GIS Definitions: M-P

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

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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).

map library
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 by layer.
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 limits.
map projection
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 system.
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 values.
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.
map section
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 and tile.
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.
marker symbol
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 globe.
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 value.
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 modeling, data model, analysis and spatial analysis.
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 and breakline 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 of features.
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 feature attribute table.
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 usage.
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 (LAN) or a wide area network (WAN).
The ArcInfo software product that performs address matching/ geocoding, allocation, routing, and pathfinding across linear networks.
network coverage
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, centers, and turns.
network link
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 node
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 of data.
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 operations.
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 node.
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 Windows platform.
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 specification (OGIS).
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.
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 INTEGRATOR.
See operating system.
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 workstation environment.
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 mainframes).
1. The reference location for a planar coordinate system, usually represented by the values 0,0.
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 overlay.
That portion of an arc digitized past its intersection with another arc. See also dangling arc.

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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 also zoom.
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 identifier.
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 attribute table.
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).
persistent lock
A long-term database lock required when users wish to maintain a consistent view of their data while doing modifications over a long transaction.
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.
polygon-arc topology
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 on identity, intersect, or union.
POSIX character set
POSIX character sets support languages which only require 7-bit characters (such as US English). See also extended character set.
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 applications.
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 (RIP) software.
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 precision.
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 foreign key.
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 line.
See map projection.
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.
proximal tolerance
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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