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Glossary

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Accuracy

1. If applied to paper maps or map databases, degree of conformity with a standard if accepted value. Accuracy relates to the quality of a result and is distinguished from precision.
2. If applied to data collection devices such as digitizers, degree of obtaining the correct value.

Address matching

A mechanism for relating two files using address as the key item. Geographic coordinates and attributes subsequently can be transferred from one address to the other.

Analog Map

Any direct viewable map on which graphic symbols portray features and values: contrast with digital map.

Annotation

Text on a drawing map associated with identifying or explaining graphic entities shown.

Arc

See line.

Arc-node structure

The coordinate and topological data structure used in most vector GIS. Arcs represent lines that can define linear features or the boundary of areas or polygons. In arc-node structures, there is an implied direction to the line so that it may have a left and right side. In this way the area bounded by the arc can also be described, and it is not necessary to double-store coordinates for arcs that define a boundary between two ares.

Area

A closed figure (polygon) bounded by one or more lines enclosing a homogenous area and usually represented only in two dimensions. Examples are provinces, lakes, census tracts and forest stands.

ASCII

American Standard Code for Information Interchange, ASCII is 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 a computer system's text editor, are often referred to as ASCII files.

Aspect

A position facing a particular direction. Usually referring to in compass directions such as degrees or as cardinal directions (e.g., north, west).

Attribute

1. A numerical, text, or image field in a relational database table that describes a spatial feature such as a point, line, area, or cell.
2. A characteristic of a geographic feature described by numbers or characters, typically stored in tabular format, and linked to the feature by an identifier. For example, attributes of a well (represented by a point) might include depth, pump type, location and gallons per minute.

Automated mapping/facilites management (AM/FM)

A GIS technology focused on the specific segment facility information application and management, such as roads, pipes, and wires.

Axis

A reference line in a coordinate system.

Band: One layer of a multispectral image representing data values for a specific range of the electromagnetic spectrum of reflected light or heat. Also, other user-specified values derived by manipulation of original image bands. A standard color display of multispectral image displays 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.
Base map: A map showing planimetric, topographic, geological, political and/or cadastral information that may appear in many different types of maps. The base map information is drawn with other types of changing thematic information. Base map information may include major political boundaries, major hydrographic data, or major roads. The changing thematic information may be bus routes, population distribution or caribou migration routes.
Benchmark tests: Various standard tests, easily duplicated, for assisting in measurement of product performance under typical conditions of use.
Bit: The smallest unit of information that can be stored and processed in a computer. A bit has two possible values, 0, or 1, which can be interpreted as YES/NO, TRUE/FALSE, or ON/OFF.
Boolean expression: 1. A type of expression based upon, or reducible to, a true or false condition. 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, the use of AND in "DEPTH > 100 and GPM > 500."
2. Loosely, but erroneously, used to refer to logical expressions such as "DEPTH greater than 100."
Breakline: A line that defines and controls the surface behavior of a triangulated irregular network (TIN) in terms of smoothness and continuity. Physical examples of breaklines are ridge lines, streams, and lake shorelines.
Buffer: A zone of a given distance around a physical entity such as a point, line, or polygon.
Byte: A group of eight contiguous bits, that is a memory and data storage unit. For Example, file sizes are measured in bytes or megabytes (one million bytes). Bytes contain values of 0 to 255 and are most often used to represent integers or ASCII characters. A collection of bytes (most often 8 bytes) is used to represent real numbers and integers larger than 255.

CAD

See computer-aided design or drafting.

Cadastre

A record of interests in land, encompassing both the nature and extent of interests. Generally, this means maps and other descriptions of land parcels as well as the identification of who owns certain legal rights to the land (such as ownership, liens, easements, mortgages, and other legal interests). Cadastral information often includes other descriptive information about land parcels.

Cartesian coordinate system

A concept from a french philosopher and mathematician Rene Descartes (1596-1650). A system of two or three mutually perpendicular axes along which any point can be precisely located with reference to any other point often referred to as x, y, and z coordinates. Relative measure of distance, area, and direction are constant throughout the system.

Cell

The basic element of spatial information in a grid data set. Cells are always square. A group of cells form a grid.

Centroid

The "center of gravity" or mathematically exact center of a irregular shaped polygon; often given as x, y coordinate of a parcel of land.

Character

1. A letter, number, or special graphic symbol (*,@,-) treated as a single unit of data.
2. A data type referring to text columns in an attribute table (such as NAME).

Clip

The spatial extraction of physical entities from a GIS file that reside within the boundary of a polygon. The bounding polygon the works much like a cookie cutter.

Cluster

A spatial grouping of geographic entities on a map. When there are clustered on a map, there is usually some phenomenon causing a relationship among them (such as incidents of disease, crime, pollution, etc.).

COGO

See coordinate geometry.

Column

A vertical field in a relational database management system data file. It may store one to many bytes of information.

Command

An instruction, usually one word or concatenated words or letters that perform an action using the software. A command may also have extra options or parameters that define more specific applications of the action.

Computer-Aided Design or Drafting (CAD)

A computer program software package for creating graphic documents.

Computer-Aided Mapping (CAM)

The application of computer technology to automate the map compilation and drafting process.

Configuration

The physical arrangement and connections of a computer and its related peripheral devices. This can also pertain to many computers and peripherals.

Conflation A set of functions and procedures that aligns the arcs of one GIS file 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.

Conformality

Small areas on a map are represented in their true shape and angles are preserved, a characteristic of some map projections.

Connectivity

1. The ability to find a path or "trace" through a network from a source to a given point. For example, connectivity is necessary to find the path along a network of streets to find the shortest or best route from a fire station to a fire.
2. A topographical construct.

Contiguity

The topographical identification of adjacent polygons by recording the left and right polygons of each arc.

Continuous Data

Usually a reference to grid or raster data representing surface data such as elevation. In this instance, the data can be any value, positive or negative; sometimes referred to as real data. In contrast, see discrete data.

Contour

A line connecting points of equal value. Often in reference to a horizontal datum such as mean sea level.

Conversion

1. The translation of data from one format to another (e.g. TRIM to DXF; a map to digital files).

Coordinate

The position of a point in space with respect to a cartesian coordinate system (x, y, and/or z values). In GIS, a coordinate often represents locations on the earth's surface relative to other locations.

Coordinate Geometry (COGO)

A computerized surveying - plotting calculation methodology created at MIT in the 1950's. Positions are measured relative to a given base position by distance and azimuth, rather than an absolute coordinate.

Coordinate System

The system used to measure horizontal and vertical distance in a planimetric map. In a GIS, it is the system whose units and characteristics are defined by a map projection. A common coordinate system is used to spatially register geographic data for the same area. See Map Projection.erminous

Having the same or coincident boundaries. Two adjacent polygons are coterminous when they share the same boundary (such a street centerline dividing two block).

Curve Fitting

An automated mapping function that converts a series of short connected straight lines into smooth curves to represented entities that do not have precise mathematical definitions (such as rivers, shorelines, and contour lines).

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.

Data

A general term used to denote any or all facts, numbers, letters, and symbols that refer to or describe an object, idea, condition, situation, or other factors. These may be line graphics, imagery, and/or alphanumerics. It connotes basic elements of information that can be processed, stored, or produced by a computer.

Data Dictionary

A coded catalog of all data types, or a list of items giving data names and structures. May be on-line (referred to as an automated data dictionary), in which case the codes for the data types are carried in the database. Also referred to as DD/D for data dictionary/directory.

Data Integration

The combination of databases or data files from different functional units of an organization or from different organizations that collect information about the same entities (such as properties, census tracts, street segments). In combining the data, added intelligence is derived.

Data Model

1. A generalized, user-defined view of the data related to applications .
2. A formal method for arranging data to mimic the behavior of the real world entities they represent. Fully developed data models describe data types, integrity rules for the data types, and operations on the data types. Some data models are triangulated irregular networks, images, ad georelational or relational models for tabular data.

Database

Usually a computerized file or series of files or information, most, diagrams, listings, location records, abstracts, or references on a particular subject or subjects organized by data sets and governed by a scheme of organization. "Hierarchical" and "relational" define two popular structural schemes in use in a GIS. For example, a GIS database includes data about the spatial location and shape of geographic entities as well as their attributes.

Database Management System (DBMS)

1. The software for managing and manipulating the whole GIS including the graphic and tabular data.
2. Often used to describe the software for managing (e.g., input, verify, store, retrieve, query, and manipulate) the tabular information. Many GISs use a DBMS made by another software vendor, and the GIS interfaces with that software.

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 corresponding datum is the basis for a planar coordinate system.

Densify

A process of adding vertices to arcs at a given distance without altering the arc's shape. See spline for a different method for adding vertices.

Digital

Usually refers to data that is in computer-readable format.

Digital Elevation Model (DEM)

A digital representation of a topographic surface. Elevation values may be stored in a regular grid of values, one for each cell, a regular lattice of points or in a 'triangulated irregular network' (TIN) of points.

Digital Exchange Format (DXF)

1. ASCII text files defined by Autodesk, Inc. Originally used in CAD, now showing up in a third party GIS software.
2. An intermediate file format for exchanging data from one software package to another, neither of which has a direct translation for the other but where both can read and convert DXF data files into their format. This often saves time and preserves accuracy of the data by not reautomating the original.

Digital Line Graph (DLG)

1. In reference to data, the geographic and tabular data files obtained from the USGS that may include base categories such as transportation, hydrography, contours, and public land survey boundaries.
2. In reference to data format, the formal standards developed and published by the USGS for exchange of cartographic and associated tabular data files. Many non-DLG data may be formatted in DLG format.

Digital Map

A machine-readable representation of a geographic phenomenon stored for display or analysis by a digital computer; contrast with analog map.

Digitize

A means of converting or encoding map data that are represented in analogue form into digital information of x and y coordinates.

Digitizer

A device used to capture planar coordinate data, usually as x and y coordinates, from existing analog maps for digital use within a computerized program such as a GIS. Also called a digitizing table.

Discrete Data

Categorical data such as types of vegetation, or class data such as speed zones. In geographical terms, discrete data can be represented by polygons. Sometimes referred to as integer data. In contrast, see continuous data.

Dots Per Inch (DPI)

Often referred to in printing/plotting processes, it related to how sharply an image may be represented. More dots per inch implies that edges of images will be more precisely represented.

Double-Precision

Refers to a level of coordinate accuracy based on the possible number of significant digits that can be stored for each coordinate. Whereas single-precision coverages can store up to 7 significant digits for each coordinate and thus retain a precision of 1 metre in an extent of 1,000,000 metres, double precision coverages can store up to 15 significant digits per coordinate (typically 13-14 significant digits) and therefore retain the accuracy of much less than 1 metre at a global extent.

Eastings

The x-coordinates in a plane coordinate system; see northings.

Edge Match

An editing procedure to ensure that all features crossing adjacent map sheets have the same edge locations attribute descriptions, and feature classes.

Feature: A representation of a geographic entity, as a point, line, or polygon.
File: A single set of related information in a computer that can be accessed by a unique name (e.g., a text file created with a text editor, 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.
Flat File: A structure for storing data in a computer system in which each record in the file has the same data items, or fields. Usually, one field is designed as a "key" that is used by computer programs for locating a particular record or for sorting the entire file in particular order.
Font: A logical set of related patterns representing text characters or point symbology (e.g., A,B,C). A font pattern is the basic building block for markers and text symbols.
Foreign Key: In a relational database management system terms, the item or column of a data that is used to relate one file to another.
Format: 1. The pattern in which data are systematically arranged for use on a computer.
2. A file format is the specific design of how information is organized in the file.

Gap: The distance between two objects that should be connected. Often occurs during the digitizing process or in the edge-matching process.
Generalize: 1. Reduce the number of points, or vertices, used to represent a line.
2. Increase the cell size and resample data in a raster format GIS.
Geographic Data: The composite locations and descriptions of geographic entities.
Geographic Database: Efficiently stored and organized spatial data and possibly related descriptive data.
Geographic Information System (GIS): 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. Certain complex, time-consuming, or impractical otherwise.
Geographical Resource Analysis Support System (GRASS): 1. A public-domain raster GIS modeling product of the US army corps of Engineers Construction Engineering Research Laboratory.
2. A raster data format that can be used as an exchange format between two GIS's.
Georeference: To establish the relationship between page coordinates on a paper map or manuscript and known real-world coordinates.
Graduated Circle: A circular symbol whose area or some other dimension, represents a quantity.
Graphical User Interface (GUI): A graphical method used to control how a user interacts with a computer to perform various tasks. Instead of issuing commands at a prompt, the user is presented with a "dashboard" of graphical buttons and other functions in the form of icons and objects on the display screen. The user interacts with the system using a mouse to point and click.
Graticule: The designated grid of parallels and meridians on the earth and a map.
Grid Data: 1. One of many data structures commonly used to represent geographic entities. A raster based data structure composed of square cells of equal size arranged in columns and rows. The value of each cells, or group of cells, represents the entity value.
2. A set of regularly spaced reference lines on the earth's surface, a display screen, a map, or any other object.

Hardware: Components of a computer system, such as the CPU, terminals, plotters, digitizers, printers...
Hierarchical: This type of data storage refers to data linked together in a tree-like fashion, similar to the concept of family lines, where data relations can be traced through particular arms of the hierarchy. Knowledge about these data is dependant on the data structure.
Hierarchy: Refers to information that has order and priority

Image: A graphic representation or description of an object that is typically produced by an optical or electronic device. Common examples include remotely sensed data such as satellite data, scanned data, and photographs. An image is stored as a raster data set of binary or integer values representing the intensity of reflected light, heat, or another range of values on the electromagnetic spectrum. Remotely sensed images are digital representations of the earth.
Impedance: The amount of resistance (or cost) required to traverse through a portion of a network such as a line, or through one cell in a grid system. Resistance may be any number of factors defined by the user such as travel distance, time, speed of travel times the length, slope, or cost.
Integer: A number without a decimal. Integer values can be less than, equal to, or greater than zero.
Item: A field or column of information within an RDBMS

Jaggies: A jargon term for curved lines that have a stepped or saw-tooth appearance on a display device.
Join: To connect two or more separate geographic data sets.

Key: An Item or column within an RDBMS that contains a unique value for each record in the database.

Layer: A logical set of thematic data, usually organized by subject matter.
Library: A collection of repeatedly used items such as a symbol library-often used graphic objects shown on a map-or often used program subroutines.
Line: 1. set of ordered coordinates that represents the shape of a geographic entity too narrow to be displayed as an area (e.g., contours, roads, and streams). A digital line begins and ends with a node.
2. A line on a map (e.g., a neatline).
Local Area Network (LAN): Computer data communications technology that connects computers at the same site. Computers and terminals on a LAN can freely share data and peripheral devices, such as printers and plotters. LAN's are composed of cabling and special data communications hardware of software.

Macro: A set of instructions used by a computer program or programs. Macros are usually stored in a text file and invoked from a program that reads the text files as if the commands were typed interactively.
Many-To-One Relate: A relate in which many records can be related to a single record. A typical goal in relational database design is use many-to-one relates to reduce data storage and redundancy.
Map Projection: A mathematical model for converting locations on the earths surface from spherical to planar coordinates, allowing flat maps to depict three dimensional features. Some map projections preserve the integrity of shape; others preserve accuracy of area, distance, or direction.
Map Units: The coordinate units in which the geographic data are stored, such as inches, feet, or metres or degrees, minutes and seconds.
Meridian: A line running vertically from the north pole to the south pole long which all locations gave the same longitude. The prime meridian runs through Greenwich, England. Moving from the prime meridian, measures of longitude are negative to the west and positive to the east up to 180 degrees halfway around the globe. On maps they are graded in degrees E and W from the Prime Meridian.
Minimum Bounding Rectangle: The rectangle defined by the map extent of a geographic data set and specified by two coordinates: xmin, ymin, and xmanx, ymax.
Model: 1. An abstraction of reality. Models can include a combination of logical expressions, mathematical equations, and criteria that are applied or the purpose of simulating a process, predicting an outcome, or characterizing a phenomenon. The terms modeling and analysis are often used interchangeably, although the former is more limited in space.
2. Data representation of reality (e.g., spatial data models include the arc-node, georelational model, rasters or grids, and TINs).

Neatline: A border line commonly drawn around the extent of a map to enclose the map, legend, scale, title, and other information, keeping all the information pertaining to that map in one "neat" box.
Network: 1. system of interconnected elements through which resources can be passed or transmitted-for example, a street network with cars as the resource, or electric network with power as the resource.
2. In computer operations, the means by which computers connect and communicate with each other or with peripherals.
Network Analysis: The technique utilized in calculating and determining relationships and locations arranged in networks, such as in transportation, water and electrical distribution facilities.
Node: 1.  The beginning or ending location of a line.
2.  The location where lines connect.
3.  In graph theory, the location at which three or more lines connect.
4.  In computers, the point at which one computer attaches to a communications network.
Northings: The y-coordinates in a plane-coordinate system; see eastings.

Operating System (OS)

Computer software designed to allow communication between the computer and the user. For larger computers, it is usually supplied by the manufacturer. The operating system controls the flow of data, the interpretation of other programs, the organization and management of files, and the display of information. Commonly known operating systems are VMS, Windows 95, UNIX, DOS and OS/2.

Overshoot

That portion of a line digitized past its intersection with another line. Sometimes referred to as a dangling line.

Parallel: 1. A property of two or more lines that are 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 degrees. Digital measures of latitude are positive up to 90 degrees above the equator and negative below. On maps they are identified by degrees N or S from the equator
Pathname: The directions to a file or directory location on a disk. Pathnames are always specific to the computer operating system. Computer operating systems use directories and files to organize data. Directories are organized in a tree structure; each branch on the tree represents a subdirectory or file. Pathnames indicate locations in this hierarchy.
Peripheral: A component such as a digitizer, plotter, or printer that is not part of the central computer but is attached through communication cables.
Pixel: One picture element of a uniform raster or grid file. Often used synonymously with cell.
Plane-Coordinate System: A system for determining location in which two groups of straight lines intersect at right angles and have as a point of origin a selected perpendicular intersection.
Planimetric Map: A large-scale map with all features projected perpendicularly onto a horizontal datum place so that horizontal distances can be measured on the map with accuracy.
Point: 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. Some GIS systems also use a point to identify the interior of a polygon
Polygon: A vector representation of an enclosed region, described by a sequential list of vertices or mathematical functions.
Precision: 1.  If applied to paper maps or map databases, it means exactness and accuracy of definition and correctness of arrangement.
2.  If applied to data collection devices such as digitizers, it is the exactness of the determined value (i.e., the number 134.98988 is more precise than the number 134.9).
3.  The number of significant digits used to store numbers.
Primary Key: The central item or column within an RDBMS that contains a unique value for each record in the database, such as the unique number assigned to each parcel within a country.
Projection: See Map Projection

Quadtree: A spatial index that breaks a spatial dataset into homogenous cells of regularly decreasing size. Each decrement in size is 1/4 the area of the previous cell. The quadtree segmentation process continues until the entire map is partitioned. Quadtrees are often used for storing raster data.

Raster Data: Machine-readable data that represents values usually stored for maps or images and organized sequentially by rows and columns. Each "cell" must be rectangular but not necessarily square, as with grid data.
Record: In an attribute table, a single "row" of thematic descriptors.
Rectify: The process by which an image or grid is converted from image coordinates to real-world coordinates. Rectification typically involves rotation and scaling of grid cells, and thus requires resampling of values.
Relate: An operation establishing a connection between corresponding records in two tables using an item common to both. Each record in one table is connected to one or more records in the other table that share the same value for a common item.
Relational Database Management System (RDBMS): A database management system with the ability to access data organized in tabular files that may be related together by a common field (item). An RDBMS has the capability to recombine the data items from different files, thus providing powerful tools for data storage.
Relational Join: The process of connecting two tables of descriptive data by relating them by a key item, then merging the corresponding data. The common key item is not duplicated in this process.
Resolution: 1.  The accuracy at which the location and shape of map features can be depicted for a given map scale. For example, at a map scale of 1:63,360 (1 inch = 1 mile), it is difficult to represent areas smaller than 1/10 of a mile wide or 1/10 of a mile in length because they are only 1/10-invh wide or long on the map. In a larger scale map, there is less reduction, so feature resolution more closely matches real-world features. As map scale decreases, resolution also diminishes because feature boundaries must be smoothed, simplified, or not shown at all.
2.  The size of the smallest feature that can be represented in a surface.
3.  The number of points in an x and y in a grid.
Route: A process that establishes connections through a network or grid from a source to a destination. A network example would be to establish a route through a network of streets from a fire station to the fire. A grid example would be to move soil particles from a ridgetop to a stream based on equations developed by soil scientists. The determination of these routes usually take into consideration impedances.
Row: 1. A record in an attribute table
2. A horizontal group of cells in a grid or pixels in an image.
Rubber-Sheet: A procedure to adjust the entities of a geographic data set in a non-uniform manner. From and to coordinates are used to define the adjustment.

Scale: The relationship between a distance on a map and the corresponding distance on the earth. Often used in the form 1:24,000, which means that one unit of measurement on the map equals 24.000 of the same units on the earth's surface.
Scanning: Also referred to as automated digitizing or scan digitizing. A process by which information originally in hard copy format can be rapidly converted to digital raster form using optical readers.
Single-Precision: A lower level of coordinate accuracy based on the possible number of significant digits that can be stored for each coordinate. Single precision numbers can store up to seven significant digits for each coordinate and thus retain a precision of five meters in an extent of 1,000,000 meters. Double precision numbers can store up to 15 significant digits and therefore retain the accuracy of much less than one meter at a global extent.
Sliver Polygon: A relatively narrow feature commonly occurring along the borders of polygons following the overlay of two or more geographic data sets. Also occurs along map borders when two maps are joined, as a result of inaccuracies of the coordinates in either or both maps.
Smooth: A process to generalize data and remove smaller variations.
Software: A program that provides the instructions necessary for the hardware to operate correctly and to perform the desired functions. Some kinds of software are operating system, utility and applications.
Spatial Model: Analytical procedures applied with a GIS. There are three categories of spatial modeling functions that can be applied to geographic data objects within a GIS: (1) geometric models (such as calculation in Euclidian distance between objects, buffer generation, area, and perimeter calculation; (2) coincidence models; and (3) adjacency models. All three model categories support operations on geographic data objects such as points, lines, polygons, TIN's, and grids. Functions are organized in a sequence of steps to derive the desired information for analysis.
Spike: 1. An overshoot line created erroneously by a scanner and its raster software.
2. An anomalous data point that protrudes above or below an interpolated surface representing the distribution of the value of an attribute over an area.
Spline: A method to mathematically smooth spatial variation by adding vertices along a line. See densify for a slightly different method for adding vertices.
Structured Query Language: A syntax for defining and manipulating data from a relational database. Developed by IBM in the 1970's, it has since become an industry standard for query languages in most RDBMS's.
Surface: A representation of geographic information as a set of continuous data in which the map features are not spatially discrete; that is, there is an infinite set of values between any two locations. There are no clear or well defined breaks between possible values of the geographic feature. Surfaces can be represented by models built from regularly or irregularly spaced sample points on the surface.
Surface Model: Digital abstraction or approximation of a surface. Because a surface contains an infinite number of points, some subset of points must be used to represent the surface. Each model contains a formalized data structure, rules and x,y,z point measurements that can be used to represent a surface.
Syntax: A set of rules governing the way statements can be used in a computer language.

Table: 1. Usually referred to as a relational table. The data file in which the relational data reside.
2. A file that contains ASCII or other data.
Template: 1.  A geographic data set containing boundaries, such as land-water boundaries, for use as a starting place in automating other geographic data sets. Templates save time ad increase the precision of spatial overlays.
2.  A map containing neatlines, north arrow, logos, and similar map elements for a common map series, but lacking the central information that makes one map unique from another.
3.  An empty tabular data file containing only item definitions.
Thematic Map: A map that illustrates one subject or topic either quantitatively or qualitatively.
Theme: A collection of logically organized geographic objects defined by the user. Examples include streets, wells, soils, and streams.
Thiessen Polygons: Polygons whose boundaries define the area that is closest to each point relative to all other points. Thiessen polygons are generated from a set of points. They are mathematically defined by the perpendicular bisectors of the lines between all points. A triangulated irregular network structure is used to create Thiessen polygons.
Tile: A part of the database in a GIS representing a discrete part of the earths surface. By splitting a study area into tiles, considerable savings in access times and improvements in system performance can be achieved.
Topographic Map: A map of land-source features including drainage lines, roads, land marks, and usually relief or elevation.
Topology: The spatial relationships between connecting or adjacent coverage features (e.g., arc, nodes, polygons, and points). For example, the topology of an arc includes its from and to nodes and its left and right polygons. Topological relationships are built from simple elements into complex elements: points (simplest elements), arcs (sets of connected points), ares (sets of connected arcs), and routes (sets of sections that are arcs or portions of arcs). Redundant data (coordinates) are eliminated because an arc may represent a linear feature, part of the boundary of an area feature, or both. Topology is useful in GIS because many spatial modeling operations don't require coordinates, only topological information. For example, to find an optimal path between two points requires a list of which arcs connect to each other and the cost of traversing along each arc in each direction. Coordinates are only necessary to draw the path after it is cancelled.
Transformation: The process of converting data from one coordinate system to another through translation, rotation, and scaling.
Triangulated Irregular Network (TIN): A representation of a surface derived from irregularly spaced sample points and breakline features. The TIN data set includes topological relationships between points and their proximal triangles. Each sample point has an x,y coordinate and a surface or z value. These points are connected by edges to form a set of non-overlapping triangles that can be used to represent the surface. TIN's are also called irregular triangular mesh or irregular triangular surface models.

Undershoot: A digitized line that does not quite reach a line that it should intersect. As with an overshoot, this is also sometimes referred to as a dangling line.

Vector Data: A coordinate-based data structure commonly used to represent map features. Each linear feature is represented as a list of ordered x,y coordinates. Attributes are associated with the feature (as opposed to a raster data structure, which associates attributes with a grid cell). Traditional vector data structures include double-digitized polygons and arc-node models.
Vertex: One point along a line.

Z-value: The elevation value of a surface at a particular x, y location. May also be referred to as a spot height or spot elevation.
Zoom: To display a smaller or larger region instead of the present spatial data set extent in order to show greater or lesser detail.