttons (icons) and lists. Some GUI tools are dynamic and the user
must manipulate a graphical object on the screen to invoke a function;
for example, moving a slider bar to set a parameter value (e.g., setting
the scale of a map).
- graphics display terminal
- A computer terminal used
to view and manipulate graphic information. It can also be used for
graphic selection (e.g., identifying a feature on the display), digitizing
- That area on the graphics
display device reserved for map display, or simulating the plotter
page area. Page units are typically in centimeters or inches instead
of real-world coordinates such as meters or feet. Maps are composed
on the graphics page.
- Geographical Resource
Analysis Support System. A public-domain raster GIS modeling product
of the U.S. Army Corp of Engineers' Construction Engineering Research
- gravity model
- A methodology used in
the geography, engineering and social sciences to model the behavior
of populations. The underlying assumption of the gravity model is
that the influence of populations on one another is inversely proportional
to the distance between them. This approach is analogous to the view
of gravitational attraction from Newtonian physics.
- A fully integrated grid
(cell-based) geoprocessing system for use with ArcInfo. GRID supports
a Map Algebra spatial language that allows sophisticated spatial modeling
- A geographic data model
representing information as an array of equally sized square cells
arranged in rows and columns. Each grid cell is referenced by its
geographic x,y location. See also raster
and grid cell.
- A discretely uniform unit
that represents a portion of the Earth, such as a square meter or
square mile. Each grid cell has a value that corresponds to the feature
or characteristic at that site, such as a soil type, census tract,
or vegetation class. Additional values of the cell can be stored in
a value attribute
- See graphical
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- The physical components
of a computer system-the computer, plotters, printers, terminals,
digitizers, and so on.
- A computational method
that uses trial and error methods to approximate a solution for computationally
- historical view
- In ArcStorm, a snapshot
of the state of a given data source at a given time. In an historical
view, the database is not modified, and no data is created locally,
it is simply a 'read-only' view of the past.
- A mechanism in ArcStorm
to enable the tracking of changes made to a data source. This enables
the creation of historical views and supports 'rolling back' the data
to a previous period in time.
- A transportation hub is
a node in a network that can be used to channel goods from origins
to destinations. Hubs are used at strategic locations in a network
to reduce transportation costs.
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Glossary pages: A-D