V
variable
(in database queries) A named entity that has an assigned value. There are four kinds of query variables: (1) those are associated with the vector elements (like numlines, class, and startnode); (2) those which are fields from a database (perhaps Name, Acres, and Yield); (3) those which control the drawing styles of vector elements (like size, color, and fillcolor); and (4) the ten temporary user variables for numeric operations (temp0, temp1, temp2 ... temp9). For example, the point element variable "size" can have a value from 1 to 64. It controls the size at which a point element is drawn. An assignment statement can change only the values of drawing style and temporary variables; it cannot change the value of variables that are associated with a vector element or a database.
VAT
Value Attribute Table. A table containing attribute for a grid. In addition to user defined attributes, the VAT contains the values assigned to cell in the grid and a count of the cells with those values.
vector
"A quantity possessing magnitude and direction, represented by an arrow, the direction of which indicates the direction of the quantity and the length of which is proportional to the magnitude" (Random House). In connection with GIS and computer graphics, this term is used more loosely to refer to a set of vectors joined end to end to make an arc or irregular line with a uniform set of properties.
vector element
A vector object is made up of three different types of elements that can have associated attributes 1) points, which are single sets of coordinates that define a point feature (such as a well); 2) lines, which are curvilinear strings of coordinates that define a curved line (such as a stream); and 3) polygons, which are collections of lines that inscribe an area (such as a lake), and a fourth type of element, nodes, which are necessary to maintain vector topology.
vector format
A data structure for representing point and line data by means of 2- or 3-dimensional geometric (Cartesian x,y or x,y,z) coordinates with exacting topological requirements. In connection with GIS and computer graphics, "vector" can refer to a set of line segments joined end-to-end to make a curved path in space. Vector objects also have a set of attributes (such as ID number, color, drawing style) attached to them.
vectorize
A general term for any technique that converts raster data into vector data.
vector object
A collection of vector elements and attributes stored in a complex topology as one entity project file. (See also: vector topology)
vector topology
A description of the relationship between point, node, line, and polygon elements in a vector object. An RVC vector object has a rigorously defined topology, which requires that points can be in at most one polygon and lines do not intersect without separation by nodes, which are required for topology maintenance.
vegetation index
The output from standard manipulations of multispectral image raster objects. The system processes the input spectral information and creates output raster objects whose cell values represent the site's biophysical properties: amount of vegetation, leaf area, greenness, brightness, and wetness.
VersaCAD
A popular, commercial microcomputer Computer Aided Design (CAD) software package developed and marketed by PRIME.
vertex
The point at which a line changes direction or terminates.
VESA
Video Electronics Standards Association. A microcomputer graphics standard with support for 256 simultaneous colors.
VGA
Video Graphics Array. A microcomputer video subsystem introduced by IBM with the PS/2 microcomputer in 1987 with support for 256 simultaneous colors. The VGA is an enhancement of the older EGA and is now also available for other AT bus based microcomputers. (See also: CGA, EGA.)
VHS
Video Home System. A popular format for low-cost video recording and playback, common in home video systems.
video capture
Some image display boards have the ability to display live broadcast or taped programming and then freeze and grab an image, storing it in the computer's memory or on disk.
video digitizing board
A video interface circuit board installed in a PC that samples or frame-grabs a video frame and constructs a digital image. Video digitizing boards can be used for non-standard, higher resolution video sources. (See also: frame-grabbing.)
video field
The image you see on a standard TV screen is composed of a set of about 480 horizontal lines. The lines are projected in two passes of the signal beam. Each pass only projects every other line of the image: the odd lines in one pass, and the even lines in the next pass. One scan takes 1/60 of a second, so the whole picture (the frame) is refreshed every 1/30th of a second.
A field contains every other line of the complete video image. The primary field contains the odd lines; the secondary field contains the even lines.
video frame
A complete video image, which consists of two interlaced fields Odd lines of the frame are contained in the primary field which is alternated with the secondary field which contains the even lines. The primary field lasts 1/60 of a second in standard broadcast video. The secondary field follows in the next 1/60 of a second. The entire frame takes 1/30 of a second to display. There is a difference of 1/60 of a second between alternate lines in the image.
viewshed
The boundaries of sight from a single vantage point, assuming an unobstructed surface (disregard trees and buildings
vignetting
Darkening at the edges and corners of an optical image. Vignetting occurs because lenses are physically unable to pass as much light to the fringe of an optical field as they do to its center. Several techniques compensate for vignetting, but it is almost always present to some degree. For color rasters, vignetting means that the color of a feature at the edge or corner of an image may not match the color of that same feature in the center.
volume
An individual storage device such as a tape or floppy disk.