Names/Extensions for Geodetic Catalog Files
The Coordinate System tools determine the sequence of grid file searches by listing eligible files in various .gdc files (Geodetic Catalog files). Grid files in turn contain latitude/longitude values of one datum that corresponding to a second datum. These are used for lookup purposes in order to perform a datum shift using the grid file based shift indicated by a datum definition. In cases where overlap of gridfile data is encountered, the first file containing coverage for a value being shifted is used. By sequencing grid data files within a .gdc file, the user can control which value will be encountered. Example: NAD shifts for Canada and the United States overlap. Depending on your national mapping standard, you will sequence the grid data differently or even remove files from the search entirely. Coordinate System tools can (optionally) notify the user in the event that no coverage exists for one or more points being shifted. Also see: Preferences>Projection Transform options.
A data catalog file consists of lines of ordinary text. The pound sign character (‘#’) introduces a comment that extends to the end of the text line. Each (non-comment) line in a data catalog file contains the path to a data file, a buffer size, a flags word, and a density. The buffer size, flags, and density specifications are optional. If present, they are separated from the path name, and each other, by commas.