GIS Definitions: M-P
Definitions of some commonly used GIS (Geographic Information System) terms.
M N O
P Full glossary:
- A text file containing
a sequence of commands that can be executed as one command. Macros
can be built to perform frequently used, as well as complex, operations.
The ARC Macro Language (AML)
is used to create macros for ArcInfo.
- many-to-one relate
- A relate in which many
records in one table are related to a single record in another table.
- An abstract representation
of the physical features of a portion of the Earth's surface graphically
displayed on a planar surface. Maps display signs, symbols, and spatial
relationships among the features. They typically emphasize, generalize,
and omit certain features from the display to meet design objectives
(e.g., railroad features might be included in a transportation map
but omitted from a highway map).
- map extent
- 1. The rectangular limits
(xmin,ymin and xmax,ymax) of the area of the Earth's surface displayed
using ArcInfo. Map extent is specified in the coordinate system of
the coverage or other geographic data set used. Typically, the extent
of the geographic database (or a portion of it defined by a zoomed-in
view) defines the map extent for display.
2. The geographic extent of a geographic data set specified by
the minimum bounding rectangle (i.e., xmin,ymin and xmax,ymax).
- An organized, uniformly
defined collection of spatial data partitioned by layers and tiles
into component parts called map
sections. A map library organizes geographic data spatially as
a set of tiles and thematically as a set of layers. The data in a
map library are indexed by location for optimal spatial access. A
map library organizes coverages spatially by tiles and thematically
- map limits
- The rectangular area on
the graphics page in which geographic features are displayed. All
geographic data are drawn within the map limits, and none outside
the map limits. Map titles and legends can be drawn outside the map
- A mathematical model that
transforms the locations of features on the Earth's surface to locations
on a two-dimensional surface. Because the Earth is three-dimensional,
some method must be used to depict a map in two dimensions. Some projections
preserve shape; others preserve accuracy of area, distance, or direction.
See also coordinate
Map projections project the Earth's surface onto a flat plane. However,
any such representation distorts some parameter of the Earth's surface
be it distance, area, shape, or direction.
- map query
- The process of selecting
information from a GIS by asking spatial or logical questions of the
geographic data. Spatial query is the process of selecting features
based on location or spatial relationship (e.g., select all features
within 300 feet of another; point at a set of features to select them).
Logical query is the process of selecting features whose attributes
meet specific logical criteria (e.g., select all polygons whose value
for AREA is greater than 10,000 or select all streets whose name is
'Main St.'). Once selected, additional operations can be performed,
such as drawing them, listing their attributes or summarizing attribute
- map scale
- The reduction needed to
display a representation of the Earth's surface on a map. A statement
of a measure on the map and the equivalent measure on the Earth's
surface, often expressed as a representative fraction of distance,
such as 1:24,000 (one unit of distance on the map represents 24,000
of the same units of distance on the Earth). Map scale can also be
expressed as a statement of equivalence using different units; for
example, 1 inch = 1 mile or 1 inch = 2,000 feet.
- The unit of data storage
in a map library. A map section is the data for one layer in one tile
of a map library. Map sections are implemented as ArcInfo coverages.
See also map library, layer
- map-to-page transformation
- The process of positioning
and scaling a map on a graphic page. It controls how coverage coordinates
are transformed into graphics on the display screen or plotter page.
(Coverages are not maps; they contain the unscaled coordinates that
ArcInfo uses to draw maps.)
- map units
- The coordinate units in
which a geographic data set (e.g., a coverage) is stored in ArcInfo.
Map units can be inches, centimeters, feet, meters, or decimal degrees.
- A symbol used to represent
a point location such as an airport.
- mass point
- Irregularly distributed
sample points, each with an x,y location and a z value, which are
used as the basic elements to build a tin.
Each mass point has important, yet equal, significance in terms defining
the tin surface. Ideally, the location of each mass point is intelligently
chosen to capture important variations in the surface's morphology.
- Multiple Document Interface
was developed by Microsoft, with menus, buttons, tools, and windows
called documents. ArcView is based on the MDI standard, including
multiple document types: Project View, Table, Layout, Chart, and Scripts.
Documents can be organized and manipulated in a variety of standard
ways: tiled, cascaded, iconified, resized, or closed.
- A line running vertically
from the north pole to the south pole along which all locations have
the same longitude. The Prime Meridian (0) runs through Greenwich,
England. From the Prime Meridian, measures of longitude are negative
to the west and positive to the east up to 180, halfway around the
- mesh point
- One sample point in the
array of sample points in a lattice.
Each mesh point is located at a constant sampling interval in the
x and y directions relative to a common origin, and contains the z
value for the surface at that location. Mesh points outside the surface,
or which represent holes in the surface, are assigned a null
- minimum bounding rectangle
- A rectangle, oriented
to the x and y axes, which bounds a geographic feature
or a geographic data set. It is specified by two coordinates: xmin,ymin
and xmax,ymax. For example, the BND defines a minimum
bounding rectangle for a coverage.
- minimum mapping units
- For a given map scale,
the size or dimension below which a long narrow feature is represented
as a line and a small area as a point. For example, streams and rivers
will be represented as lines if their width is less than .10 inch,
and polygons smaller than .125 inch on a side will be represented
as a point.
- A representation of reality
used to simulate a process, understand a situation, predict an outcome,
or analyze a problem. A model is structured as a set of rules and
procedures, including spatial modeling tools available in a geographic
information system (GIS). See also spatial
- See model.
- The moment is the time
when all the tasks associated with a transaction
have completed. A feature's creation, deletion or update date is set
to the transaction's moment.
- The form and structure
of a surface.
In tins, the morphology of a surface is defined by the sample points
features used to build the tin. Breaklines, when properly located
at locations of significant change in surface behavior, play a major
role in defining surface morphology. In lattices, the morphology of
a surface cannot be directly represented by sample points and linear
features; it must be implied from the mesh point z values.
- Software used to access
resources on the Internet. The Mosaic project has been conducted by
the National Center for Supercomputing Applications (NCSA) in Illinois
to provide a single front-end or user interface to many information
services. The goal is to present the query to, and results from, each
of these different information resources in a similar way to minimize
the number of "systems" one must be familiar with to successfully
navigate the Internet. Mosaic relies on the existence of servers;
it does not serve information of its own. Mosaic client software exists
for X Windows, Macintosh, and Microsoft Windows. Computers must be
connected to the Internet to use Mosaic.
- A hand-controlled hardware
device for interacting with a computer terminal or entering data from
a digitizer. A mouse is used to make selections and position the cursor
to fields in computer forms when interacting with graphical user interfaces.
A digitizer mouse is used to trace features and enter x,y coordinates
- Multispectral scanner.
An instrument on some satellites used for imaging the earth. An MSS
image will have data recorded by the scanner from three or more bands
of the electromagnetic spectrum. ArcInfo can read multispectal images
in various formats.
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- Arc attribute table. A
table containing attributes for node coverage features. For each node,
the NAT contains a reference to one of the arcs it connects to, an
internal node sequence number and node feature identifier. See also
- native mode usage
- A statement syntax entered
in ArcInfo in the language of an external system. Inclusion of an
SQL WHERE clause in an ArcInfo native mode SQL selection operation,
or the declaration of a DBMSCURSOR are examples of native mode SQL
- National Bureau of Standards,
now known as NIST.
- A border line commonly
drawn around the extent of a map.
- 1. An interconnected set
of arcs representing possible paths for the movement of resources
from one location to another.
2. A coverage representing linear features containing arcs or a route-system.
Also known as network coverage.
3. When referring to computer hardware systems, a local area network
or a wide area network (WAN).
- The ArcInfo software product
that performs address matching/ geocoding, allocation, routing, and
pathfinding across linear networks.
- In ArcInfo, a line coverage
on which network tools such as PATH and ALLOCATE can operate.
- network element
- The components of a network
in ArcInfo, including network
links and network nodes,
of which there are three types: stops,
- Network links are interconnected
linear entities which represent the conduits for transportation (e.g.,
vehicles, fluids, electricity) and communication networks, for example,
highways and electrical transmission lines. In ArcInfo, links are
represented as arcs with attributes stored in the AAT.
- Network nodes are the
endpoints and connecting points of network links, for example, intersections
and interchanges of a road network, the confluence of streams in a
hydrologic network, or switches in a power grid. In ArcInfo, network
nodes are used to model stops, centers, and turns. Network nodes are
represented as nodes, with attributes stored in an NAT.
- networking protocols
- A networking protocol
is software that provides a communication gateway (link) allowing
the exchange of data between various networking systems. Protocols
are a fixed set of rules used to specify the format of an exchange
- The Network File System
(NFS) protocol allows a given computer to access a disk on another
computer over a network in a transparent fashion. The hard disk can
be accessed just as easily as if it were local to the user's machine.
To get access to a disk across a network, the disk must be NFS mounted
on the user's local machine. See your system administrator for such
- National Institute of
Standards & Technology is the agency that produces the Federal
Information Processing Standards (FIPS) for all U.S.A. government
agencies except the Department of Defense.
- National map accuracy
standards are specifications of accuracy standards for well-defined
map points on published maps that are specified by the U.S. Geological
Survey and revised by the U.S. Bureau of the Budget.
- 1. The beginning and ending
locations of an arc. A node is topologically linked to all arcs that
meet at the node. See also network
2. In graph theory, the location at which three or more lines connect.
3. The three corner points of each triangle in a tin. Every sample
point input to a tin becomes a node in the triangulation. A triangle
node is topologically linked to all triangles that meet at the node.
- node match tolerance
- The minimum radial distance
within which two nodes will be joined (matched) to form one node.
- A conceptual database
design task that involves applying data dependency to a data model
to avoid data inconsistencies by prohibiting redundancy.
- National Transfer Format
(British Standard BS 7567). NTF is an exchange format that permits
the transfer of vector data with five different levels of complexity.
NTF is the format used by the British Ordinance Survey.
- null value
- The absence of a value.
If a particular column of a row in a table is null, that means there
is no value stored. Null is not the same as blank or zero.
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- Optical Character Recognition
is the automatic recognition and interpretation of text.
- Open Database Communication.
A standard API (application program interface) used to communicate
with database management systems, developed by Microsoft, and incorporated
in ArcView Version 2. ArcView supports ODBC for DBMSs on the Microsoft
- The Open GIS Consortium,
a group composed of software vendors, academics, government agencies,
consultants and software integrators, dedicated to open systems geoprocessing.
Their first project is to develop an open geodata interoperability
- The Open Geodata Interoperability
Specification being developed by OGC to support interoperability of
GIS systems in a heterogeneous computing environment.
- Object Linking and Embedding
developed by Microsoft. Allows objects from one application to be
embedded within another (e.g., taking an Excel spreadsheet and
putting it into a Word document). ArcView Version 2 does not support
the still evolving OLE standard. Instead, it supports DDE (see DDE).
OLE support is planned for future releases of ArcView.
- The Object Management
Group is a computing industry collaboration to promote object-oriented
interoperability among heterogeneous computing environments. They
continue to develop specifications which address the many aspects
of this problem, the most popular of which is the Common Object Request
Broker Architecture (CORBA).
- A relate in which one
record in a table is related to many records in another table.
- OPEN LOOK
- A graphical user interface
(GUI) for the X Window system developed by AT&T (Open Look) and
Sun Microsystems (OPEN LOOK). (See also OSF/Motif).
- online access
- Direct access to data
that does not involve file transfer.
- optical disk
- A digital data storage
technology that uses optical media to store information. Optical disks
are slower, but store more data and cost less per unit of stored data
than magnetic disks. Several optical platters can be installed in
a single device called a jukebox. Optical disks are used when very
large amounts of data need to be stored.
- operating system
- Computer software designed
to allow communication between the computer and the user. The operating
system controls the flow of data, the application of other programs,
the organization and management of files, and the display of information.
- A relational database
management system to which ArcInfo has access through the DATABASE
- See operating
- The Open Software Foundation
is an international consortium that promotes the standardization of
the UNIX operating system.
- An industry-standard graphical
user interface developed by the Open Software Foundation for the UNIX
- Open Systems Interconnect,
a seven-layer hierarchical reference interface and communications
model sponsored by ISO, 1984, known as the OSI Reference Model: layer
7--applications, layer 6--presentations, 5--session, 4--transport,
3--network, 2--data link, 1--physical. This model is incorporated
at the operating system level. The OSI model is used to develop interfaces
and integrate two dissimilar systems (i.e., PCs and UNIX or UNIX and
- 1. The reference location
for a planar coordinate system, usually represented by the values
2. The place where a trip starts. This is usually the home for most
consumers. For a population group, an origin could be a census tract
or a city. Origins are represented as nodes in a network coverage,
as points in a point coverage, and as label points in a polygon coverage.
- See topological
- That portion of an arc
digitized past its intersection with another arc. See also dangling
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- page extent
- Defines a rectangular
portion of the graphics
page to be displayed.
- To move the viewing window
up, down, or sideways to display areas in a geographic data set which,
at the current viewing scale, lie outside the viewing window. See
- 1. A property of two or
more lines that is separated at all points by the same distance.
2. A horizontal line encircling the Earth at a constant latitude.
The Equator is a parallel whose latitude is 0. Measures of latitude
range from 0 to 90 north of the Equator and from 0 to -90 to the south.
- Point attribute table
or polygon attribute table. A coverage can have either a point attribute
table or a polygon attribute table, but not both. In addition to user-defined
attributes, a PAT contains data on area and perimeter of a polygon
(values are 0 for points), an internal sequence number and feature
The PAT is also used for regions. The same attributes are maintained,
however, the name of the attribute table is PAT<REGION>, where
<REGION> is the name of the region for which attributes are
stored. One polygon and many region attribute tables can be stored
in the same coverage. See feature
- An ordered set of network
links and network nodes
which connects an origin to
a destination (center).
- The process of finding
a path between an origin and destination, which usually involves determining
a least-cost path.
- The path to a file or
directory located on a disk. Pathnames are always specific to the
computer operating system.
- Paul Revere tour
- A traveling salesman tour
in which the start is different from the ending stop. The name is
derived from American history, after the famous ride of Paul Revere.
- A point around which all
slopes are negative (i.e., downward).
- peripheral device
- A hardware device not
part of the central computer (e.g., digitizers, plotters, and printers).
- A long-term database
lock required when users wish to maintain a consistent view of
their data while doing modifications over a long
- A measure of data size.
One petabyte is equivalent to 1,000 terabytes.
- A point around which all
slopes are positive (i.e., upward).
- A contraction of the words
picture element. The smallest unit of information in an image or raster
map. Referred to as a cell in an image or grid.
- 1. A single x,y coordinate
that represents a geographic feature too small to be displayed as
a line or area; for example, the location of a mountain peak or a
building location on a small-scale map.
2. A coverage feature class used to represent point features or to
identify polygons. It is not possible to have point and polygon features
in the same coverage. When representing point features, the x,y location
of the label point describes the location of the feature. When identifying
polygons, the label point can be located anywhere within the polygon.
Attributes for points are stored in a PAT.
- A topological
overlay procedure which determines the spatial coincidence of
points and polygons. Points are assigned the attributes of the polygons
within which they fall.
- point event
- See event.
- A coverage feature class
used to represent areas.
A polygon is defined by the arcs that make up its boundary and a point
inside its boundary for identification. Polygons have attributes (PAT)
that describe the geographic feature they represent.
- The topological data structure
ArcInfo uses to represent connectivity between arcs to form polygons.
Polygon-arc topology supports the definition of polygons and analysis
functions such as topological
overlay. See also topology.
- polygon overlay
- A topological
overlay procedure which determines the spatial coincidence of
two sets of polygon features and creates a new set of polygons based
- POSIX character set
- POSIX character sets support
languages which only require 7-bit characters (such as US English).
See also extended character
- POSIX 1003.1A
- A revision to the POSIX
1003.1 standard that defines a set of standard operating system interfaces
and an environment for application programs written in C.
- POSIX 1003.4a
- Defines thread support
to facilitate writing multitasking operations, particularly server
- PostScript is a page-description
computer language developed, marketed, and trademarked by Adobe Systems,
Inc. PostScript is supported on most LaserWriter printers. PostScript
is particularly useful in computerized typesetting applications and
desktop publishing with graphics. PostScript files can be plotted
on non-PostScript plotting devices by means of Raster Image Processor
- Refers to the number of
significant digits used to store numbers, and in particular, coordinate
values. Precision is important for accurate feature representation,
analysis and mapping. ArcInfo supports single
precision and double
- preliminary topology
- Refers to incomplete region
topology. Region topology defines region-arc and region-polygon relationships.
A topological region has both the region-arc relationship and the
region-polygon relationship. A preliminary region has the region-arc
relationship but not the region-polygon relationship. In other words,
preliminary regions have no polygon topology.
- primary key
- One or more attributes
whose values uniquely identify a row in a database table. See also
- The property of an origin
which produces a trip for a particular activity. This is usually a
function of the population at the origin. For example, a household
production for two adults and one child might be one trip per week
for groceries, two trips per day for work, three trips per six months
to see a dentist, and so on. Interactions between an origin and a
center are estimated bases on how much of an origin's production will
be attracted to a particular center.
- A vertical sectional view
of a surface
derived by sampling surface values along a section
- See map
- projection file
- 1. A coverage file that
stores the parameters for the map projection and coordinate system
of a geographic data set (e.g., a coverage).
2. A text file containing input and output projection parameters that
can be used to convert a geographic data file from one coordinate
system to another.
- The minimum distance in
ground units separating all point locations on the horizontal plane.
If two or more points are found within the proximal tolerance distance
of each other, only the first point read is passed for further processing.
- pseudo node
- A node where two, and
only two, arcs intersect, or a single arc that connects with itself.
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Glossary pages: A-D