Identify the celestial body in GeoTIFF

[desruisseaux]

Defines a new GeoKey for identifying the celestial body (Mars, Venus, etc.) in a GeoTIFF file. It would be used in User-defined Model Coordinate Reference Systems. Some sections from GeoTIFF specification to edit are:

• User-defined geographic 2D CRS
• User-defined geocentric CRS

In particular the following list of items should be updated with the item given in italic characters:

• user-defined geodetic datum name and other defining information:
  • the geodetic datum name (through the GeodeticCitationGeoKey),
  • the celestial body name (through the CelestialBodyGeoKey),

CelestialBodyGeoKey would be optional. If omitted, default value would be "Earth".

Proposed numerical value for CelestialBodyGeoKey is 2063.