G
gadget
A graphical X object, like a widget, that does not have a window of its own (as most widgets do) but appears inside a parent window.
gap
A line segment undershoot leaves a blank space in the T-junction of two lines. The raster thinning process often leaves gaps that should be repaired by raster editing before final vectorization.
Gazetteer
A work of geographic reference that supplies place name and locatian information when a place is known, a gazetteer can provide the coordinates of the place. Most atlases cantain gazetteer. Well known digital gazetteers are the USGS Geographic Names Information System (GNIS) and the gazetteer in the Digital chart of the world (DEW) In ARC/INFO the Gazetter spatial index is done as a grid of alpha and numeric reference which is converted into a polygon coverage. Places (Points or polygon) are then overlaid with this grid, then sorted alphabetically. This produces a list of place names sorted both alphabetically and by reference grid number.
GBF/DIME
For the 1980 census, the US census Bureau produced Geographic Base Files (GBF) and Dual Independent Map Encoding (DIME) files, containing census Geographic statistical codes and coordinates of line segments for most metropolitan areas. DIME files provide a schematic map of a city's states, address ranges and geostatistical codes relating to the census Bureau's tabular statistical data. DIME was replaced by TIGER in 1990 census.
GEM
A window and mouse desktop operating system developed and marketed by Digital Research, Inc.
genlock
To synchronize two video signals (usually NTSC) to match the color video frames. Genlocking is necessary to mix and combine two signals, like a live video image and a graphic overlay. Some display boards can output an NTSC signal but cannot genlock, or mix, that signal with another video signal.
Geo centric latitude
One of the many forms of latitude used shown dealing with an ellipsoid. The geocentric latitude of a point is defined as the angle a line from the center of the earth through the point maps with the plane of the equator.
Geocode
The process of identifiying the coordinates of a location given its address. For example, an address can be matched against a TIGER street network to determine the location of a home. Also referred to as address Geocoding.
geodesy
Investigation of scientific questions connected with the shape and dimensions of the earth.
Geodetic
Of or relating to the study of the shape and size of the earth: A geodetic coordinate is the specification of a praise location on the surface of the earth.
Geodetic Reference System
The true technical name for a datum. The combination of an ellipsoid, which specifies the size and shape of the earth, and a base point from which the latitude and langitude of all other points are referenced.
Geographic data
The locations and descriptions of geographic features. The composite of spatial data and descriptive data.
Geographic Database
A collection spatial data and related descriptive data organized for efficient storage and retirieval by many users.
Geographic data set
One of the seven geographic data types supported by ARC/INFO. Geographic data sets in ARC/INFO include coverage, grids, DBMS tables, tins, images, Lattices and CAD drawings.
Geographic feature
A user - defined geographic phenomenon that can be modeled or represented using geographic data sets in ARC/INFO. Examples of geographic features include streets, sewer lines, Manhole cover, accidents, lot line and parcels.
geoid
The figure that represents the irregular spheroidal shape of the earth is called the geoid. The geoid is an equipotential surface (which means the direction of gravity is perpendicular to all points on the surface) at mean sea level. Because of variations in the distribution of the mass and density of the earth's constituents, the geoid generally rises over the continents and is depressed in oceanic areas ; the geoid also conforms to various lumps and depressions that depart from average smoothness on the earth's surface. For mapping purposes, the attributes found on the geoid must be transferred to a more regular geometric surface, an ellipsoid.
Geoid Height
The height of the geoid above the ellipsoid in use. New days this usually refers to the hight of the geoid above the was ellipsoid. Upon which the global Positioning System is based.
geometric transformation (or rectification)
A process in which an image is stretched differentially so as to change its internal geometry is said to have undergone geometric transformation or rectification. A transformation specifically refers to the process of projecting an image from its plane onto another plane by translation, rotation, and/or scale change.
georeference or geographic calibration
1. Information that relates raster cells or vector/CAD elements to a specified coordinate system or map projection. The RVC file format keeps the information needed to relate every raster cell or vector coordinate point to some geographic coordinate system (like a particular map projection) in subobjects associated with the parent object. Geographic calibration may be established when creating an object (like extracting an image map from a Landsat or SPOT satellite image). Calibration can also be added to an existing object either by entering control points or by associating it with some calibrated object (like overlaying a calibrated vector to calibrate a raster). An RVC subobject identifies a calibrated object's map projection.
2. To establish the relationship between page coordinates on a planar map and known real world coordinates.
georeferenced image map
See - image map.
Geo-relational model
A Geophic data model that me presents geographic features as an inter related set of spatial and descriptive data. The geore lational model is the fundamental data model used in ARC/INFO
gigabyte, Gbyte, or GB
A computer unit of measurement for (approximately) 1,000,000,000 bytes, 1,000,000 kilobytes, 1,000 megabytes, or .001 terabytes. (See also: bit, byte, exabyte, kilobyte, megabyte, pecabyte, terabyte.)
GIS
Geographic Information System. "A Geographic Information System is a computer system designed to allow users to collect, manage, and analyze large volumes of spatially referenced and associated attribute data. The major components of a GIS are: a user interface; system/database management capabilities; database creation/data entry capacity; spatial data manipulation and analysis packages; and display/product generation functions." - from USGS Open File Report 88-105 [A process for evaluating Geographic Information Systems]. In the broadest sense, a GIS is any integrated system of information that includes a geographic component.
global variable
(SML) All variables not explicitly declared local are called "global variables" and are insulated from the operations of a function or procedure only if the function or procedure declares a local variable of the same name.
glyph
Characters, letters, numerals, and symbols that make up a font typeface. Glyphs can include alpha-numeric characters, ideographs, international currency symbols, and directive symbols and icons such as curved arrows and dingbats.
gnomonic projection
A type of perspective azimuthal map projection. The point of the projection is the center of the sphere. Gnomonic is the only projection on which all great circles represented are straight lines.
GPIB
General Purpose Interface Board. An interface standard for computers and a peripheral devices, such as a scanner or printer.
GPS
Global Positioning System. A network of radio-emitting satellites deployed by the US Department of Defense. Ground-based GPS receivers can automatically derive accurate surface coordinates for all kinds of GIS, mapping, and surveying data collection.
gradient filtering
A method of edge detection using two filters. One filter enhances horizontal edges and the other enhances vertical edges. The results are then combined into a single output cell value. Gradient filtering usually produces better results than a single high-pass filter. (See also: filtering)
GRASS
Geographic Resources Analysis Support System. A public domain GIS system developed at the U.S. Army's Construction Engineering Research Laboratory in Urbana, Illinois. It is now managed by an inter-agency governmental committee. The system was designed for minicomputers and has since been implemented on various microcomputers. It is a raster-based system that can interface to vector data sets for data preparation and import/export.
Graphic Tablet
A small digitizer (usually 28 cms x 26 cms) for interactive work with a GIS or CAD/CAM system.
graticule
A network of parallels and meridians for equal intervals of latitude and longitude on the earth's surface. The form and appearance of a graticule depends on the map projection employed. The lines comprising a graticule may form a square / rectangular network; a graticule may also be comprised of two families of curves having variable separation between them. The mathematical framework of a map is comprised of a grid, or a graticule, or both. A grid is always comprised of a square network of parallel lines representing particular linear distances on the ground.
gray out
To dim a label that is nonfunctional / unavailable in current context.
graytone or grayscale
An image or raster object that contains tone levels and intensities that grade gradually from black (no intensity) to white (high intensity). Differences between one tone level and another correspond to differences between one data level and another.
graytoned image or raster
An image or raster object that contains tone levels and intensities that grade gradually from black (no intensity) to white (high intensity). Differences between one tone level and another correspond to differences between one data level and another.
great circle
"A circle on a spherical surface such that the plane containing the circle passes through the center of the sphere" (Random House). For example, the equator is a great circle.
greeking
An on-screen substitution, such as a wide gray line or an empty box, for graphic text that is too small to be drawn and viewed distinctly.
green biomass or phytomass
The amount of wet weight or dry weight of growing, chlorophyll-containing plant material per unit ground area. It is usually expressed in grams per square meter, tons per acre, or metric tons per hectare.
greenness
The biophysical property of the surface of the earth that indicates its greenness in a biological sense and gives a qualitative estimate of green biomass. It measures plant vigor, water wellness, and chlorophyll content. Greenness can be computed from Landsat MSS or TM images and stored in a raster object by Kauth's greenness, brightness, wetness transformations.
GRID
A fully intergrated grid (cell based) geoprocessing system for use with ARC/INFO. GRID supports a Map Algebra Spatial language that allows sophisticated spatial modelling and analysis.
A geographic data model representing information on an array of equally sized square cells arranged in rows and columns. Each grid cell is referenced by its geographic X, Y location.
Grid Cell
A discreetly uniform unit that represents a portion of the Earth, such as a sq. meter or sq. mile, Each grid cell has a value that corresponds to the feature or characteristic at the site such as a soil type, census tract or vegetation class.
ground control
Targeted terrain surface features, such as road intersections, of known (surveyed) horizontal and vertical coordinates used to orient and otherwise relate airphotos and satellite images to a map projection.
ground resolution
The limit of detail clarity in an image of the earth's surface collected by some remote sensing device, usually measured in meters. An image with a ground resolution of 10 meters shows no ground features smaller than 10 x 10 meters. Each data cell in such an image contains a value for a distinct 10 x 10 meter surface area.
ground truth
Information collected at the same site and at the same time as a remote sensing system is collecting data. Ground truth is considered more accurate, and is used to interpret and calibrate remotely sensed observations.
group or display group
Each view window in the display process can include multiple groups of objects, and each group can contain multiple layers of objects. Groups allow side-by-side display of related items in a single window.
group list
A list in the control window of the display process that shows the currently defined groups.
GUI
Graphical User Interface. A system program that provides windows and icons and a way for the user to manipulate them with keyboard and mouse. In the X Window System, the GUI is a client and not part of the X server. Thus several different GUI's are possible under X.