GIS Definitions: E-H

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

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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 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.
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 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.

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feature attribute table
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.
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.
Synonymous term for Cover-ID and User-ID.
feature selection by attribute
See logical selection.
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, DGM, and SDTS.
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 program.
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.
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.
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.
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 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.
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 grid number.
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 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 ArcInfo. Geographic data sets in ArcInfo 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 ArcInfo. Examples of geographic features include streets, sewer lines, manhole covers, accidents, lot lines, and parcels.

geographic information system
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 features.

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 information.

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.
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.

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.

A fully integrated grid (cell-based) geoprocessing system for use with ArcInfo. GRID supports a Map Algebra spatial language that allows sophisticated spatial modeling and analysis.

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).
See graphical user interface.

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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 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.
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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