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The GIS Glossary -- E-H
This glossary will help familiarize you with terms associated with GIS and ESRI
software. Select a letter to go to that point in the glossary, or use the find tool from
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This page: E F G H
- 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.
- edit
- 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.
- entity
- A collection of objects (persons, places, things) described by the same attributes.
Entities are identified during the conceptual design phase of database and application
design.
- entity relationship diagram
- 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.
- environment
- 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'.
- EOS
- 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
ARC/INFO 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.
- Ethernet
- 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).
- Equator
- The parallel of reference 0 north or south.
- event
- 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 ARC/INFO, 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 set
- Extended character sets support languages which require 8-bit characters or double-byte
characters, such as Chinese and French. Compare with POSIX
character set.
- 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 polygon.
- feature attribute table
- A table used to store attribute information for a specific coverage feature class.
ARC/INFO 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.
- FDDI
- 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 ARC/INFO 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 ARC/INFO applications
and data.
- feature class
- 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, and surface.
- Feature-ID
- Synonymous term for Cover-ID and User-ID.
- feature selection by attribute
- See logical selection.
- FGCC
- 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 ARC/INFO.
- FGDC
- 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, DGM, and SDTS.
- field
- 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 ARC/INFO GENERATE is often used to convert GPS coordinates while ARC/INFO COGO has
a FIELDDATA conversion program.
- file
- 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 disks.
- file transfer
- The process of copying data from one computer to another or one DBMS to another.
- FIPS
- 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.
- font
- A logical set of related patterns representing text characters or point symbols.
Courier, Helvetica and Times are three types of font.
- foreign key
- 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.
- format
- 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,
ARC/INFO has specific, proprietary formats used to store coverages. DLG, DEM, and TIGER
are geographic data sets with different file formats.
- forms interface
- A graphic user interface characterized by user-controlled movement of a cursor from one
data field to another. Contrast to command line interface.
- from-node
- 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.
- gazetteer
- 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
ARC/INFO 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 grid number.
- GBF/DIME
- 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.
- generalization
- In general, reducing the number of points used to represent a line. In ARC/INFO, the
process of removing vertices from arcs according to a specified tolerance.
- geocode
- 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 data.
- 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 ARC/INFO. Geographic data sets in
ARC/INFO include coverages, grids, DBMS tables, tins, images, lattices, and CAD drawings.
- geographic feature
- A user-defined geographic phenomenon that can be modeled or represented using geographic
data sets in ARC/INFO. Examples of geographic features include streets, sewer lines,
manhole covers, accidents, lot lines, and parcels.
- geographic information system
- See GIS.
- geometry
- Geometry deals with the measures and properties of points, lines and surfaces. In
ARC/INFO, geometry is used to represent the spatial component of geographic features.
- georeference
- 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 ARC/INFO.
- GIRAS
- 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.
- GIS
- 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 information.
- GOSIP
- 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 compliance.
- global positioning system
- 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.
- GPS
- See global positioning system.
- grain tolerance
- 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. Unlike densify tolerance, grain tolerance can affect the shape of
curves.
- graphical 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 and
editing.
- graphics page
- 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.
- GRASS
- Geographical Resource Analysis Support System. A public-domain raster GIS modeling
product of the U.S. Army Corp of Engineers' Construction Engineering Research Laboratory
(USACERL).
- 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.
- GRID
- A fully integrated grid (cell-based) geoprocessing system for use
with ARC/INFO. GRID supports a Map Algebra spatial language that allows sophisticated
spatial modeling and analysis.
- grid
- 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.
- 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
table (VAT).
- GUI
- See graphical user interface.
- hardware
- The physical components of a computer system-the computer, plotters, printers,
terminals, digitizers, and so on.
- heuristic
- A computational method that uses trial and error methods to approximate a solution for
computationally difficult problems.
- 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.
- history
- 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.
- hub
- 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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