Fallback Specification for Geodetic Catalog Files
A fallback specification can appear in the Geodetic data catalog File.
The specification takes the form:
where KEYNAME is the key name of a datum definition defined in the Datum Dictionary.
In the event that the listed files do not cover a point to be converted, this definition is used to calculate the appropriate datum shift. Obviously, the conversion technique of the fallback datum definition must not be one that relies on a grid data file. A Seven Parameter or Molodensky transformation is usually used.
A fallback specification can appear anywhere in the catalog file, and is case insensitive. If more than one specification is encountered, only the last one will be recognized.
If you do not use a fallback method, you can optionally be informed during the transformation if griddata coverage was not found.