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## 7 Glossary

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ASCII:
  [American Standard Code for Information Interchange] The predominant character set encoding of present-day computers.
Cell:
  A rectangular area in Raster space, in which a single pixel value is filled.
Code:
  In GeoTIFF, a code is a value assigned to a GeoKey, and has one of 65536 possible values.
Coordinate System:
  A systematic way of assigning real (x,y,z..) coordinates to a surface or volume. In Geodetics the surface is an ellipsoid used to model the earth.
Datum:
  a mathematical approximation to all or part of the earth's surface. Defining a datum requires the definition of an ellipsoid, its location and orientation, as well as the area for which the datum is valid.
Device Space
  A coordinate space referencing scanner, printers and display devices.
DOUBLE:
  8-byte IEEE double precision floating point.
Ellipsoid:
  A mathematically defined quadratic surface used to model the earth.
EPSG:
  European Petroleum Survey Group.
Flattening:
  For an ellipsoid with major and minor axis lengths (a,b), the flattening is defined by:,

                                           f = (a - b)/a

  For the earth, the value of f is approximately 1/298.3
Geocoding:
  An image is geocoded if a precise algorithm for determining the earth-location of each point in the image is defined.
Geographic Coordinate System:
  A Geographic CS consists of a well-defined ellipsoidal datum, a Prime Meridian, and an angular unit, allowing the assignment of a Latitude-Longitude (and optionally, geodetic height) vector to a location on earth.
GeoKey
  In GeoTIFF, a GeoKey is equivalent in function to a TIFF tag, but uses a different storage mechanism.
Georeferencing:
  An image is georeferenced if the location of its pixels in some model space is defined, but the transformation tying model space to the earth is not known.
GeoTIFF:
  A standard for storing georeference and geocoding information in a TIFF 6.0 compliant raster file.
Grid
  A coordinate mesh upon which pixels are placed
IEEE
  Institute of Electrical and Electronics Engineers, Inc.
IFD:
  In TIFF format, an Image File Directory, containing all the TIFF tags for one image in the file (there may be more than one).
Meridian:
  Arc of constant longitude, passing through the poles.
Model Space
  A flat geometrical space used to model a portion of the earth.
Parallel:
  Lines of constant latitude, parallel to the equator.
Pixel:
  A dimensionless point-measurement, stored in a raster file.
POSC:
  Petrotechnical Open Software Corporation.
Prime Meridian:
  An arbitrarily chosen meridian, used as reference for all others, and defined as 0 degrees longitude.
Projection
  A projection in GeoTIFF consists of a linear (X,Y) coordinate system, and a coordinate transformation method (such as Transverse Mercator) to tie this system to an unspecified Geographic CS..
Projected Coordinate System
  The result of the application of a projection transformation of a Geographic coordinate system
Raster Space:
  A continuous planar space in which pixel values are visually realized.
RATIONAL:
  In TIFF format, a RATIONAL value is a fractional value represented by the ratio of two unsigned 4-byte integers.
SDTS
  The USGS Spatial Data Transmission Standard.
Tag:
  In TIFF format, a tag is packet of numerical or ASCII values, which have a numerical "Tag" ID indicating their information content.
TIFF:
  Acronym for Tagged Image File Format; a platform-independent, extensive specification for storing raster data and ancillary information in a single file.
USGS
  US Geological Survey

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# END OF SPECIFICATION

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