T
tab group
(also, "Field") A component of a window such as a button, list, scroll bar, pane, sash, or slider (sometimes "scale") that can be selected with the location box by pressing the Tab key. Some tab groups, such as a selection list, can be composed of multiple items through which the location cursor can be moved by the arrow keys.
table
A database is organized into tables that contain records. Tables cover different topics related to the same common theme. The theme and its extent of development determine the number of tables that comprise the database. For example, the Crow Butte soil polygon database contains twelve tables, in such areas as yield, crop potential, and statistical information. (See also: database object, field, record.)
TARGA file
Truevision Advanced Raster Graphics Adapter file. A file saved or loaded from an ICB, TARGA, or VISTA display board in one of several related TARGA formats. These formats are widely used with other boards and software to transport color images between microcomputer software and systems.
TAT
Text AttributeTable for an annotation subclass in a coverage. In addition to user defined attributes, the TAT contains a sequence number and text feature identifier.
terabyte, Tbyte, or TB
A unit of measurement for (approximately) 1,000,000,000,000 bytes, 1,000,000,000 kilobytes, 1,000,000 megabytes, or 1000 gigabytes. (See also: bit, byte, exabyte, gigabyte, kilobyte, megabyte, pecabyte)
Terra-Mar
A vendor of a second-generation raster-based microcomputer image processing and GIS system.
text
A string of characters grouped in a file.
text cursor
A graphic mark such as a blinking underscore, vertical line or character-sized box on the screen that indicates the focus of keyboard text entry and editing.
text field
A component of a window that can be selected with the location box for textual information that can be entered and edited from the keyboard.
text monitor or text screen
The B/W or color monitor used to display text materials (like menus, database records, and keyboard input).
text object
Anything from a simple string of ASCII characters to a more complex, formatted page description.
thinning
(rasters) To remove cells from wide line images in a raster object. When a scanner creates a raster object, the lines in the drawing typically are several cells wide in the result. Before automatic vectorization techniques can work on the data, the line images in the raster object must be thinned to make the line images just one cell wide.
thinning
(vectors) Reducing the number of coordinate pairs that describe a vector's line and polygon elements. Some of the coordinate pairs will be discarded when straight line lengths replace curved or noisy segments in the original lines.
threshold
(binary) The separation point for converting grayscale raster cell data into binary (black and white) data. For a raster object scanned with 256 levels of gray, a threshold might be set at 180. Then the output binary raster object would have a 1 for every cell in the input raster object that had the value 180 or higher, and 0 everywhere else.
thresholding
Setting a data conversion separation limit such that any incoming value above the designated limit, or threshold, is assigned one value (in binary thresholding, a "1") and any incoming value below or equal to the limit is assigned another value (in binary thresholding, a "0"). Thresholding is often used to generate a binary raster from a 16-gray-level scan, or to find feature edges during the steps of raster to vector conversion.
tie point
A point that is co-located on two raster objects that have an overlapping geographic extent. Tie points are used in the manual mosaic process to establish the relationship between adjacent pieces of the mosaic that provide no absolute ground control information, but do show some common ground feature in each piece. For example, multiple frames of airvideo can be tied together with tie points (farm buildings, field corners, bushes, rock outcroppings, bends in streams) even if the map coordinates are not known for such features. Then, the map calibration for the entire sequence can be established from control points sparsely located in the mosaic sequence (such as road intersections) for which the map coordinates are known. (See also: control point .)
TIFF
Tag Image File Format. A series of standard color image file formats adopted by Microsoft, Aldus, and others to transfer images between different software packages.
TIGA
Texas Instruments Graphics Architecture. A software interface that standardizes communication between application software and display boards that use one of the TMS340 chips and can be used to interface with several manufacturers' image display boards. TIGA divides tasks between the TMS340 display processor and the CPU of the host microcomputer to improve performance.
TIGER files
Topologically Integrated Geographic Encoding and Referencing files compiled and distributed by the U.S. Census Bureau.
tiles
Rectangular areas of defined size in a raster object used for storage of the information. Storing raster objects in tiled format generally increases display speed when viewing less than the entire object.
A third usage of the word tiles occurs in the Color Map Editor where the upper part of the window displays the colors in the map with small sample squares or tiles. The active or selected tile is outlined in white, and its cell value and a larger sample is displayed below.
tiling
Assembling large images from smaller segments that have common angular orientations, cell sizes, and map projections. The process is similar to that of assembling floor tiles, except that the raster tiles may overlap at the onset. (See also: montage, mosaic.)
tin
Triangulated Irregularr network. A surface representation derived from irregularly spaces, sample point and breakline features. The tin dataset includes topological relationship between points and their neighbouring 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 used to represent the surface. Tins are also called irregular triangular mesh or irregular triangular surface model.
TIN Densification
A DEM extraction method that builds a TIN surface from the initial set of user-supplied tie points, and then iteratively densifies the TIN object. The process looks in the middle of each existing triangle and adds a new node when it finds a point of high correlation. Then it recomputes the local topology and goes on to the next triangle.
TIN elements
TIN objects contain four element types: nodes, edges, triangles, and hulls. Nodes are the most fundamental element of a TIN and the only element type that references spatial coordinates (x,y,z). An edge is an oriented line segment that connects two nodes. Three edges connect three nodes to form a triangle that satisfies the Delaunay criterion. Triangles represent elementary areas of the surface that describe topological relationships between all other elements of the TIN data. A hull represents the area covered by a TIN structure. Interpolation of z-values is only valid within a hull region.
TIN object
A TIN, or Triangulated Irregular Network, object represents a continuous surface as a set of conterminous triangles computed from irregularly spaced 3D points. TIN topology is more restrictive than the topology of other coordinate data objects; every node is part of some polygon, every polygon is a triangle, and every triangle satisfies the Delaunay criterion. (See also: Delaunay criterion.)
title bar
The area across the top of a window that displays the name of the window, the resize graphic, and the iconifying button.
TM
Thematic Mapper. A sensing device on the Landsat satellite that scans and stores 7 individual images in spectral bands ranging from the blue wavelengths up to those in the thermal infrared.
toggle button
A toggle button offers an on/off control option for the current process. A toggle button's label tells what state it controls, and it is preceded by a graphic indicator of the on/off state of the button.
toolTip
A ToolTip pops in naming the menu item selection that diplicates the function of the icon button when the mouse cursor pauses over an icon button.
topographic map or topo map
A map that uses colors and symbolic patterns to represent the general surface features of the earth, such as grassland, forest, marsh, agricultural, urban, and barren rock.
topography
The features of the actual surface of the earth, considered collectively according to their form (for example, grassland, cultivated, desert, forest, swamp). A single feature, such as one mountain or one valley, is called a topographic feature.
topology or vector topology
A description of the relationship between node, line, and polygon elements in a vector object. An RVC vector object has a rigorously defined topology, which keeps track of things like lines that intersect at nodes, polygon elements on either side of a line element, line elements that form a polygon, island polygons within polygons, and parent polygons for island polygons.
topology errors
Violations of vector topology, such as line elements that intersect without a node at the point of intersection, or polygons that overlap without defining an intersection polygon.
trace
To create a vector line element by manually or interactively tracing over line images in a raster object.
training set or prototype
A group of sample cells in an image known to represent a feature type or ground cover of interest defined by the user from his or her knowledge of the site (perhaps through ground visitation or detailed airphoto interpretation).
transcoder
A device that converts video signal formats from an input type (like SVHS) to an output type (like RGB analog).
transfer rate
The number of XY coordinate pairs produced per second when you are digitizing with the streaming method.
Transformed Vegetation Index
A commonly used vegetation index derived from images of certain spectral bands. The TVI is equal to the square root of the quotient of the photo-infrared minus the red band, and the photo-infrared plus the red band {SQRT[(IR - red) / (IR + red)]}.
translation curve
The curve used to adjust a raster object's cell values to the brightness values used for display.
transliterate
To represent the words of one language, with the alphabet of another language. Usually the language is foreign to the reader, and the alphabet is familiar.
transparent color
The ability to overlay one color image over another image or map so that the spatial details of both are revealed for comparison is achieved with transparent color. The colors that result from such an overlay depend on whether the color in the images is additive or subtractive.
transparent patterns
Patterns that only partially obscure the image upon which they are superimposed are transparent. For example, a pattern could consist of a drawing of a duck that leaves the remainder of the pattern tile transparent. When this pattern is used to fill a polygon that contains a lake, the lake colors and features will be visible through the transparent portions of each tile.
true scale
At large sizes, every map projection distorts the scale of distance, especially towards the edges. The location of the true scale of a projection identifies the position where map measurements correctly correspond to actual surface distances.
TSR
Terminate and Stay Resident. A software process that becomes resident in memory when executed and monitors your input or the actions of other programs and performs some special activity when the appropriate conditions are detected. For example, A TSR program may be loaded to memory and wait for you to enter a particular non-standard series of keystrokes to initiate printing your text screen out on paper.
TVI
See - Transformed Vegetation Index
TYDAC
A raster-based commercial GIS system.