2011

Table Of Contents

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Grid Data Files for Australia and New Zeal-

and (page 109)

Specify a new grid data file for Australia or

New Zealand

Australian Datum Transformation and Co-

ordinate System Changes (page 110)

Use the high accuracy GDA94 datum for

Australia

Datum Shift Issues for North American

Users (page 112)

Specify a grid data version to resolve differ-

ences between source and destination co-

ordinate systems for Canada

Coordinate System Files (page 113)Modify a coordinate system file

Grid Data Catalog (GDC) Files

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

datum (page 2059) 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.