AutoCAD Map 3D uses Grid Data Catalog (GDC) files to determine the order in which grid data files are used in areas of overlap when converting from one to another in certain parts of the world, such as North America, France, Australia, and New Zealand.
If there is an overlapping coverage, AutoCAD Map 3D selects the grid data file with the smallest grid cell in the region of the conversion. If the grid cell sizes are the same, AutoCAD Map 3D selects the grid data file that appears first in the GDC file.
Grid Data Catalog files are ASCII text files with a .gdc extension that are stored, by default, in the C:\Documents and Settings\All Users\Application Data\Autodesk\Geospatial Coordinate Systems folder. The base name of a GDC file is based on the transformation with which it is associated. For example, the GDC file for the NAD27 to NAD83 transformation is Nad27toNad83.gdc.
Each line in a GDC file points to a grid data file (also known as a datum shift data file). GDC files also often include comments that are preceded by a pound sign (#) and a fallback datum definition that is used when none of the grid data files provide coverage for a point being converted.