GAC: Global Area Coverage; produced by sub-sampling AVHRR LAC imagery to simulate a 5 x 3 km pixel
size.
Gamut: Total range of colours which can be output to a device such
as a display monitor or plotter.
Gaussian: A statistical term that refers to a normal distribution
of values.
GCP: Ground Control Point.
Geocoding: Geographical referencing or coding of the location(s)
of data items.
Geographic information system (GIS): A data-handling and analysis
system based on sets of spatial attributes. The data sets may be map-oriented,
when they comprise qualitative attributes of an area recorded as lines,
points and areas (often in vector format), or image-oriented, when the data
are quantitative attributes referring to cells in a rectangular grid (usually
in raster format). It is also known as a geobased, or geocoded, information
system.
Geometric distortion: Distortion in an image which is due to movements
or non-linear characteristics of either sensor, platform, or the Earth's
surface.
Geometrical transformations: Adjustments made in the image data to
change its geometrical character, usually to improve its geometrical consistency
or cartographic utility.
Geosat: US Navy geodetic satellite.
Geostationary: A satellite which orbits in such a way that it remains
over the same position on the Earth's surface, that is, its orbital period
exactly matches the Earth's rotation time.
Geomathematics: Mathematical and statistical methods for handling
data related to the earth sciences.
Geostatistics: Specialised branch of statistics, originally developed
for estimating oil reserves and now includes spatial statistics and mathematical
morphology.
Giga: Standard prefix indicating 10^9.
GIS: Geographic Information System.
Global Positioning System (GPS): US Navy network of satellites and
ground stations for accurate location fixing of ground receivers and other
satellites around the globe.
GMS: Geostationary Meteorological Satellite.
GOES: Geostationary Operational Environmental Satellite.
GPS: Global Positioning System.
Graded series: A scale of colours used in graphics to present change
in a variable. A graded series may be composed of progressive change in
either lightness or saturation of one hue, in hue steps around the hue circle,
or along the grey scale.
Graticule: The network of longitude and latitude lines upon which
a map is drawn.
Grey scale: A calibrated sequence of grey tones ranging from black
to white.
Grey scale image: Represents pixel values as a grey scale.
Grid format: The result of interpolation from values of a variable
measured at irregularly distributed points, or along survey lines, to values
referring to square cells in a rectangular array. It forms a step in the
process of contouring data, but can also be used as the basis for raster
format to be displayed and analysed digitally after the values have been
rescaled to the 0-255 range.
Grid line: One of the lines in a grid system; a line used to divide
a map into squares. East-west lines in a grid system are x-lines, and north-south
lines are y-lines.
Ground Control Points (GCPs): Points which are used to obtain external
reference information when tracking and spacecraft attitude data are not
known precisely. They may be any features in the study area that can be
detected and precisely located on both a map and image.
Ground information: Information derived from ground data and surveys
to support interpretation of remotely sensed data.
Ground range: The distance from the ground track (nadir) to a given
object.
Ground receiving station: A facility that records data from satellite-borne
sensing systems.
Ground resolution cell: The area on the terrain that is covered by
the instantaneous field of view of a detector. The size of the ground resolution
cell is determined by the altitude of the remote sensing system and the
instantaneous field of view of the detector.
Ground sampling element: The area on the Earth's surface that is
covered by the instantaneous field of view of a sensor.
Ground track: The vertical projection of the actual flight path of
an aerial or space vehicle onto the surface of the Earth or other body.
Ground truth: Term coined for data and information obtained on surface
or sub-surface features to aid with the interpretation of remotely sensed
data.
GVI: Global Vegetation Index.
