GIS Glossary/E

GIS Glossary

0-9 | 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

early binding
A COM technique an application uses to access an object. In early binding, an object's properties and methods are discovered from an interface at compile time, instead of being checked at run time as in late binding. This difference often gives early binding performance benefits over late binding. There are two types of early binding: DispID binding and vTable binding.

earth-centered datum
A horizontal geodetic datum based on an ellipsoid that has its origin at the earth's center of mass. Examples are the World Geodetic System of 1984, the North American Datum of 1983, and the Geodetic Datum of Australia of 1994. The first uses the WGS84 ellipsoid; the latter two use the GRS80 ellipsoid. Geocentric datums are more compatible with satellite positioning systems, such as GPS, than are local datums.

easting
The distance east of the origin that a point in a Cartesian coordinate system lies, measured in that system's units.

eccentricity
A measure of how much an ellipse deviates from a circle, expressed as the ratio of the distance between the center and one focus of an ellipsoid to the length of its semimajor axis. The square of the eccentricity (e2) is commonly used with the semimajor axis a to define a spheroid.

ecliptic
The great circle formed by the intersection of the plane of the earth's orbit around the sun (or apparent orbit of the sun around the earth) and the celestial sphere.

ecological fallacy
The assumption that an individual from a specific group or area will exhibit a trait that is predominant in the group as a whole.

economic geography
The field of geography concerning the distribution and variation of economic factors by location, including how economic factors interact with geographic factors such as climate, land use, and geology.

edge
A line between two points that forms a boundary. In a geometric shape, an edge forms the boundary between two faces. In an image, edges separate areas of different tones or colors. In topology, an edge defines lines or polygon boundaries.

edge connectivity policy
In network datasets, a connectivity policy that defines how one edge may connect to another edge mid-span. There are two edge-edge connectivity policies: endpoint connectivity and any-vertex connectivity.

edge detection
A digital image processing technique for isolating edges in a digital image by examining it for abrupt changes in pixel value.

edge element
A line between two points that forms a boundary. In a geometric shape, an edge forms the boundary between two faces. In an image, edges separate areas of different tones or colors. In topology, an edge defines lines or polygon boundaries.

edge enhancement
A digital image processing technique for emphasizing the appearance of edges and lines in an image.

edge-edge rule
In geometric networks, a connectivity rule that defines how one edge may connect to another edge through a junction.

edge-junction cardinality
In connectivity relationships for networks, the number of edges of one type that may be associated with junctions of another type. Edge-junction cardinality defines a range of permissible connections that may occur in a one-to-many relationship between a single junction and many edges.

edge-junction rule
In geometric networks, a connectivity rule that defines how an edge may connect to a junction.

edgematching
A spatial adjustment process that aligns features along the edge of an extent to the corresponding features in an adjacent extent.

Edit Annotation tool
A tool on the Annotation toolbar that is used to manipulate geodatabase annotation. Text can be interactively moved, scaled, and rotated. Shortcut menu options allow control of the position, orientation, symbology, content, size, and style of text.

edit box
An entity that displays text entered by a user or derived from another source for editing purposes.

edit cache
A setting used in ArcMap that allows temporary storage of geodatabase or ArcIMS feature service features from a given map extent in the desktop computer's RAM, which may result in performance improvements in ArcMap for editing, feature rendering, and labeling.

edit mask
The portion of a coverage where the geometry (or geographical features) has been altered, but where topology has not yet been restored.

edit session
In ArcMap, the environment in which spatial and attribute editing take place. After starting an edit session, a user can modify feature locations, geometry, or attributes. Modifications are not saved unless the user explicitly chooses to save them.

edit sketch
In ArcMap, a temporary sketch that is used to perform a variety of tasks. Creating an edit sketch is a standard way to edit feature geometry.

edit task
During editing in ArcMap, a setting in the Task drop-down list that determines which operation the sketch construction tools will perform. Examples of edit tasks include creating new features and modifying existing features. The edit task is set by clicking a task in the Task drop-down list.

Editor toolbar
In ArcMap, a set of tools that allows the creation and modification of features and their attributes.

EDMS
Acronym for electronic document management system. A computer-based system for organizing, maintaining, and retrieving digital and hard-copy documents. An EDMS usually includes a check-in, check-out system for document tracking, versioning, and search-and-retrieval capabilities.

EDN
Acronym for ESRI Developer Network. A subscription program used to acquire ESRI software for developers, which includes a special program for supporting the ESRI developer community. EDN is intended to promote collaboration and interaction among GIS developers and ESRI staff.

EJB
The server-side component architecture for the J2EE platform. EJB enables development of distributed, transactional, secure, and portable Java applications.

elastic transformation
A procedure for adjusting the coordinates of all the data points in a dataset to allow a more accurate match between known locations and a few data points within the dataset. Rubber sheeting preserves the interconnectivity between points and objects through stretching, shrinking, or reorienting their interconnecting lines.

electromagnetic radiation
Energy that moves through space at the speed of light as different wavelengths of time-varying electric and magnetic fields. Types of electromagnetic radiation include gamma, x, ultraviolet, visible, infrared, microwave, and radio.

electromagnetic spectrum
The entire range of wavelengths (frequencies) over which electromagnetic radiation extends.

electronic atlas
A mapping system that displays but does not allow for the spatial analysis of data.

electronic document management system
A computer-based system for organizing, maintaining, and retrieving digital and hard-copy documents. An EDMS usually includes a check-in, check-out system for document tracking, versioning, and search-and-retrieval capabilities.

electronic navigational chart
A vector data product used for nautical navigation. ENC data is produced by nautical charting agencies throughout the world and uses the IHO (International Hydrographic Organization) S-57 standard for its database structure and attribution.

element
In geoprocessing in ArcGIS, a component of a model. Elements can be variables, such as input and derived data, or tools.

elevation
The vertical distance of a point or object above or below a reference surface or datum (generally mean sea level). Elevation generally refers to the vertical height of land.

elevation guide
A map element that displays a simplified representation of the terrain within a map's extent. Elevation guides are designed to provide a quick overview of topography, including the high and low points.

elevation layer
A layer in ArcGlobe that has been categorized to help define the geometry of the globe surface.

elevation tints
Hypsometric tint bands based on elevation ranges used in an elevation guide.

ellipse
A geometric shape described mathematically as the collection of points whose distances from two given points (the foci) add up to the same sum. An ellipse is shaped like a circle viewed obliquely.

ellipsoid
A three-dimensional, closed geometric shape, all planar sections of which are ellipses or circles. An ellipsoid has three independent axes, and is usually specified by the lengths a,b,c of the three semi-axes. If an ellipsoid is made by rotating an ellipse about one of its axes, then two axes of the ellipsoid are the same, and it is called an ellipsoid of revolution, or spheroid. If the lengths of all three of its axes are the same, it is a sphere.

ellipticity
A measure of how much an ellipse deviates from a circle, expressed as the ratio of the distance between the center and one focus of an ellipsoid to the length of its semimajor axis. The square of the eccentricity (e2) is commonly used with the semimajor axis a to define a spheroid.

embedded feature class
A multipoint feature class embedded into a terrain dataset. When a feature class is embedded, it is incorporated directly into the terrain pyramid and the terrain becomes the sole container of the data. Embedded feature classes can be used to reduce the amount of disk space required by mass point data such as lidar.

EMF
Acronym for Enhanced Metafile. A spool file format used in printing by the Windows operating system.

empirical
That property of a quantity that indicates that the quantity depends on data, observations, or experiment only; that is, it is not a model or part of a model. An empirical semivariogram is computed on data only, in contrast to a theoretical semivariogram model.

enabled feature
In geometric networks, a network feature that allows flow to pass through it.

ENC
Acronym for electronic navigational chart. A vector data product used for nautical navigation. ENC data is produced by nautical charting agencies throughout the world and uses the IHO (International Hydrographic Organization) S-57 standard for its database structure and attribution.

enclosure
A file describing the contents of an item included in metadata. Enclosing files in metadata works the same way as enclosing files in an e-mail message.

encoding
The recording or reformatting of data into a computer format. Data may be encoded to reduce storage, increase security, or to transfer it between systems using different file formats. In GIS, analog graphic data, such as paper maps and images, are encoded into computer formats by scanning or digitizing.

end hatch definition
In linear referencing, a special type of hatch definition that draws hatch marks only at the low and high measure of a linear feature.

end offset
An adjustable value that dictates how far away from the end of a line an address location should be placed. Using an end offset prevents the point from being placed directly over the intersection of cross streets if the address happens to fall on the beginning or end of the street.

endpoint connectivity
In network datasets, a type of edge connectivity policy that states that an edge may only connect to another edge at its endpoints.

enhancement
In remote sensing, applying operations to raster data to improve appearance or usability by making specific features more detectable. Such operations can include contrast stretching, edge enhancement, filtering, smoothing, and sharpening.

enterprise geodatabase
A geodatabase managed in an RDBMS server by ArcSDE. Multiuser geodatabases can be very large and support multiple concurrent editors. They are supported on a variety of commercial RDBMS, including IBM DB2, IBM Informix, Oracle, Microsoft SQL Server, and PostgreSQL.

enterprise GIS
A geographic information system that is integrated through an entire organization so that a large number of users can manage, share, and use spatial data and related information to address a variety of needs, including data creation, modification, visualization, analysis, and dissemination.

Enterprise JavaBeans
The server-side component architecture for the J2EE platform. EJB enables development of distributed, transactional, secure, and portable Java applications.

envelope
The rectangle surrounding one or more geographical features in coordinate space, determined by the minimum and maximum coordinates in the x and y directions, as well as the ranges of any z- or m-values that the features may have. An envelope can be used to filter data for analysis.

environment settings
Settings that can apply to all tools within the application, all tools within a model or script, or a particular process within a model or script. Environment settings include current workspace, output spatial reference, output spatial grids, cell size, and tile size. They are generally set before running tools.

environment variable
A variable maintained by the operating system and shared among programs. Environment variables function as placeholders for environment information, such as a drive, path or file name.

environmental model
An abstract representation of a complex environmental process, emphasizing relationships and patterns in natural systems. Environmental models allow decision makers to better understand the effects of natural systems or the impact of human activities on natural systems.

EOBrowser
An ArcGIS utility application that can be used to investigate the contents of object libraries.

ephemeris
A table of the predicted positions of a satellite within its orbit for each day of the year, or for other regular intervals.

EPSG ID
A coordinate system identification created by the European Petroleum Survey Group.

equal competition area
A trade area boundary set halfway between a store or service point and its neighboring stores or service points.

equal-area classification
A data classification method that divides polygon features into groups so that the total area of the polygons in each group is approximately the same.

equal-area projection
A projection in which the whole of the map as well as each part has the same proportional area as the corresponding part of the earth. An equal-area projection may distort shape, angle, scale, or any combination thereof. No flat map can be both equal-area and conformal.

equal-interval classification
A data classification method that divides a set of attribute values into groups that contain an equal range of values.

equator
The parallel of reference that is equidistant from the poles and defines the origin of latitude values.

equatorial aspect
A planar (or azimuthal) projection with its central point located at the equator.

equidistant projection
A projection that maintains scale along one or more lines, or from one or two points to all other points on the map. Lines along which scale (distance) is correct are the same proportional length as the lines they reference on the globe. In the sinusoidal projections, for example, the central meridian and all parallels are their true lengths. An azimuthal equidistant projection centered on Chicago shows the correct distance between Chicago and any other point on the projection, but not between any other two points.

equivalent projection
A projection in which the whole of the map as well as each part has the same proportional area as the corresponding part of the earth. An equal-area projection may distort shape, angle, scale, or any combination thereof. No flat map can be both equal-area and conformal.

erase
In ArcInfo, a command that removes or deletes features from one coverage that overlap features in another coverage.

error
A measured, observed, calculated, or interpreted value that differs from the true value or the value that would be obtained by a perfect observer using perfect equipment and perfect methods under perfect conditions.

error propagation
In GIS data processing, the persistence of an error into new datasets calculated or created using datasets that originally contained errors. The study of error propagation is concerned with the effects of combined and accumulated errors throughout a series of data processing operations.

error table
A geodatabase table used by the GIS Data ReViewer to track error information through the quality control process. Defects are recorded, resolved and verified in the error table.

ESRI Adapters for IBM LAS
A translator that allows users to access ArcWeb Services through the IBM WebSphere Everywhere Access (WEA) Location Aware Services (LAS).

ESRI Developer Network
A subscription program used to acquire ESRI software for developers, which includes a special program for supporting the ESRI developer community. EDN is intended to promote collaboration and interaction among GIS developers and ESRI staff.

ESRI Grid
An ESRI data format for storing raster data that defines geographic space as an array of equally sized square cells arranged in rows and columns. Each cell stores a numeric value that represents a geographic attribute (such as elevation) for that unit of space. When the grid is drawn as a map, cells are assigned colors according to their numeric values. Each grid cell is referenced by its x,y coordinate location.

ESRI Maplex Label Engine
In Maplex for ArcGIS, extended software that allows finer control of how labels are placed.

ESRI Standard Label Engine
In ArcMap, the software used to place labels.

EsriMapCatalog.ser
A binary serialized file created in the ArcIMS 3.x AppServer directory to save site parameters. This file has now been replaced by ArcIMSSite.sez.

EsriMapCookies.ser
A binary serialized file created in the ArcIMS 3.x AppServer directory to store ArcIMS Folders and any submitted MapNotes and EditNotes. This file has now been replaced by ArcIMSFolders.sez.

estimation
In spatial modeling, the process of forming a statistic from observed data to assign optimal parameters in a model or distribution.

Euclidean distance
The straight-line distance between two points on a plane. Euclidean distance, or distance "as the crow flies," can be calculated using the Pythagorean theorem.

Euclidean distance analysis
In ArcGIS Spatial Analyst, a description of each raster cell's relationship to the closest source.

evaluator
A function that determines attribute values for network elements in a network dataset. If a network source does not have an evaluator, the default evaluator for its element type is used.

event
A geographic location stored in tabular rather than spatial form. Event types include address events, route events, x,y events, and temporal events.

event handling
Watching for events that are broadcast by another class, and taking action when they occur.

event layer
In ArcGIS, a layer created from an event table.

event location
A geographic location stored in tabular rather than spatial form. Event types include address events, route events, x,y events, and temporal events.

event overlay
In linear referencing, an operation that produces a route event table that is the logical intersection or union of two input route event tables. Event overlay is one way to perform line-on-line, line-on-point, and event point-on-point overlays.

event table
A data source containing location information in tabular format (called events) that is used to create a spatial dataset. For example, an event table might contain x,y coordinates or routes.

event theme
In ArcView 3.x, a spatial data theme created from an event table.

event transform
In linear referencing, an operation that produces a new table by copying and transforming events from one route reference to another. This allows the events to be used with a route reference having different route identifiers and/or measures.

exception
An error that is an acceptable violation of a topology rule. In ArcMap, for example, a cul-de-sac is a legitimate exception to the rule that prohibits dangles.

executable file
A binary file containing a program that can be run as a stand-alone application. In the Microsoft Windows program, executable files are designated with an .exe extension.

exit state
The condition of a tool upon closure. If a tool fails due to a programming bug or command failure, the exit state will be "failed."

explanatory variable
One or a set of variables used to model or predict the dependent variable. For example, a prediction of annual purchases for a proposed store (the dependent variable) might include independent variables representing the number of potential customers, distance to competition, store visibility, and local spending patterns. In the regression equation, independent variables appear on the right side of the equal sign and are often referred to as explanatory variables.

explode
An editing process that separates a multipart feature into its component features, which become independent features.

exponent
In mathematics, a number that indicates how many times a base value is multiplied by itself. Exponents are usually indicated with superscripts.

export
To move data from one computer system to another, and often, in the process, from one file format to another.

exposure station
In aerial photography, each point in the flight path at which the camera exposes the film.

expression
A sequence of operands and operators constructed according to the syntactic rules of a symbolic language that evaluates to a single number, string, or value.

Extended Entity Data
In AutoCAD, extra optional data attached to an AutoCAD drawing element.

extended postal code
An enhanced ZIP Code that consists of the five-digit ZIP Code plus four additional digits that identify a specific geographic segment within the five-digit delivery area, such as a city block, office building, or other unit.

Extensible Markup Language
Developed by the W3C, a standardized general purpose markup language for designing text formats that facilitates the interchange of data between computer applications. XML is a set of rules for creating standard information formats using customized tags and sharing both the format and the data across applications.

Extensible Style Language
A set of standards for defining XML document presentation and transformation. An XSL style sheet may contain information about how to display tagged content in an XML document, such as font size, background color, and text alignment. An XSL style sheet may also contain XSLT code that describes how to transform the tagged content in an XML document into an output document with another format. The W3C maintains the XSL standards.

Extensible Style Language Transformations
A language for transforming the tagged content in an XML document into an output document with another format. An XSL style sheet contains the XSLT code that defines each transformation to be applied. Transforming a document requires the original XML document, an XSL document containing XSLT code, and an XSLT processor (parser) to execute the transformations. The W3C maintains the XSLT standard.

extension
In ArcGIS, an optional software module that adds specialized tools and functionality to ArcGIS Desktop. ArcGIS Network Analyst, ArcGIS StreetMap, and ArcGIS Business Analyst are examples of ArcGIS extensions.

extent
The minimum bounding rectangle (xmin, ymin and xmax, ymax) defined by coordinate pairs of a data source. All coordinates for the data source fall within this boundary.

extent rectangle
A rectangle that is displayed in one data frame, showing the size and position of another data frame.

external polygon
In coverages, the first record in a polygon attribute table, representing the area beyond the outer boundary of the coverage.

extract server
A private ArcIMS virtual server that clips data and saves it to a Zip file.

Extract wizard
An ArcToolbox wizard that selects features from a coverage based on attribute values to create a new coverage.

extraction guide
A specification that defines parameters for feature extraction and attribution. Specifications typically include the size of features to be collected, density of feature collection, scale ranges, and attribute assignment.

extrapolation
Using known or observed data to infer or calculate values for unobserved times, locations or other variables outside a sampled area. In the absence of data, extrapolation is a common method for making predictions, but it is not always accurate. For example, based on observed economic indicators, an economist can make predictions about the state of the economy at a future time. These predictions may not be accurate because they cannot take into account seemingly random events such as natural disasters.

extrusion
The process of projecting features in a two-dimensional data source into a three-dimensional representation: points become vertical lines, lines become planes, and polygons become three-dimensional blocks. Uses of extrusion include showing the depth of well point features or the height of building-footprint polygons.