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The GIS Glossary -- A-D
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
your browser to find a word.
You may also download this glossary as a PDF
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This page: A B C D
- AAT
- Arc attribute table. A table containing attributes for arc coverage features. In
addition to user-defined attributes, the AAT contains the from and to nodes, the left and
right polygons, the length, an internal sequence number and a feature identifier. See also
feature attribute table.
- ACCESS directory
- The system directory that LIBRARIAN uses to store the files that manage access to the
library. Each library has an ACCESS directory located in the library's DATABASE directory.
- accessibility
- An aggregate measure of how reachable locations are from a given location. The
ACCESSIBILITY command computes values for accessibility as a function of the distance
between locations and an empirically derived distance decay parameter.
- access rights
- The privileges accorded a user for reading, writing, deleting, updating and executing
files on a disk. Access rights are stated as 'no access', 'read only' and 'read/write'.
- ACODE file
- An INFO data file storing arc attributes for coverages created from TIGER, DIME, IGDS
and Etak files. ACODE stands for 'Arc CODE'. The ACODE file is related by Cover-ID to the
Arc Attribute Table (AAT) of the coverage.
- address matching
- A mechanism for relating two files using address as the relate item. Geographic
coordinates and attributes can be transferred from one address to the other. For example,
a data file containing student addresses can be matched to a street coverage that contains
addresses creating a point coverage of where the students live.
- ADS
- 1. Arc Digitizing System. A simple digitizing and editing system used to add arcs and
label points to a coverage.
2. A command at the Arc: prompt that starts an ADS session.
- allocation
- The process of assigning arcs in a network to the closest center until the maximum
impedance or resource capacity of the center is reached.
- AML
- ARC Macro Language. A high-level algorithmic language for generating end-user
applications. Features include the ability to create on-screen menus, use and assign
variables, control statement execution, and get and use map or page unit coordinates. AML
includes an extensive set of commands that can be used interactively or in AML programs
(macros) as well as commands that report on the status of ARC/INFO environment settings.
- analysis
- Analysis is the process of identifying a question or issue to be addressed, modeling the
issue, investigating model results, interpreting the results, and possibly making a
recommendation. See model and spatial
analysis.
- annotation
- 1. Descriptive text used to label coverage features. It is used for display, not for
analysis.
2. One of the feature classes in a coverage used to label other features.
Information stored for annotation includes a text string, the location at which it is
displayed, and a text symbol (color, font, size, etc.) for display. See also TAT.
- ANSI
- American National Standards Institute is a national coordinator of voluntary standards
activities, and an approval organization and clearinghouse for consensus standards in the
United States. ANSI works closely with international organizations, particularly ISO, for the development and approval of international standards.
While ANSI standards apply to every facet of today's world, their efforts in the area of
SQL and spatial extensions to SQL are of particular interest to the GIS community.
-
- Application program interface (API). An API is a set of system calls or routines for
application programs to access services from operating systems or other programs. An API
allows your program to work with other programs, possibly on other computers. API is
fundamental to client/server computing. ArcView provides this service to ARC/INFO users.
- arc
- 1. An ordered string of vertices (x,y coordinate pairs) that begin at one location and
end at another. Connecting the arc's vertices creates a line. The vertices at each
endpoint of an arc are called nodes.
2. A coverage feature class used to represent
linear features and polygon boundaries. One line feature can contain many arcs. Arcs are
topologically linked to nodes (arc-node topology) and to
polygons (polygon-arc topology). The descriptive
attributes of arcs are stored in the arc attribute table (AAT). See also node.
- arc-node topology
- The topological data structure ARC/INFO uses to represent connectivity between arcs and
nodes. Arc-node topology supports the definition of linear feature and polygon boundaries,
and supports analysis functions such as network tracing. See also topology.
- archive
- A preserved collection of historical information purged from an ArcStorm database.
- ArcStorm
- ArcStorm (ArcStorageManager) is a data storage facility and transaction manager for
ARC/INFO data. ArcStorm manages a feature-oriented database that can be closely integrated
with database systems supported by ARC/INFO's DATABASE INTEGRATOR.
- ArcStorm database
- An ArcStorm database is a collection of libraries, layers, INFO tables and external DBMS
tables. Data stored in an ArcStorm database benefit from the transaction management and
data archiving capabilities of ArcStorm.
- ArcTools
- ArcTools is a collection of ARC/INFO productivity tools implemented through an AML-based (ARC Macro Language) graphical user
interface. ArcTools provides a user-friendly approach to commonly used ARC/INFO
operations and functions.
- area
- 1. A homogeneous extent of the Earth bounded by one or more arc features (polygon) or represented as a set of polygons (region). Examples: states, counties, lakes, land-use areas, and
census tracts.
2. The size of a geographic feature measured in unit squares. ARC/INFO
stores an area measure for each polygon and region.
- ASCII
- American Standard Code for Information Interchange. A set of codes for representing
alphanumeric information (e.g., a byte with a value of 77 represents a capital M). Text
files, such as those created with the text editor of a computer system, are often referred
to as ASCII files.
- aspect
- The compass direction toward which a slope faces, measured in degrees from North in a
clockwise direction.
- ASRG
- ARC Standard Raster Graphic. Raster graphic data transformed to the Equal ARC-second
Raster Chart/Map (ARC) Projection System. See SRG. ASRG (using
RGB) is very similar to DMA (Defense Mapping Agency) ADRG (ARC Digitized Raster Graphic).
ADRG files can be imported into ARC/INFO with the ADRGGRID command. The ASRG (using RGB)
can be converted with the ADRGGRID command if it is in a true DMA ADRG format. The ASRG
permits color codes, which the ADRGGRID command does not handle.
- attractiveness
- The property or properties of a location that create an incentive for trips to be made
to that location. For example, the attractiveness of a retail store could be a function of
the retail floor space, number of parking spaces, product pricing, or a combination of
these factors.
- attribute
- 1. A characteristic of a geographic feature described by numbers, characters, images and
CAD drawings, typically stored in tabular format and linked to the feature by a
user-assigned identifier (e.g., the attributes of a well might include depth and gallons
per minute).
2. A column in a database table. See also item.
- attribute table
- An INFO or other tabular file containing rows and columns. In ARC/INFO, attribute tables
are associated with a class of geographic features, such as wells or roads. Each row
represents a geographic feature. Each column represents one attribute of a feature, with
the same column representing the same attribute in each row. See also feature attribute table.
- azimuth
- The horizontal direction of a vector, measured clockwise in degrees of rotation from the
positive y-axis, for example, degrees on a compass.
- backup
- A copy of a file, a set of files, or whole disk for safekeeping in case the original is
lost or damaged.
- band
- One layer of a multispectral image representing data values for a specific range of the
electromagnetic spectrum of reflected light or heat (e.g., ultraviolet, blue, green, red,
near-infrared, infrared, thermal, radar, etc.). Also, other user-specified values derived
by manipulation of original image bands. A standard color display of a multispectral image
shows three bands, one each for red, green and blue. Satellite imagery such as LANDSAT TM
and SPOT provide multispectral images of the Earth, some containing seven or more bands.
- band separate
- An image format that stores each band of data collected by multispectral satellite
scanning instruments in a separate file.
- bandwidth
- A measure of the volume of data that can flow through a communications link. Image data
tend to exist as large data sets; thus moving image data sets from one computer to another
requires high bandwidth or performance will be slowed. Also known as throughput.
- base map
- A map containing geographic features used for locational reference. Roads, for example,
are commonly found on base maps.
- base table
- A table that is physically stored in the database. Compare with view.
- baud rate
- A measure of the speed of data transmission between computer and other devices, measured
in bits per second.
- bit
- The smallest unit of information that a computer can store and process. A bit has two
possible values, 0 or 1, which can be interpreted as YES/NO, TRUE/FALSE, or ON/OFF. See
also byte.
- BLOB
- Binary large object. The data type of a column in an RDBMS table which can store large
image or textual data as attributes.
- BND
- The file in a coverage or grid which contains the coverage extent.
- Boolean expression
- 1. A type of expression that reduces to a true or false (logical) condition. A Boolean
expression contains logical expressions (e.g., DEPTH > 100) and Boolean operators. A
Boolean operator is a keyword that specifies how to combine simple logical expressions
into complex expressions. Boolean operators negate a predicate (NOT), specify a
combination of predicates (AND), or specify a list of alternative predicates (OR). For
example, DEPTH > 100 AND DIAMETER > 20. See also logical
selection.
- border arcs
- 1. The arcs that create the outer edge boundary of a polygon coverage.
2. In
LIBRARIAN, the tile boundary arcs that split a polygon coverage into tiles.
- breakline
- A linear feature that defines and controls the surface behavior of a tin in terms of smoothness and continuity. Breaklines are always
maintained as linear features in a tin. Stereo-digitized features containing x,y,z values
such as streams and shorelines containing an elevation attribute are often stored as
breakline features.
- buffer
- A zone of a specified distance around coverage features. Both constant- and
variable-width buffers can be generated for a set of coverage features based on each
feature's attribute values. The resulting buffer zones form polygons-areas that are either
inside or outside the specified buffer distance from each feature. Buffers are useful for
proximity analysis (e.g., find all stream segments within 300 feet of a proposed logging
area).
- bug
- An error in a computer program or in a piece of electronics that causes it to
malfunction.
- byte
- A memory and data storage unit composed of contiguous bits, usually
eight. For example, file sizes are measured in bytes or megabytes (one million bytes).
Bytes contain values of 0 to 255 and most often represent integer numbers or ASCII
characters (e.g., a byte with an ASCII value of 77 represents a
capital M). A collection of bytes (often 4 or 8 bytes) represents real numbers and
integers larger than 255.
- CAD
- Computer-aided design. An automated system for the design, drafting, and display of
graphically oriented information.
- CAD drawing
- The digital equivalent of a drawing, figure or schematic created using a CAD system. For
example, a drawing file or DWG file in AutoCAD.
- calibration
- The process of choosing attribute values and computational parameters so that a model
properly represents the real-world situation being analyzed. For example, in pathfinding
and allocation, calibration generally refers to assigning or calculating appropriate
values to be entered in impedance and demand items.
- capacity
- The maximum resource that can be assigned (allocated) to or be serviced by a center. For example, the capacity of a school is the number of students
that can be enrolled there.
- Cartesian coordinate system
- A two-dimensional, planar coordinate system in which x measures horizontal distance and
y measures vertical distance. Each point on the plane is defined by an x,y coordinate.
Relative measures of distance, area, and direction are constant throughout the Cartesian
coordinate plane.
- CASE
- Computer-Aided Software Engineering. CASE tools are defined programming rules for
applying engineering principles, methods, techniques, and concepts. These tools assist in
accomplishing a definable engineering task in the software design process by automating
manual activities through structured prototyping. This technique reduces customized
development time, supplying consistent code sets and supporting the entire software life
cycle process.
- CCITT
- Comité Consultatif Internationale de Télégraphique et Téléphonique
(Consultative Committee on International Telephone and Telegraph). CCITT is a technical
committee of the International Telecommunications Union, a United Nations organization in
Geneva. It sets international communications recommendations concerning standardization of
data interfaces, modems, and data networks. ARC/INFO is fully compliant with CCITT Group
IV, the Standard for raster data compression. ARC/INFO supports the following TIFF compression schemes: CCITT Group 4 for black-and-white data
(read only); CCITT Group 3, one-dimensional encoding for black-and-white data; and
PackBits.
- CD-ROM
- Compact Disk-Read Only Memory. CD-ROM is an optical media. A CD-ROM 5.25-inch disk can
hold about 650 megabytes of information. The ISO 9660 standard defines the format of data
held on CD-ROM.
- cell
- See grid cell.
- center
- A discrete location that has a supply of a resource or commodity. In spatial
interaction, a center is consider to have attractiveness.
- CGM
- Computer Graphics Metafile is a graphic image exchange standard, ANSI: x3.122-1986, ISO:
8632-1986, for graphic output file format. ARC/INFO, ArcView Version 2, and PC ARC/INFO
support CGM.
- character
- 1. A letter (e.g., a, b, c, or d), digit (e.g., 1, 2, or 3), or special graphic symbol
(e.g., *, |, or -) treated as a single unit of data.
2. A data type for an attribute
designating that values for the attribute will be represented using characters. For
example, the character data type would be appropriate for the attribute COUNTRY, if the
values assigned are like United States, Brazil, Canada, Thailand, and so on.
- checkin
- Checkin is the act of returning ArcStorm data which was previously checked out for
update purposes. When modified data is checked in, all locks on the data are released.
- checkout
- Checkout is the act of taking selected data out of an ArcStorm database into a local
coverage for editing purposes. When data is checked out, it is locked to prevent updates
from other users. ArcStorm data cannot be modified directly, it must first be checked out.
- client/server
- A software system is said to have a client/server architecture when there is a central
process (server) which accepts requests from multiple user processes (clients). ArcStorm
is one example of a client/server architecture within ARC/INFO.
- clip
- The spatial extraction of those features from one coverage that reside entirely within a
boundary defined by features in another coverage (called the clip coverage)-clipping works
much like a cookie cutter.
- COGO
- 1. Abbreviation of the term COordinate GeOmetry. Land surveyors use COGO functions to
enter survey data, to calculate precise locations and boundaries, to define curves, and so
on.
2. The name of the ARC/INFO coordinate geometry software product.
- column
- The vertical dimension of a table. A column has a name and a data type applied to all
values in the column.
- command
- A specific instruction to a computer program, issued by the user to perform a desired
action.
- command line interface
- A software product that allows the user to type in commands at a prompt. Contrast to forms interface.
- commit
- To make permanent any changes made during a database transaction. Compare with roll back.
- concurrency management
- A database management process for maintaining consistency of the data while supporting
simultaneous access by more than one user. A typical technique is to allow any number of
users read access but to allow only one user to have write access. A second user wanting
write access will have to wait until the first person completes their transaction.
- conditional operator
- A symbol or keyword specifying how to compare values. Conditional operators are used to
query a database. Examples from SQL include: = (equal to) BETWEEN < (LESS THAN) LIKE
> (greater than) CONTAINING
- conflation
- A set of functions and procedures that aligns the arcs of one coverage with those of
another and then transfers the attributes of one to the other. Alignment precedes the
transfer of attributes and is most commonly performed by rubber-sheeting operations.
- connectivity
- The topological identification of connected arcs by recording the from- and to-node for
each arc. Arcs that share a common node are connected. See also arc-node
topology.
- constraints
- Limits imposed on a model. For example, in an interaction model, specifying that the
number of trips generated from an origin to all destinations cannot exceed the origin's
production capacity.
- contiguity
- The topological identification of adjacent polygons by recording the left and right
polygons of each arc. See also polygon-arc topology.
- continuous data
- A surface for which each location has a specified or derivable value. Typically
represented by a tin or lattice (e.g., surface elevation).
- contour
- A line connecting points of equal surface value.
- contour interval
- The difference in surface values between contours.
- coordinate
- A set of numbers that designate location in a given reference system, such as x,y in a
planar coordinate system or an x,y,z in a three-dimensional coordinate system. Coordinates
represent locations on the Earth's surface relative to other locations. See also vector and Cartesian coordinate system.
- coordinate geometry
- See COGO.
- coordinate system
- A reference system used to measure horizontal and vertical distances on a planimetric
map. A coordinate system is usually defined by a map projection, a spheroid of reference,
a datum, one or more standard parallels, a central meridian, and possible shifts in the x-
and y-directions to locate x,y positions of point, line, and area features. In ARC/INFO, a
system with units and characteristics defined by a map projection. A common coordinate
system is used to spatially register geographic data for the same area.
- Cover#
- A unique sequence number automatically generated by ARC/INFO for each coverage feature.
This internal number is used to directly access features and to describe topological
relationships between coverage features. It is often referred to as the 'record number'.
- Cover-ID
- An integer identifier, assigned by the user, to relate geographic features and
corresponding attribute data. Cover-ID is an item found in feature
attribute tables, with 'Cover' replaced by the coverage name (e.g., for a soils
coverage, the Cover-ID would be SOILS-ID). Feature-ID and User-ID are synonymous terms to
Cover-ID.
- coverage
- 1. A digital version of a map forming the basic unit of vector data storage in ARC/INFO.
A coverage stores geographic features as primary features (such as arcs, nodes, polygons,
and label points) and secondary features (such as tics, map extent, links, and
annotation). Associated feature attribute tables describe and store attributes of the
geographic features.
2. A set of thematically associated data considered as a unit. A
coverage usually represents a single theme such as soils, streams, roads, or land use.
- coverage extent
- The coordinates defining the minimum bounding rectangle (i.e., xmin,ymin
and xmax,ymax) of a coverage or grid. All coordinates for the coverage or
grid fall within this boundary. In ARCPLOT and ARCEDIT, map extent is often set from the
coverage extent. See also BND.
- coverage units
- The units (e.g., feet, meters, inches) of the coordinate system in which a coverage is
stored.
- cross-tile indexing
- A method used to index features that cross tile boundaries. Features that cross tile
boundaries are stored as one or more features in each tile instead of as a single feature.
- CSSM
- The Content Standards for Spatial Metadata. A document produced by the Federal
Geographic Data Committee (FGDC) that describes spatial
metadata.
- cursor
- 1. A graphic pointer used with a mouse to point to a location on a terminal screen.
2.
An internal pointer to a record in a table which provides a mechanism for processing a
selected set of records. The cursor is moved one by one through the set while operations
such as display, query and update are performed.
- cycle
- 1. In pathfinding, a cycle is a path or tour beginning and ending at the same node.
2.
In tracing, a cycle is a set of arcs forming a closed polygon. Upstream and downstream
directionality are undefinable in a cycle.
- DAL
- Data Access Language. Apple's former standard to allow applications to communicate with
relational databases. DAL is middleware on a network. It is a program installed on the
database server to provide a common SQL access for all database servers on a network.
Apple has licensed this technology to Independence Technologies, Inc.
- dangle length
- Minimum length allowed for dangling arcs during the CLEAN process. CLEAN removes
dangling arcs that are shorter than the dangle length.
- dangling arc
- An arc having the same polygon on both its left and right sides and having at least one
node that does not connect to any other arc. It often identifies where a polygon does not
close properly (e.g., undershoot), where arcs don't connect properly, or where an arc was
digitized past its intersection with another arc (i.e., overshoot). A dangling arc is not
always an error. For example, dangling arcs can represent cul-de-sacs in street centerline
maps. See also dangling node.
- dangling node
- The endpoint of a dangling arc not connected to another arc.
- data access security
- Measures taken to control system users' ability to view or modify data. These measures
can include logical views of data and explicit access rights by group or individual users.
See also access rights.
- database
- A logical collection of interrelated information, managed and stored as a unit, usually
on some form of mass-storage system such as magnetic tape or disk. A GIS database includes
data about the spatial location and shape of geographic features recorded as points,
lines, areas, pixels, grid cells, or tins, as well as their attributes.
- database design
- The formal process of analyzing facts about the real world into a structured database
model. Database design is characterized by the following phases: requirement analysis,
logical design and physical design.
- DATABASE directory
- The same as the Library Reference workspace. It is the system directory that LIBRARIAN
uses to manage information about a map library. Each map library has one database
directory named DATABASE.
- data conversion
- The translation of data from one format to another. ARC/INFO supports data conversion
from many geographic data formats such as DLG, TIGER, DXF, and DEM.
- data dictionary
- A catalog of all data held in a database, or a list of items giving data names and
structures. Also referred to as DD/D for data dictionary/directory. Commercial RDBMSs have
online data dictionaries stored in special tables called system
tables.
- data integrity
- Maintenance of data values according to data model and data type. For example, to
maintain integrity, numeric columns will not accept alphabetic data. See referential integrity.
- data model
- 1. The result of the conceptual design process. A generalized, user-defined view of the
data related to applications.
2. A formal method of describing the behavior of the
real-world entities. A fully developed data model specifies entity classes, relationships
between entities, integrity rules and operations on the entities.
3. ARC/INFO coverages and grids use a georelational data model, a hybrid data model
that combines spatial data (in coverages or grids) and attribute data (in tables). Other
data models used in ARC/INFO include tins, images, and grid.
- data set
- A named collection of logically related data items arranged in a prescribed manner.
- data type
- The characteristic of columns and variables that defines what types of data values they
can store. Examples include character, floating point and integer.
- DATABASE INTEGRATOR (DBI)
- ARC/INFO software's link to relational database management systems (RDBMS). DBI enables
ARC/INFO users to access existing commercial databases and take advantage of the power and
capabilities of the RDBMS.
- database lock
- Locking is a mechanism by which database systems can prevent conflicting access to data
when multiple users are making requests to the data. See also persistent lock.
- database management system (DBMS)
- A set of computer programs for organizing the information in a database. A DBMS supports
the structuring of the database in a standard format and provides tools for data input,
verification, storage, retrieval, query, and manipulation.
- datum
- A set of parameters and control points used to accurately define the three-dimensional
shape of the Earth (e.g., as a spheroid). The datum is the basis for a planar coordinate
system. For example, the North American Datum for 1983 (NAD83) is the datum for map
projections and coordinates within the United States and throughout North America.
- DBI
- See DATABASE INTEGRATOR.
- DBMS
- See database management system.
- DBMS table
- See attribute table.
- DCW
- "Digital Chart of the World." The first 1:1,000,000-scale digital basemap of
the world. The DCW contains topologically based vector data digitized from the U.S.
Defense Mapping Agency's Operational Navigation Charts.
- DDE
- Dynamic Data Exchange. An IAC protocol developed by Microsoft for Windows-based
applications. DDE allows one application to send messages to, and get information from,
other applications in Windows. This is specific to Windows only (RPC, Remote Procedure
Calls, is used in the UNIX environment). DDE is supported in ArcView Version 2 for the
exchange of data with other business applications without having to convert the data or
leave ArcView. (See IAC.)
- DDL
- Data definition language. SQL statements that can be used either interactively or within
programming language source code to define databases and their components.
- DEM
- See digital elevation model.
- demand
- 1. In allocation, the potential for using a portion of the supply of a resource or
commodity.
2. In spatial interaction, demand is the measure of the need for a
particular type of service or goods that generates a trip to a destination. For example,
the demand for a gallon of milk may generate a trip to a grocery store.
- denormalization
- The process of restructuring a normalized data
model to accommodate operational constraints or system limitations.
- densify
- A process of adding vertices to an arc at specified distances, without altering the
arc's shape. Compare with spline and grain tolerance.
- descriptive data
- Tabular data describing the characteristics of geographic features. Can include numbers,
text, images, and CAD drawings about features. ARC/INFO stores descriptive data in feature attribute tables and in related tables. Also referred to
as attribute data.
- destination
- In spatial interaction, the location of the end of a trip. For example, a shop or an
office where a consumer or a worker is going. Destinations are represented as centers in a
network coverage, as points in a point coverage, and as label points in a polygon
coverage.
- digital elevation model
- 1. A digital representation of a continuous variable over a two- dimensional surface by
a regular array of z values referenced to a common datum. Digital elevation models are
typically used to represent terrain relief. Also referred to as 'digital terrain model'
(DTM).
2. An elevation database for elevation data by map sheet from the National
Mapping Division of the U.S. Geological Survey (USGS).
3. The format of the USGS digital elevation data sets.
- DGM
- Digital Geospatial Metadata. DGM was approved in June 1994 by the Federal Geographic
Data Committee (FGDC). DGM describes the specifications for
the content, quality, condition, and other characteristics of metadata (data about data).
The standard provides a common set of terminology and definitions for the documentation of
geospatial data. DGM establishes the names of data elements and groups of data elements to
be used for these purposes, definitions of these data elements and groups, and information
about the values that are to be provided for the data elements.
- DIGEST
- The Digital Geographic Information Exchange Standard is produced under authority of
NATO's Digital Geographic Information Working Group. DIGEST is a standard for digital
geographic information which will enable interoperability and compatibility among national
and multinational systems and users. DIGEST is composed of standards for two digital
geographic formats: ARC Standard Raster Graphic (ASRG) and vector relational format (VRF).
ASRG is very similar to ADRG and can be imported into ARC/INFO as an ADRG file with the
ADRGGRID command. The ARC/INFO VPFIMPORT and VPFEXPORT convertors will process the VRF
data.
- digital terrain model
- See digital elevation model.
- digitize
- 1. To encode geographic features in digital form as x,y coordinates.
2. The process
of using a digitizer to encode the locations of geographic features by converting their
map positions to a series of x,y coordinates stored in computer files. Pushing a digitizer
button records an x,y coordinate. A digitized line is created by recording a series of x,y
coordinates.
- digitizer
- 1. A device that consists of a table and a cursor with crosshairs and keys used to digitize geographic features.
2. Title of the person who uses a
digitizing device.
- digitizing
- See digitize.
- DIME
- See GBF/DIME.
- directed network
- A network in which each arc has an associated direction of flow. Direction of flow can
be determined by arc direction (e.g., each arc is digitized so that it is oriented
downstream), a value in an item in the AAT, or through the use of a selection file.
- directory
- A computer term identifying a location on a disk containing a set of data files and
other directories (subdirectories). Operating systems use directories to organize data.
The location of a directory is specified with a pathname.
- discrete data
- Geographic features containing boundaries: point, line or area boundaries.
- disk
- A storage medium consisting of a spinning disk coated with a magnetic material for
recording digital information.
- diskette
- An inexpensive, low-capacity storage medium, usually measuring 3.5 inches in diameter,
often referred to as a floppy disk.
- dissolve
- The process of removing boundaries between adjacent polygons that have the same values
for a specified attribute.
- distance-decay function
- In spatial interaction, the mathematical representation of the effect of distance on the
accessibility and number of interactions between locations. It can be either a power or an
exponential function.
- DLG
- 1. Digital Line Graph files from the U.S. Geological Survey (USGS), including data from
the base map categories such as transportation, hydrography, contours, and public land
survey boundaries.
2. The digital format standards published by USGS for exchanging
cartographic data files and in which the USGS delivers Digital Line Graph data sets.
- DML
- Data manipulation language. SQL statements that can be used either interactively or
within programming language source code to access and retrieve data stored in a database
management system.
- domain
- In a database, the set of allowed values for a table column, for example all positive
integers.
- double precision
- Refers to a high level of coordinate accuracy based on the possible number of
significant digits that can be stored for each coordinate. ARC/INFO data sets can be
stored in either single- or double-precision coordinates. Double-precision coverages store
up to 15 significant digits per coordinate (typically, 13 to 14 significant digits),
retaining the accuracy of much less than one meter at a global extent. See also single precision.
- downstream
- In tracing, downstream is the direction along the arcs that is the same as the direction
of flow. Direction of flow is determined by a user-defined convention. See also directed network.
- drape
- A perspective or panoramic rendering of two-dimensional features superimposed on a
surface.
- DTM
- Digital terrain model. See digital elevation model.
- DXF
- Data Exchange Format. A format for storing vector data in ASCII or binary files. Used by
AutoCAD and other CAD software for data interchange. DXF files are convertible to ARC/INFO
coverages.
- dynamic segmentation
- The process of computing the locations of events on linear features at run time based on
event tables for which distance measures are available. Route-system features and
event-handling commands provide the dynamic segmentation capability within ARC/INFO.
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