GIS Glossary/V

GIS Glossary

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vagueness
In GIS, a state of uncertainty in data classification that exists when an attribute applies to an indeterminate quality of an object or describes an indefinite quantity. For example, the classification of an area of land as the range of golden-winged warblers (a rare species of bird) is vague for two reasons. The area populated by the birds is indefinite: it is changing constantly and can never be precisely defined. The term "range" is also somewhat vague since the birds migrate and occupy the territory for only part of the year.

valency
In coverages, the number of arcs that begin or end at a node.

valency table
A table that lists the nodes in a data layer along with their valencies.

valid value table
A component of the PLTS knowledge base that contains the valid attribute combinations and rendering information for the features in a feature class. VVTs are used to symbolize layers in batch mode, and provide a means to symbolize features based on more than three attributes.

validation
The process, using formal methods, of evaluating a system or software component to determine whether it functions as expected and achieves the intended results.

validation rule
A rule applied to objects in the geodatabase to ensure that their state is consistent with the system that the database is modeling. The geodatabase supports attribute, connectivity, relationship, and custom validation rules.

value
A measurable quantity that may be passed to a function. Values are either assigned or determined by calculation.

value attribute table
A table containing attributes for a grid, including user-defined attributes, the values assigned to cells in the grid, and a count of the cells with those values.

variable
A symbol or placeholder that represents a changeable value or a value that has not yet been assigned.

variable depth masking
A drawing technique for hiding part of one layer using another set of features. Variable depth masking allows a layer to be drawn with gaps at specific locations without affecting other layers at these locations.

variance
A numeric description of how values in a distribution vary or deviate from the mean. The larger the variance, the greater the dispersion of values around the mean. The standard deviation for a distribution is the square root of the variance.

variance-covariance matrix
In surveying, the symmetric 3x3 matrix that mathematically expresses the correlation between errors in coordinates x, y, and z.

variant
A data type that can contain any kind of data.

variogram
A function of the distance and direction separating two locations that is used to quantify dependence. The variogram is defined as the variance of the difference between two variables at two locations. The variogram generally increases with distance and is described by nugget, sill, and range parameters. If the data is stationary, then the variogram and the covariance are theoretically related to each other.

variography
The process of examining spatial dependence using a variogram; a set of procedures (as much art as science) for interpreting variograms.

VAT
Acronym for value attribute table. A table containing attributes for a grid, including user-defined attributes, the values assigned to cells in the grid, and a count of the cells with those values.

VB
A programming language developed by Microsoft based on an object-oriented form of the BASIC language and intended for application development. Visual Basic runs on Microsoft Windows platforms.

VBA
The embedded programming environment for automating, customizing, and extending ESRI applications, such as ArcMap and ArcCatalog. It offers the same tools as Visual Basic in the context of an existing application. A VBA program operates on objects that represent the application and can be used to create custom symbols, workspace extensions, commands, tools, dockable windows, and other objects that can be plugged in to the ArcGIS framework.

VBVM
Acronym for Visual Basic Virtual Machine. The runtime environment used by Visual Basic code when it runs.

VDOP
Acronym for dilution of precision. An indicator of satellite geometry for a constellation of satellites used to determine a position. Positions with a lower DOP value generally constitute better measurement results than those with higher DOP. Factors determining the total GDOP (geometric DOP) for a set of satellites include PDOP (positional DOP), HDOP (horizontal DOP), VDOP (vertical DOP), and TDOP (time DOP).

vector
A coordinate-based data model that represents geographic features as points, lines, and polygons. Each point feature is represented as a single coordinate pair, while line and polygon features are represented as ordered lists of vertices. Attributes are associated with each vector feature, as opposed to a raster data model, which associates attributes with grid cells.

vector data model
A representation of the world using points, lines, and polygons. Vector models are useful for storing data that has discrete boundaries, such as country borders, land parcels, and streets.

vector model
A representation of the world using points, lines, and polygons. Vector models are useful for storing data that has discrete boundaries, such as country borders, land parcels, and streets.

Vector Product Format
A vendor-neutral data format used to structure, store, and access geographic data according to a defined standard.

vectorization
The conversion of raster data (an array of cell values) to vector data (a series of points, lines, and polygons).

vectorization settings
GIS software settings that allow users to control the conversion of raster data to vector data. For example, vectorization settings may allow users to select which raster cells are eligible for vectorization or specify how the geometry of the output vector data will be constructed during vectorization.

Vectorization Trace tool
An ArcScan tool that allows users to manually trace raster cells and generate features to be converted from raster to vector data.

vehicle routing problem
In ArcGIS Network Analyst, a type of network analysis for routing a fleet of vehicles to service a set of orders with the goal of minimizing some objective (e.g., operating cost), while satisfying certain constraints. These constraints may include time windows, multiple route capacities, travel duration constraints, route zone and route seed point constraints, specialties constraints, and paired order constraints.

verbal scale
A map scale that expresses the relationship between distance on the map and distance on the ground in words; for example, "One inch represents 20 miles."

verification
The process, using formal methods, of evaluating a system or software component to determine whether it satisfies the requirements imposed at the start of development.

version
In databases, an alternative state of the database that has an owner, a description, a permission (private, protected, or public), and a parent version. Versions are not affected by changes occurring in other versions of the database.

version merging
The process of reconciling two versions of a dataset into a common version. If conflicting edits have been made in either of the merged versions, these conflicts are resolved, either automatically or by an interactive process.

version reconciliation
The process of updating a version of a dataset with changes made in another version. Using this technique, a version can remain up to date with changes even if it is within a long transaction lasting many months.

vertex
One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature.

vertical control
A basic element of a software application's GUI. Examples of controls include menus, buttons, tools, check boxes, slider bars, text input boxes, and combo boxes.

vertical control datum
A geodetic datum for any extensive measurement system of heights on, above, or below the earth's surface. Traditionally, a vertical geodetic datum defines zero height as the mean sea level at a particular location or set of locations; other heights are measured relative to a level surface passing through this point. Examples include the North American Vertical Datum of 1988; the Ordnance Datum Newlyn (used in Great Britain); and the Australian Height Datum.

vertical coordinate system
A reference system that defines the location of z-values relative to a surface. The surface may be gravity related, such as a geoid, or a more regular surface like a spheroid or sphere.

vertical exaggeration
A multiplier applied uniformly to the z-values of a three-dimensional model to enhance the natural variations of its surface. Scenes may appear too flat when the range of x- and y-values is much larger than the z-values. Setting vertical exaggeration can compensate for this apparent flattening by increasing relief.

vertical geodetic datum
A geodetic datum for any extensive measurement system of heights on, above, or below the earth's surface. Traditionally, a vertical geodetic datum defines zero height as the mean sea level at a particular location or set of locations; other heights are measured relative to a level surface passing through this point. Examples include the North American Vertical Datum of 1988; the Ordnance Datum Newlyn (used in Great Britain); and the Australian Height Datum.

vertical line
A line that corresponds to the direction of gravity at a point on the earth's surface; the line along which an object will fall when dropped.

vertical photograph
An aerial photograph taken with the camera lens pointed straight down.

vertical shift
In ArcGIS, a parameter that offsets the z-origin from the surface of a vertical coordinate system. The vertical shift is similar in effect to the false easting or false northing parameters of a projected coordinate system.

view
In ArcGIS, a way to see the contents of a selected item in the Catalog tree in ArcCatalog.

viewer
In ArcGlobe and 3D Analyst, an additional window that allows a user to view 3D data in a scene from another angle. Users can have multiple viewers in a scene.

viewshed
The locations visible from one or more specified points or lines. Viewshed maps are useful for such applications as finding well-exposed places for communication towers, or hidden places for parking lots.

virtual directory
A directory name, used in a URL, that corresponds to a real or actual directory on a Web server.

virtual page
The map page as seen in layout view in ArcMap.

virtual study area
The current extent of a map document in ArcGIS. The virtual study area never has a boundary.

virtual table
A logical table in a database that stores a pointer to the data rather than the data itself.

visible scale range
A minimum and maximum value that a map scale must fall between in order for the map layers to be displayed.

Visual Basic
A programming language developed by Microsoft based on an object-oriented form of the BASIC language and intended for application development. Visual Basic runs on Microsoft Windows platforms.

Visual Basic for Applications
The embedded programming environment for automating, customizing, and extending ESRI applications, such as ArcMap and ArcCatalog. It offers the same tools as Visual Basic in the context of an existing application. A VBA program operates on objects that represent the application and can be used to create custom symbols, workspace extensions, commands, tools, dockable windows, and other objects that can be plugged in to the ArcGIS framework.

Visual Basic Virtual Machine
The runtime environment used by Visual Basic code when it runs.

Visual C++
A Microsoft implementation of the C++ programming language which is part of Microsoft's Visual Studio Development environment, used to compile and develop Windows software.

visual center
The point on a rectangular map or image to which the eye is drawn. The visual center lies slightly (about 5 percent of the total height) above the geometric center of the page.

visual hierarchy
The presentation of features on a map in a way that implies relative importance, usually achieved with visual contrast.

visualization
The representation of data in a viewable medium or format. In GIS, visualization is used to organize spatial data and related information into layers that can be analyzed or displayed as maps, three-dimensional scenes, summary charts, tables, time-based views, and schematics.

VMap
Acronym for Vector Map. A vector-based data product in vector product format (VPF) at several scales divided into groups, referred to as levels. For example, VMap Level 1 includes vector maps at a scale of 1:250,000, and VMap Level 2 includes vector maps at a scale of 1:50,000.

volume
In a TIN, the space (measured in cubic units) between a TIN surface and a plane at a specified elevation. Volume may be calculated above or below the plane.

Voronoi diagram
A partition of space into areas, or cells, that surround a set of geometric objects (usually points). These cells, or polygons, must satisfy the criteria for Delaunay triangles. All locations within an area are closer to the object it surrounds than to any other object in the set. Voronoi diagrams are often used to delineate areas of influence around geographic features. Voronoi diagrams are named for the Ukrainian mathematician Georgy Fedoseevich Voronoi (1868-1908).

voxel
A three-dimensional pixel used to display and rotate three-dimensional images.

VPF
Acronym for Vector Product Format. A vendor-neutral data format used to structure, store, and access geographic data according to a defined standard.

VPF dataset
Acronym for Vector Product Format. A vendor-neutral data format used to structure, store, and access geographic data according to a defined standard.

VPF feature class
In ArcGIS, a collection of geographic features with the same geometry type (such as point, line, or polygon), the same attributes, and the same spatial reference. Feature classes can be stored in geodatabases, shapefiles, coverages, or other data formats. Feature classes allow homogeneous features to be grouped into a single unit for data storage purposes. For example, highways, primary roads, and secondary roads can be grouped into a line feature class named "roads." In a geodatabase, feature classes can also store annotation and dimensions.

VPF layer
A layer that references a set of VPF data. VPF data is a standard format, structure, and organization for large geographic databases that are based on a georelational data model.

VRP
In ArcGIS Network Analyst, a type of network analysis for routing a fleet of vehicles to service a set of orders with the goal of minimizing some objective (e.g., operating cost), while satisfying certain constraints. These constraints may include time windows, multiple route capacities, travel duration constraints, route zone and route seed point constraints, specialties constraints, and paired order constraints.

vTable binding
The fastest form of early binding, during which client code that uses a COM object (such as Visual Basic) binds to a method by indexing to a virtual function table, or vtable.

VVT
A component of the PLTS knowledge base that contains the valid attribute combinations and rendering information for the features in a feature class. VVTs are used to symbolize layers in batch mode, and provide a means to symbolize features based on more than three attributes.