GCP
Ground-control point.

Geocoding
Also known as georeferencing. Allocating planimetric coordinates (X, Y) to a point or converting image coordinates to map coordinates.

Geographic Information System (GIS)
A data-handling and analysis system based on sets of data distributed spatially in two dimensions. 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 correction
Image-processing procedure that corrects spatial distortions in an image.

Geostationary
Refers to satellites travelling at the angular velocity at which the earth rotates; as a result,
they remain above the same point on earth at all times.

Geostationary Operational Environmental Satellite
A NOAA satellite that acquires visible and thermal IR images for meteorologic purposes.

Geostationary orbit
An orbit at 41 000 km in the direction of the Earth's rotation, which matches speed so that a satellite
remains over a fixed point on the Earth's surface.

Geothermal
Refers to heat from sources within the earth.

G.I.S.
In the strictest sense, a GIS is a computer system capable of assembling, storing, manipulating, and displaying geographically referenced information, i.e. data identified according to their locations. Practitioners also regard the total GIS as including operating personnel and the data that go into the system.

Goddard Space Flight Center
The NASA facility at Greenbelt, Maryland, that is also a Landsat ground receiving station.

GMT
Greenwich mean time. This international 24-h system is used to designate the time at which
Landsat images are acquired.

GOES
Geostationary Operational Environmental Satellite.

Global Positioning System (GPS)
The GPS system relies on a network of 24 satellites, approximately 11,000 miles up in space, in precisely controlled orbits. By locking on to the signals these satellites send, any GPS receiver can, with great precision, triangulate its precise latitude and longitude coordinates.

Gossan
Surface occurrence of iron oxide formed by the weathering of metallic sulfide ore minerals.

Granularity
Graininess of developed photographic film that is determined by the texture of the silver grains.

Gray scale
A sequence of gray tones ranging from black to white.

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 a raster format to be displayed and analyzed digitally after the values have been rescaled to the 0-255 range.

Ground Control Point (GCP)
A geographic feature of known location that is recognizable on images and can be used to determine geometric corrections.

Ground range
On radar images, the distance from the ground track to an object.

Ground-Range image
Radar image in which the scale in the range direction is constant.

Ground receiving station
Facility that records data transmitted by a satellite, such as Landsat.

Ground resolution cell
Area on the terrain that is covered by the IFOV of a detector.

Ground swath
Width of the strip of terrain that is imaged by a scanner system.